DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.
DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.
Although the words “he,” “him,” and “his” are used sparingly in this course to enhance communication, they are not intended to be gender driven or to affront or discriminate against anyone.
i
PREFACE By enrolling in this self-study course, you have demonstrated a desire to improve yourself and the Navy. Remember, however, this self-study course is only one part of the total Navy training program. Practical experience, schools, selected reading, and your desire to succeed are also necessary to successfully round out a fully meaningful training program.
COURSE OVERVIEW: To introduce the student to the subject of Magnetic Recording who needs such a background in accomplishing daily work and/or in preparing for further study.
THE COURSE: This self-study course is organized into subject matter areas, each containing learning objectives to help you determine what you should learn along with text and illustrations to help you understand the information. The subject matter reflects day-to-day requirements and experiences of personnel in the rating or skill area. It also reflects guidance provided by Enlisted Community Managers (ECMs) and other senior personnel, technical references, instructions, etc., and either the occupational or naval standards, which are listed in the Manual of Navy Enlisted Manpower Personnel Classifications and Occupational Standards, NAVPERS 18068.
THE QUESTIONS: The questions that appear in this course are designed to help you understand the material in the text.
VALUE: In completing this course, you will improve your military and professional knowledge. Importantly, it can also help you study for the Navy-wide advancement in rate examination. If you are studying and discover a reference in the text to another publication for further information, look it up.
ii
“I am a United States Sailor.
I will support and defend the Constitution of the United States of America and I will obey the orders of those appointed over me.
I represent the fighting spirit of the Navy and those who have gone before me to defend freedom and democracy around the world.
I proudly serve my country’s Navy combat team with honor, courage and commitment.
I am committed to excellence and the fair treatment of all.”
iii
1. Introduction to Magnetic Recording......................................................................... 1-1
2. Magnetic Tape .......................................................................................................... 2-1
3. Magnetic Tape Recorder Heads ............................................................................... 3-1
4. Magnetic Tape Recorder Transports ........................................................................ 4-1
5. Magnetic Tape Recorder Record and Reproduce Electronics .................................. 5-1
6. Magnetic Tape Recorder Specifications................................................................... 6-1
7. Digital Magnetic Tape Recording ............................................................................ 7-1
8. Magnetic Disk Recording......................................................................................... 8-1
I. Glossary.................................................................................................................. AI-1
II. References .............................................................................................................. AII-1
INDEX ...................................................................................................................... INDEX-1
iv
The figures listed below and included in this edition of NEETS, Module 23, Magnetic Recording, were provided by Datatape, Inc. Permission to use these illustrations is gratefully acknowledged.
Datatape, Inc. 3-2, 3-3, 3-4, 5-3, 6-3, 6-4 A&B, 7-1, 7-4, 7-5, 7-6
v
The Navy Electricity and Electronics Training Series (NEETS) was developed for use by personnel in many electrical- and electronic-related Navy ratings. Written by, and with the advice of, senior technicians in these ratings, this series provides beginners with fundamental electrical and electronic concepts through self-study. The presentation of this series is not oriented to any specific rating structure, but is divided into modules containing related information organized into traditional paths of instruction.
The series is designed to give small amounts of information that can be easily digested before advancing further into the more complex material. For a student just becoming acquainted with electricity or electronics, it is highly recommended that the modules be studied in their suggested sequence. While there is a listing of NEETS by module title, the following brief descriptions give a quick overview of how the individual modules flow together.
Module 1, Introduction to Matter, Energy, and Direct Current, introduces the course with a short history of electricity and electronics and proceeds into the characteristics of matter, energy, and direct current (dc). It also describes some of the general safety precautions and first-aid procedures that should be common knowledge for a person working in the field of electricity. Related safety hints are located throughout the rest of the series, as well.
Module 2, Introduction to Alternating Current and Transformers, is an introduction to alternating current (ac) and transformers, including basic ac theory and fundamentals of electromagnetism, inductance, capacitance, impedance, and transformers.
Module 3, Introduction to Circuit Protection, Control, and Measurement, encompasses circuit breakers, fuses, and current limiters used in circuit protection, as well as the theory and use of meters as electrical measuring devices.
Module 4, Introduction to Electrical Conductors, Wiring Techniques, and Schematic Reading, presents conductor usage, insulation used as wire covering, splicing, termination of wiring, soldering, and reading electrical wiring diagrams.
Module 5, Introduction to Generators and Motors, is an introduction to generators and motors, and covers the uses of ac and dc generators and motors in the conversion of electrical and mechanical energies.
Module 6, Introduction to Electronic Emission, Tubes, and Power Supplies, ties the first five modules together in an introduction to vacuum tubes and vacuum-tube power supplies.
Module 7, Introduction to Solid-State Devices and Power Supplies, is similar to module 6, but it is in reference to solid-state devices.
Module 8, Introduction to Amplifiers, covers amplifiers. Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits, discusses wave generation and wave-shaping circuits.
Module 10, Introduction to Wave Propagation, Transmission Lines, and Antennas, presents the characteristics of wave propagation, transmission lines, and antennas.
vi
Module 11,Microwave Principles, explains microwave oscillators, amplifiers, and waveguides.
Module 12,Modulation Principles, discusses the principles of modulation.
Module 13, Introduction to Number Systems and Logic Circuits, presents the fundamental concepts of number systems, Boolean algebra, and logic circuits, all of which pertain to digital computers.
Module 14, Introduction to Microelectronics, covers microelectronics technology and miniature and microminiature circuit repair.
Module 15, Principles of Synchros, Servos, and Gyros, provides the basic principles, operations, functions, and applications of synchro, servo, and gyro mechanisms.
Module 16, Introduction to Test Equipment, is an introduction to some of the more commonly used test equipments and their applications.
Module 17, Radio-Frequency Communications Principles, presents the fundamentals of a radio- frequency communications system.
Module 18, Radar Principles, covers the fundamentals of a radar system.
Module 19, The Technician's Handbook, is a handy reference of commonly used general information, such as electrical and electronic formulas, color coding, and naval supply system data.
Module 20,Master Glossary, is the glossary of terms for the series.
Module 21, Test Methods and Practices, describes basic test methods and practices.
Module 22, Introduction to Digital Computers, is an introduction to digital computers.
Module 23,Magnetic Recording, is an introduction to the use and maintenance of magnetic recorders and the concepts of recording on magnetic tape and disks.
Module 24, Introduction to Fiber Optics, is an introduction to fiber optics.
Embedded questions are inserted throughout each module, except for modules 19 and 20, which are reference books. If you have any difficulty in answering any of the questions, restudy the applicable section.
Although an attempt has been made to use simple language, various technical words and phrases have necessarily been included. Specific terms are defined in Module 20,Master Glossary.
Considerable emphasis has been placed on illustrations to provide a maximum amount of information. In some instances, a knowledge of basic algebra may be required.
Assignments are provided for each module, with the exceptions of Module 19, The Technician's Handbook; and Module 20, Master Glossary. Course descriptions and ordering information are in NAVEDTRA 12061, Catalog of Nonresident Training Courses.
vii
Throughout the text of this course and while using technical manuals associated with the equipment you will be working on, you will find the below notations at the end of some paragraphs. The notations are used to emphasize that safety hazards exist and care must be taken or observed.
AN OPERATING PROCEDURE, PRACTICE, OR CONDITION, ETC., WHICH MAY RESULT IN INJURY OR DEATH IF NOT CAREFULLY OBSERVED OR FOLLOWED.
AN OPERATING PROCEDURE, PRACTICE, OR CONDITION, ETC., WHICH MAY RESULT IN DAMAGE TO EQUIPMENT IF NOT CAREFULLY OBSERVED OR FOLLOWED.
An operating procedure, practice, or condition, etc., which is essential to emphasize.
viii
The text pages that you are to study are listed at the beginning of each assignment. Study these pages carefully before attempting to answer the questions. Pay close attention to tables and illustrations and read the learning objectives. The learning objectives state what you should be able to do after studying the material. Answering the questions correctly helps you accomplish the objectives.
Read each question carefully, then select the BEST answer. You may refer freely to the text. The answers must be the result of your own work and decisions. You are prohibited from referring to or copying the answers of others and from giving answers to anyone else taking the course.
To have your assignments graded, you must be enrolled in the course with the Nonresident Training Course Administration Branch at the Naval Education and Training Professional Development and Technology Center (NETPDTC). Following enrollment, there are two ways of having your assignments graded: (1) use the Internet to submit your assignments as you complete them, or (2) send all the assignments at one time by mail to NETPDTC.
Grading on the Internet: Advantages to Internet grading are:
• you get your results faster; usually by the next working day (approximately 24 hours).
assignments. To submit your assignment answers via the Internet, go to:
http://courses.cnet.navy.mil
Grading by Mail: When you submit answer sheets by mail, send all of your assignments at one time. Do NOT submit individual answer sheets for grading. Mail all of your assignments in an envelope, which you either provide yourself or obtain from your nearest Educational Services Officer (ESO). Submit answer sheets to:
Answer Sheets: All courses include one “scannable” answer sheet for each assignment. These answer sheets are preprinted with your SSN, name, assignment number, and course number. Explanations for completing the answer sheets are on the answer sheet.
Do not use answer sheet reproductions: Use only the original answer sheets that we provide—reproductions will not work with our scanning equipment and cannot be processed.
Follow the instructions for marking your answers on the answer sheet. Be sure that blocks 1, 2, and 3 are filled in correctly. This information is necessary for your course to be properly processed and for you to receive credit for your work.
Courses must be completed within 12 months from the date of enrollment. This includes time required to resubmit failed assignments.
ix
If your overall course score is 3.2 or higher, you will pass the course and will not be required to resubmit assignments. Once your assignments have been graded you will receive course completion confirmation.
If you receive less than a 3.2 on any assignment and your overall course score is below 3.2, you will be given the opportunity to resubmit failed assignments. You may resubmit failed assignments only once. Internet students will receive notification when they have failed an assignment--they may then resubmit failed assignments on the web site. Internet students may view and print results for failed assignments from the web site. Students who submit by mail will receive a failing result letter and a new answer sheet for resubmission of each failed assignment.
After successfully completing this course, you will receive a letter of completion.
Errata are used to correct minor errors or delete obsolete information in a course. Errata may also be used to provide instructions to the student. If a course has an errata, it will be included as the first page(s) after the front cover. Errata for all courses can be accessed and viewed/downloaded at:
http://www.advancement.cnet.navy.mil
We value your suggestions, questions, and criticisms on our courses. If you would like to communicate with us regarding this course, we encourage you, if possible, to use e-mail. If you write or fax, please use a copy of the Student Comment form that follows this page.
For subject matter questions:
E-mail<EMAIL_ADDRESS>Phone: Comm:<PHONE_NUMBER>, ext. 1728
DSN: 922-1001, ext. 1728 FAX:<PHONE_NUMBER> (Do not fax answer sheets.)
E-mail<EMAIL_ADDRESS>Phone: Toll Free:<PHONE_NUMBER>
Comm:<PHONE_NUMBER>/1181/1859 DSN: 922-1511/1181/1859 FAX:<PHONE_NUMBER> (Do not fax answer sheets.)
If you are a member of the Naval Reserve, you will receive retirement points if you are authorized to receive them under current directives governing retirement of Naval Reserve personnel. For Naval Reserve retirement, this course is evaluated at 2 points. (Refer to Administrative Procedures for Naval Reservists on Inactive Duty, BUPERSINST 1001.39, for more information about retirement points.)
xi
NAVEDTRA: 14195 Date:
We need some information about you:
Your comments, suggestions, etc.:
Privacy Act Statement: Under authority of Title 5, USC 301, information regarding your military status is requested in processing your comments and in preparing a reply. This information will not be divulged without written authorization to anyone other than those within DOD for official use in determining performance.
1-1
Learning objectives are stated at the beginning of each chapter. They serve as a preview of the information you are expected to learn in the chapter. The comprehension check questions placed within the text are based on the objectives. By successfully completing those questions and the associated NRTC, you show that you have met the objectives and have learned the information. The learning objectives for this chapter are listed below.
After completing this chapter, you will be able to do the following:
1. Describe the history and purpose of magnetic recording.
2. State the prerequisites for magnetic recording.
3. Describe a magnetic recording head, how it’s constructed, and how it operates.
Have you ever wondered how a whole album of your favorite music got onto one of those little cassette tapes? Or, what about computer floppy disks; have you ever wondered how they can hold 180 or more pages of typed text? The answer to both of these questions is magnetic recording.
Magnetic recording devices seldom get much attention until they fail to work. But without magnetic recording, recording your favorite television show on a video cassette recorder would be impossible, portable tape players wouldn’t exist, and you wouldn’t be able to get money from an automated bank teller machine at two o’clock in the morning.
Now what about the Navy? Could it operate without magnetic recording? The answer is definitely no. Without it:
• Computer programs and data would have to be stored on either paper cards or on rolls of paper tape. Both of these methods need a lot of storage space, and they take much longer to load into and out of the computer.
• There wouldn’t be any movies to show or music to play on the ship’s entertainment system when the ship is at sea and is out of range for television and radio reception.
• Intelligence-collection missions would be impossible since you couldn’t store the collected signals for later analysis.
As you can see, magnetic recording plays a very important part both in our Navy life and in our civilian life.
1-2
In 1888, Oberlin Smith originated the idea of using permanent magnetic impressions to record sounds. Then in 1900, Vladeniar Poulsen brought Mr. Smith’s dream to reality. At the Paris Exposition, he demonstrated a Telegraphone. It was a device that recorded sounds onto a steel wire. Although everyone thought it was a great idea, they didn’t think it would succeed since you had to use an earphone to hear what was recorded. It wasn’t until 1925, when electronic amplifiers were developed, that magnetic recording started to receive the attention it deserved.
The best magnetic recording is the one that produces an output signal identical to the input signal. It didn’t take long to realize that the magnetism generated during the recording process didn’t vary directly to the current which caused it. This is because there’s a step in the magnetism curve where it crosses the zero point and changes polarity. This step causes the output signal to be distorted when compared with the input signal. Figure 1-1 shows this step.
In 1907, Mr. Poulsen discovered a solution to this problem. He discovered dc bias. He found that if a fixed dc voltage were added to the input signal, it moved the input signal away from the step in the magnetism curve. This prevented the input signal from crossing the zero-point of the magnetism curve. The result is an output signal exactly like the input signal. Figure 1-2 shows this process.
1-3
Unfortunately, dc bias had its problems. Since only a small portion of the magnetism curve was straight enough to use, the output signal was weak compared with the natural hiss of the unmagnetized tape passing the playback head. This is commonly called poor signal-to-noise ratio (SNR). We’ll explain SNR in more detail later.
From the beginning, the U.S. Naval Research Laboratories (NRL) saw great potential in magnetic recording. They were especially interested in using it to transmit telegraph signals at high speed. After electronic amplifiers were invented around 1925, W.L. Carlson and G.W. Carpenter at the NRL made the next important magnetic recording discovery. They found that adding an ac bias voltage to the input signal instead of a fixed dc bias voltage would
To make ac bias work, they used an ac frequency for the bias voltage that was well above what could be heard, and a level that placed the original input signal away from both steps in the magnetism curve. This resulted in two undistorted output signals that could be combined into one strong output. See figure 1-3.
1-4
Until 1935, all magnetic recording was on steel wire. Then, at the 1935 German Annual Radio Exposition in Berlin, Fritz Pfleumer demonstrated his Magnetophone. It used a cellulose acetate tape coated with soft iron powder. The Magnetophone and its "paper" tapes were used until 1947 when the 3M Company introduced the first plastic-based magnetic tape.
In 1956, IBM introduced the next major contribution to magnetic recording—the hard disk drive. The disk was a 24-inch solid metal platter and stored 4.4 megabytes of information. Later, in 1963, IBM reduced the platter size and introduced a 14-inch hard disk drive.
Until 1966, all hard disk drives were "fixed" drives. Their platters couldn't be removed. Then in 1966, IBM introduced the first removable-pack hard disk drive. It also used a 14-inch solid metal platter.
In 1971, magnetic tape became popular again when the 3M Company introduced the first 1/4-inch magnetic tape cartridge and tape drive. In that same year, IBM invented the 8-inch floppy disk and disk drive. It used a flexible 8-inch platter of the same material as magnetic tape. Its main goal was to replace punched cards as a program-loading device.
The next contribution to magnetic recording literally started the personal computer (PC) revolution. In 1980, a little-known company named Seagate Technology invented the 5-1/4-inch floppy disk drive. Without it, PCs as we know them today would not exist.
From then on, it was all downhill. Magnetic tape became more sophisticated. Floppy disks and disk drives became smaller, while their capacities grew bigger. And hard disk capacities just went through the roof. All of the major hurdles affecting magnetic recording had been successfully cleared, and it was just a matter of refining both its methods and materials.
Q-1. Why did the early inventors of magnetic recording find it necessary to add a fixed dc bias to the input signal?
Q-2. How does dc bias added to the input signal correct the distortion in the output signal?
1-5
Q-3. Why does adding dc vice ac bias voltage to the input signal result in a poor signal-to-noise ratio (SNR)?
Q-4. What are three advantages of adding an ac bias voltage to the input signal instead of adding a fixed dc bias voltage?
Q-5. Why does using ac vice dc bias voltage result in a stronger output signal? PREREQUISITES FOR MAGNETIC RECORDING
To perform magnetic recording, you need three things:
1. An input signal you wish to record.
2. A recording medium. (This is a recording surface that will hold the signal you wish to record.) 3. A magnetic head to convert the input signal into a magnetic field so it can be recorded.
If any one of these are missing, magnetic recording cannot take place.
An input signal can come from a microphone, a radio receiver, or any other source that’s capable of producing a recordable signal. Some input signals can be recorded immediately, but some must be processed first. This processing is needed when an input signal is weak, or is out of the frequency response range of the recorder.
A recording medium is any material that has the ability to become magnetized, in varying amounts, in small sections along its entire length. Some examples of this are magnetic tape and magnetic disks. These are thoroughly discussed in chapter 2 of this module.
Magnetic heads are the heart of the magnetic recording process. They are the transducers that convert the electrical variations of your input signal into the magnetic variations that are stored on a recording medium. Without them, magnetic recording isn’t possible.
Magnetic heads actually do three different things. They transfer, or record, the signal information onto the recording medium. They recover, or reproduce, the signal information from the recording medium. And they remove, or erase, the signal information from the recording medium.
MAGNETIC HEAD CONSTRUCTION.—A magnetic head is a magnetic core wrapped with a coil of very thin wire (see figure 1-4). The core material is usually shaped like the letter C, and is made from either iron or ceramic-ferrite material. The number of turns of wire placed on the core depends on the purpose of that specific head. The gap in the core is called a head gap. It's here that magnetic recording actually takes place. We'll go into more detail of magnetic head construction in chapter 3.
1-6
MAGNETIC HEAD OPERATION.—Whether you're recording on magnetic tapes or disks, all magnetic heads operate the same way. When an electric current passes through the coil of a magnetic head, magnetic field lines associated with the electric current follow paths through the core material. When the magnetic fields get to the head gap, some of them spread outside the core to form a fringing field. When a recording medium is passed through this fringing field, it is magnetized in relation to the electric current. This is called magnetic recording. Figure 1-4 illustrates this process.
Q-6. What three things are required to perform magnetic recording? Q-7. What is the meaning of the term recording medium as it pertains to magnetic recording? Q-8. What are the three functions of the magnetic heads on a magnetic recording device?
This chapter briefly covered the historical development of magnetic recording principles and devices. The following is a summary of important points in the chapter.
The BEST MAGNETIC RECORDING is one that produces an output signal that is identical to the input signal. However, a step in the magnetic curve causes the output signal to be distorted.
In 1907, DC BIAS was added to the input signal to remove the distortion in the output signal. But the dc bias caused a weak output signal with a poor SNR. Around 1925, the NRL used AC BIAS to reproduce a stronger output signal and greatly improve the SNR.
To perform magnetic recording, you need (1) an INPUT SIGNAL, (2) a RECORDING MEDIUM, and (3) a MAGNETIC HEAD.
A RECORDING MEDIUM is any material that can become magnetized in varying amounts (such as magnetic tape and disks).
MAGNETIC HEADS are used to (1) record the signal onto the recording medium, (2) reproduce the signal from the recording medium, and (3) erase the signal from the recording medium.
1-7
ANSWERS TO QUESTIONS Q1. THROUGH Q8. A-1. Because a step in the magnetism curve where it crosses the zero point and changes polarity causes
the output signal to be distorted. See figure 1-1. A-2. The dc bias moves the input signal away from the step in the magnetism curve. This prevents the
input signal from crossing the zero-point of the magnetism curve. See figure 1-2. A-3. With dc bias, the SNR is poor because only a small portion of the magnetism curve is straight
enough to use, thus the output signal is weak compared with the natural tape hiss.
A-4.
a. Reproduces a stronger output signal.
b. Greatly improves the SNR.
c. Greatly reduces the natural tape hiss.
A-5. Because an ac bias voltage of the proper frequency and level places the input signal away from both steps in the magnetism curve. The result is two undistorted output signals that are combined into one strong output.
A-6.
a. An input signal.
b. A recording medium.
c. A magnetic head.
A-7. A recording medium is any material that has the ability to become magnetized, in varying amounts, in small sections along its entire length.
A-8.
a. Record the signal onto the recording medium.
b. Reproduce the signal from the recording medium. c. Erase the signal from the recording medium.
2-1
After completing this chapter, you’ll be able to do the following:
1. Describe the physical properties of magnetic tape in terms of:
a. The Three Basic Materials Used To Make Magnetic Tape.
2. The function of the magnetic tape’s base material, oxide coating, and binder glue.
3. Describe the two types of magnetic recording tape.
4. Describe the following types of tape errors and their effects on magnetic tape recording: signal dropout, noise, skew, and level.
5. Describe the following causes of magnetic tape failure: normal wear, accidental damage, environmental damage, and winding errors.
6. Describe the purpose and makeup of tape reels and tape cartridges.
7. Describe the two methods for erasing magnetic tape, the characteristics of automatic and manual tape degaussers, and the procedures for degaussing magnetic tape.
8. Describe the proper procedures for handling, storing, and packaging magnetic tape, tape reels, and tape cartridges.
The three basic materials used to make magnetic tape are (1) the base material, (2) the coating of magnetic oxide particles, and (3) the glue to bind the oxide particles onto the base material. See figure 2-1.
2-2
The base material for magnetic tape is made of either plastic or metal. Plastic tape is used more than metal tape because it’s very flexible, it resists mildew and fungus, and it’s very stable at high temperatures and humidity.
Oxide particles that can be magnetized are coated onto the base material. The most common magnetic particles used are either gamma ferric oxide or chromium dioxide. It’s very important that these magnetic particles are uniform in size. If they’re not, the tape’s surface will be abrasive and will reduce the life of the recorder’s magnetic heads.
An ideal magnetic particle is needle-shaped. It’s actual size depends on the frequency of the signal to be recorded. Generally, long particles are used to record long wavelength signals (low-frequency signals), and short particles are used to record short wavelength signals (high-frequency signals). GLUE
The glue used to bond the oxide particles to the base material is usually an organic resin. It must be strong enough to hold the oxide particles to the base material, yet be flexible enough not to peel or crack.
There are two basic types of magnetic recording tape in common use: analog and digital. Analog magnetic tape is used to record, store, and reproduce audio and instrumentation type signals. These signals are usually in a frequency band from very-low frequency (VLF) to 2.5 MHz. Digital magnetic tape is used to record, store, and reproduce computer programs and data. It’s base material thickness is about 50 percent thicker than analog magnetic tape. This allows the digital tape to withstand the more strenuous starts and stops associated with digital magnetic recorder search, read, and write functions.
2-3
Digital magnetic tape is also held to much stricter quality control standards. It’s important not to have any blemishes or coating flaws on the tape’s surface. Because, if you lost one digital data bit, your computer program or data would be bad. In contrast, losing one microsecond of an analog signal is not nearly as critical.
Q-1. Magnetic tape is made of what three basic materials? Q-2. Why is plastic magnetic tape used more than metal tape? Q-3. Which of the two types of magnetic tape is used to record audio and instrumentation type signals
in the VLF to 2.5MHz frequency range? Q-4. What type of magnetic tape is used to record computer programs and data, and what are the
additional thickness and quality standards for this type of tape?
Four types of tape errors that will degrade the performance of a magnetic recording system are signal dropout, noise, skew, and level (signal amplitude changes). DROPOUT ERRORS
Signal dropout is the most common and the most serious type of tape error. It’s a temporary, sharp drop (50% or more) in signal strength caused by either contaminates on the magnetic tape or by missing oxide coating on part of the tape.
During recording and playback, the oxide particles on the tape can flake off and stick to the recorder’s guides, rollers, and heads. After collecting for awhile, the oxide deposits (now oxide lumps) break loose and stick to the magnetic tape. As the tape with the lumps passes over the head, the lumps get between the tape and the head and lift the tape away from the head. This causes the signal dropouts. Although oxide lumps cause most signal dropouts, remember that any contaminate (such as dust, lint or oil) that gets between the tape and the head can cause signal dropouts. NOISE ERRORS
Noise errors are unwanted signals that appear when no signal should appear. They’re usually caused by a cut or a scratch on the magnetic tape. It’s the lack of oxide particles at the cut or the scratch that causes the noise error.
Skew errors only occur on multi-track magnetic tape recorders. The term skew describes the time differences that occur between individual tracks of a single magnetic head when the multi-track tape isn’t properly aligned with the magnetic head.
There are two types of skew errors: fixed and dynamic. Fixed skew happens when properly aligned magnetic tape passes an improperly aligned magnetic head. Dynamic skew happens when misaligned tape passes a properly aligned head. This type of skew is usually caused by one or more of the following:
• A misaligned or worn-out tape transport system.
• A stretched or warped magnetic tape.
2-4
• A magnetic tape that is improperly wound on a reel.
Magnetic tape is manufactured to have a specified output signal level (plus or minus some degree of error). Level errors happen when the actual output signal level either drops or rises to a level outside the expected range. For example, if a magnetic tape is rated for 10 volts ( +/- 10%), any output signal level below 9 volts or above 11 volts is a level error. Level errors are caused by an uneven oxide coating on the magnetic tape. This can come from either the original manufacturing process or from normal wear and tear.
Some causes of level errors are permanent and cannot be removed by any means. For example, a crease in the tape, a hole in the oxide, or a damaged edge. Other causes of level errors are removable and may be cleaned off the tape. For example, oxide flakes or clumps, metallic particles, or dirt are removable.
Q-5. What are four types of tape errors that can degrade a magnetic recording system’s performance? Q-6. What are signal dropouts, and what are two tape defects that can cause signal dropouts? Q-7. What is the most common and most serious type of signal dropout? Q-8. You see a build-up of dust and lint on the take-up reel of a tape recorder. This can cause which of
the four types of tape errors? Q-9. What type of tape error causes noise to appear on the tape when no signal should appear? What
causes this type of tape error? Q-10. The multi-track tape recorder in your computer system has a fixed skew error. What does this
mean and what is the probable cause?
Q-11. Some tapes you are using may have level errors. What does this mean and what is the cause?
Tape failure happens when a magnetic tape’s performance degrades to a point where it’s no longer usable. The exact point where failure occurs will vary, depending on the type of tape and how it is used.
There are four main causes for tape failure:
1. Normal wear (natural causes) 2. Accidental damage
3. Environmental damage
4. Winding errors
Normal wear occurs because the tape must come in contact with fixed surfaces, such as a recorder’s magnetic heads, rollers, and guides. Over time, this repeated contact with the fixed surfaces causes excessive dropout errors and makes the tape unusable.
2-5
Accidental tape damage that causes tape failure is any damage that wouldn’t normally occur under ideal operating and handling conditions. It can be caused by either a human operator or the tape recorder itself. Accidental tape damage caused by human operators can range from accidentally dropping a reel of magnetic tape to improperly threading a magnetic tape recorder. Accidental tape damage caused by recording equipment can occur if the recorder is poorly designed or if the tape transport mechanism is adjusted improperly. ENVIRONMENTAL DAMAGE
The negative effect of environmental extremes on tape can also cause tape failure. Magnetic tape is very flexible and can be used in a wide range of environmental conditions. It’s designed for use in a temperature range of about 2 to 130 degrees Fahrenheit ( - 20 to 55 degrees Celsius), and in a relative humidity range of about 10 to 95%. Of course, these numbers are the extreme. Ideally, magnetic tape should be used and stored at a temperature of about 60 to 80º F (room temperature), and in a relative humidity of about 40 to 60%.
Large changes from the ideal relative humidity cause tape to expand or contract and thus affect the uniformity of a tape's oxide coating. High relative humidity causes the tape to stretch and increases the tape's friction. The increased friction causes increased head wear, head clog by oxide particles, and head-to-tape sticking. Low relative humidity encourages oxide shedding and increases static build-up on tape surfaces, causing the tape to collect airborne contaminants.
The effects of exceeding the ideal temperature and humidity ranges described above can cause the following environmental damage to magnetic tape: tape deformation, oxide shedding, head-to-tape sticking, layer-to-layer sticking, dirt build-up, and excessive tape and head wear.
Magnetic tapes are wound onto tape reels with tension applied. This tension causes great layer-to- layer pressure within the reel pack. Changes in temperature and humidity can cause the backing material to expand or contract, creating even more pressure. All of this pressure causes the tape to become deformed or warped.
At temperatures above 130º F, a tape's oxide coating tends to become soft. At temperatures below 2º F, the oxide coating tends to be brittle. In both cases, the oxide coating will shed, flake off, or otherwise become separated from the base material. These free pieces of oxide will then stick to parts of the tape transport, to the magnetic heads, or back onto the tape and cause dropout or level errors.
At higher temperatures, the tape binder glue can soften to the point where it will stick to the recorder's magnetic head. This head-to-tape sticking causes jerky tape motion. Layer-to-Layer Adhesion
When reels of magnetic tape are stored at higher temperatures, the tape's binder glue may melt and cause the layers of tape to stick to one another. In very severe cases, layer-to-layer adhesion can separate the oxide coating from the base material and completely destroy a tape.
2-6
Dirt build-up happens when the relative humidity level is less than 10%. The low humidity causes static electricity that attracts dirt and dust which builds up on the magnetic tape and other parts of the magnetic tape recorder.
When the relative humidity is more than 95%, the high humidity causes increased friction as the tape passes over the heads. This, in turn, causes excessive tape and head wear.
Q-12. What is tape failure? Q-13. What are four main causes of tape failure? Q-14. How does normal wear cause tape failure? Q-15. Accidental damage to magnetic tape is normally caused by the tape recorder itself or by human
operators of the recorder. What are three frequent causes of such accidental damage? Q-16. Environmental damage to magnetic tape can occur when the tape is stored in an area that
exceeds what ideal temperature and humidity ranges?
Q-17. What six types of environmental damage can occur to tapes in storage when the ideal temperature and humidity ranges are exceeded?
Q-18. After using a tape that was stored in an area where temperatures exceeded 130º F you notice pieces of oxide sticking to the recorder's tape-transport mechanism, to its magnetic heads, and onto the tape. What is the probable cause of these symptoms?
Q-19. Your activity stores its magnetic tape in an area where the temperature is 100º F. What two types of environmental damage could occur that would make these tapes unusable?
Q-20. When the relative humidity is below 10%, what happens to magnetic tape and parts of a tape recorder that could cause environmental damage?
Q-21. How does relative humidity over 95% cause excessive tape and head wear? WINDING ERRORS
Winding errors are another cause of tape failure. They happen when improper winding practices create an excessive or uneven force as the tape is being wound onto a tape reel. The form taken by the tape after it is wound onto the reel is called the tape pack. Winding errors can cause a deformed tape pack that will prevent good head-to-tape contact.
In most cases, a deformed tape pack can be fixed simply by rewinding it onto another reel at the proper tension and at the right temperature and humidity. The four most common types of tape pack deformation are:
1. Cinching
2. Pack-slip
3. Spoking
2-7
4. Windowing
Cinching happens when a tape reel is stopped too quickly. The sudden stop causes the outer layers of magnetic tape to continue to spin after the inner layers have stopped. This causes any loosely wound tape within the pack to unwind and pile up. Figure 2-2 shows an example of a cinched tape pack (note the complete foldover of one tape strand).
Pack slip happens when the tape is loosely wound on the reel and is exposed to excessive vibration or too much heat. This causes the tape to shift (side-to-side), causing steps in the tape pack. When a tape reel with pack slip is used, the magnetic tape will unwind unevenly and rub against the sides of the tape reel or the recorder’s tape guides. This can damage the magnetic tape and cause oxide shedding. Figure 2-3 shows an example of pack slip.
2-8
Spoking happens when magnetic tape is wound onto the tape reel with the tension increasing toward the end of the winding. The higher tension on the outside of the tape pack causes the inner pack to buckle and deform. Spoking is also caused by the uneven pressures created when a tape is wound on a reel that has a distorted hub, or when the tape is wound over a small particle that is deposited on the hub. Figure 2-4 shows a spoked tape pack.
Windows are voids or see-through air gaps in the tape winding. They happen when magnetic tape is loosely wound onto a tape reel, and especially when the loosely wound reel is later exposed to extreme heat or humidity. Figure 2-5 shows a windowed tape pack.
2-9
Q-22. Tape winding errors can cause a deformed tape pack. What are four common types of tape pack deformation?
Q-23. After rewinding a tape onto its supply reel, you examine the tape pack and notice pile-ups of tape resembling the example in figure 2-2. What causes this condition?
Q-24. You notice steps in the tape pack such as those in figure 2-3. What causes this and how does it damage the magnetic tape?
Q-25. A tape pack is buckled and deformed as shown in figure 2-4. What are three possible causes for this condition?
Q-26. A tape pack has gaps in the tape winding as shown in figure 2-5. What causes this condition?
There are two types of magnetic tape carriers: tape reels and tape cartridges. Both types can be used for either analog or digital recording. Tape cartridges are normally used only for digital recording.
Tape reels are used on magnetic recorders that use a manually loaded tape supply reel and a separate take-up reel. A reel’s purpose is to protect the magnetic tape from damage and contamination. It can be made of plastic, metal, or glass. A reel has two parts, the hub and the flanges.
A tape reel is designed to hold magnetic tape on its hub without letting the magnetic tape touch the sides of the flanges. Contrary to popular belief, the flanges are not designed to guide the magnetic tape onto the tape reel.
2-10
Tape cartridges hold a spool of magnetic tape in the same way as tape reels, except that the inside of the cartridge contains both the supply reel and the take-up reel. Unlike tape reels which must be manually loaded into a recorder, when you insert a tape cartridge into a recorder, it’s automatically loaded and ready to use. Figure 2-6 shows two typical tape cartridges.
Q-27. When winding a tape onto a plastic or metal reel, should the tape ever touch the reel’s flanges?
One advantage of magnetic tape is that you can erase what you’ve previously recorded, and record on the same tape again and again. The erasing is done by demagnetizing the magnetic tape. You demagnetize a magnetic tape by exposing it to a gradually decreasing ac (alternating current) magnetic field. There are two ways to do this: (1) with an erase head that’s mounted on the magnetic recorder, or (2) with a separate tape degausser. ERASE HEADS
A magnetic recorder’s erase head erases magnetic tape by saturating it with an ac signal that’s higher in frequency than the frequency range of the recorder itself. This method of erasing a tape works well in some cases, but it’s not the best way because:
• It’s slow; the tape must be run through the recorder to be erased.
2-11
• If the erase head is not completely demagnetized, it may not do a complete erasure.
• Some recorders do not have erase heads installed.
By far, the best way to erase a magnetic tape is to use a separate magnetic tape degausser. There are two types of degaussers: automatic and manual.
Automatic degaussers erase magnetic tape by automatically moving the whole tape reel or cartridge slowly and steadily in and out of an intense ac magnetic field. This type of degausser erases a tape very well. Some automatic degaussers are made specifically for tape reels, and some are made for both tape reels and tape cartridges. Figure 2-7 shows a typical automatic degausser.
Both manual and automatic tape degaussers use the same electronic principles for erasing magnetic tape. However, the manual version is much more portable. It’s small, hand-held, and much less expensive. Figure 2-8 shows a typical manual degausser.
2-12
To erase tapes with a manual degausser:
1. Place the tape reel or cartridge to be erased on a flat surface.
2. Hold the degausser very close to the magnetic tape and turn it on.
3. Slowly rotate the degausser in circles around the tape reel or cartridge for a few seconds.
4. Then slowly move it away until you’re about 12 to 14 inches away from the tape reel or tape cartridge.
5. Turn off the degausser.
Q-28. What are two disadvantages of using a recorder’s erase head to erase data recorded on a magnetic tape?
Q-29. What method for erasing magnetic tape is much more effective and reliable than using a recorder’s erase head?
Today’s magnetic tape coatings can store recorded signals for years. The data recorded is a permanent record that won’t fade or weaken with age. And, it’ll remain unchanged until it’s altered by another magnetic field or until the tape coating deteriorates.
2-13
When magnetic tape recordings are ruined, the cause is usually poor handling, improper storage, or shipping damage. If you want your tape recordings to last a long time, you need to know how to properly handle, store, and ship magnetic tape.
A magnetic tape reel or cartridge should always be in one of two places, either mounted on a tape recorder or in its storage container. When you handle magnetic tape, follow these rules:
• DO use extreme care when handling magnetic tape. Careless handling can damage magnetic tape, tape reels, and tape cartridges. Always hold a tape reel by the hub, NEVER by the flanges, and NEVER handle or touch the working tape surface.
• DO NOT let the magnetic tape trail on the floor. Even though the end of the tape may not have data stored on it, it can pick up dirt and dust that ends up on the recorder.
• DO clean your hands before handling magnetic tape. You can contaminate magnetic tape with dirt and oils from dirty hands.
• DO mount tape reels and cartridges properly. Improperly seated tape reels can cause unnecessary wear and tear on the magnetic tape.
• DO replace any warped take-up reels, as they can damage magnetic tape.
• DO keep the magnetic recorder and its take-up reel clean. Magnetic tape can pick up dirt and dust from the recorder itself.
• DO NOT use the top of a magnetic recorder as a work area. This can expose the magnetic tape to dirt, excessive heat, and stray magnetic fields.
• DO NOT allow eating, drinking, or smoking in areas where magnetic tape or devices are exposed.
Most magnetic tape reels and cartridges spend a lot of time in storage. It’s very important that you protect the stored tape from physical damage and the damaging effects of contamination and temperature and humidity extremes. If you don’t, damage to the tape pack such as oxide shedding, layer-to-layer sticking, and tape deformation can happen. To protect magnetic tape from damage during storage, follow these rules:
• DO make sure that magnetic tape is wound properly on the reel hub and at the proper tension.
• DO always store tape reels vertically. DO NOT lay them on their side.
• DO maintain a proper environment. Keep the storage area clean, and at a 60 to 80F degree temperature and a 40 to 60% relative humidity.
• DO NOT store magnetic tapes near any equipment that generates stray magnetic fields.
• DO handle all tape reels and cartridges as gently as possible.
• DO NOT eat, drink, or smoke in a magnetic tape storage area.
2-14
There may be times when you are asked to package magnetic tape reels or tape cartridges for shipment. If you want the tape to arrive in good condition, you must pack it properly to protect it from damage. The packaging you use must protect the tape reels or cartridges from impact, vibration, and temperature and humidity changes. Here are some simple rules to follow:
• DO always package tape reels so that they’re supported by their hub. This prevents any pressure on the reel’s flanges that might flex the flanges against the tape pack. Figure 2-9 shows a shipping box that supports the tape reel by the hub.
• DO always use reel bands where available. Reel bands are for placement around the outside edges of the reel flanges to help prevent the flanges from flexing and damaging the tape.
• DO always ship magnetic reels in a container designed so its normal positioning is with the reels in a vertical position. This will prevent the tape pack from shifting and damaging the edges of the magnetic tape.
• DO always package tape cartridges in their shipping cases. Tape cartridges are more durable than tape reels, but they still need to be protected during shipment.
Q-30. When magnetic tapes are ruined, what three factors are normally the cause? Q-31. What is the correct way to hold a magnetic tape reel? Q-32. The take-up reel on your recorder is warped. What should you do to/with the reel? Q-33. If magnetic tape is stored in areas with temperature and humidity extremes, what are three types
of tape damage that may occur? Q-34. List four rules you should follow when storing magnetic tape to protect it from damage. Q-35. When packaging tape reels or cartridges for shipping, what are four rules you should follow to
protect the tape reels from impact and vibration?
2-15
Now that you’ve finished chapter 2, you should be able to (1) describe the physical properties of magnetic tape, (2) recognize the four most common magnetic tape errors, (3) recognize the four causes of tape failure, (4) describe the two methods for erasing magnetic tape, and (5) use the proper procedures for handling, storing, and packaging magnetic tape, tape reels, and tape cartridges. The following is a summary of the important points in this chapter.
The three BASIC MATERIALS used to make magnetic tape are the (1) base material, (2) the oxide particles, and (3) the binder glue.
ANALOG and DIGITAL are the two basic types of magnetic tape in common use.
BLEMISHES OR COATING FLAWS ON DIGITAL TAPE can easily ruin the data or the computer program stored on the tape.
SIGNAL DROPOUT, NOISE, SKEW, AND LEVEL are four types of tape errors. Dropout errors are the most common.
OXIDE LUMPS accumulated on the tape cause most dropout errors. Other causes are dust or lint on the tape, or missing oxide coating on part of the tape.
MAGNETIC TAPE FAILURE has four main causes: (1) normal wear, (2) accidental damage, (3) environmental damage, and (4) winding errors.
IDEAL TEMPERATURE AND HUMIDITY RANGES for using and storing magnetic tape are 60 to 80º F and 40 to 60% relative humidity.
ENVIRONMENTAL TAPE DAMAGE caused by excessive temperature or humidity includes the following: (1) tape deformation, (2) oxide shedding, (3) head-to-tape sticking, (4) layer-to-layer sticking, (5) dirt buildup, and (6) excessive tape and head wear.
WINDING ERRORS can cause tape pack deformation. The four most common types are: (1) cinching, (2) pack slip, (3) spoking, and (4) windowing.
The TWO PARTS OF A TAPE REEL are the hub and the flanges. The tape should be wound on the hub. No part of the tape should be touching the flange sides.
ERASE HEADS AND TAPE DEGAUSSERS are two methods for erasing tape. Degaussers are the fastest and the most reliable.
Rules for HANDLING MAGNETIC TAPE are (1) always hold the reel by the hub, not the flanges, (2) never touch the working tape surface, (3) replace warped or damaged reels, and (4) mount reels and cartridges properly.
Rules for STORING MAGNETIC TAPE are (1) wind tape properly on the reel hub, (2) store tapes vertically, (3) keep storage area clean and at proper temperature and humidity levels, and (4) store tapes away from equipment that generates stray magnetic fields.
Rules for PACKAGING TAPE FOR SHIPPING are (1) support reels by their hubs, (2) use reel bands, (3) pack reels in containers vertically, and (4) keep tape cartridges in their shipping cases.
2-16
ANSWERS TO QUESTIONS Q1. THROUGH Q35. A-1.
a. Base material.
b. Coating of magnetic oxide particles. c. Glue that bonds the particles to the base.
A-2. Plastic tape is used more than metal because it’s more flexible, resists mildew and fungus, and is very stable at high temperatures and humidity.
A-3. Analog magnetic tape.
A-4. Digital magnetic tape is for computer programs and data. Its base material is about 50% thicker. The tape’s surface must not have blemishes or coating flaws because losing even one digital data bit could ruin the recorded computer program or data.
A-5. Signal dropout, noise, skew, and level. Dropout is the most common.
A-6. Dropouts are temporary, sharp drops (50% or more) in signal strength. They’re caused by contaminates that lift the tape away from the magnetic head, or when magnetic oxide coating is missing on part of the tape.
A-7. Oxide particles that get onto the magnetic tape.
A-8. Signal dropout errors and level errors. The dust and lint on the reel will eventually get onto the tape where it can get between the tape and the recorder’s heads.
A-9. Noise error is usually caused by a cut or a scratch on the magnetic tape.
A-10. Skew means there are time differences between the individual tracks of a multi-track recorder’s magnetic head. It happens when the tape isn’t properly aligned with the head. Fixed skew happens when the tape passes over an improperly aligned magnetic head.
A-11. The actual output signal level of the tape exceeds the manufacturer’s specified range for the output signal level (+ / - 10%). It’s caused by an uneven oxide coating on the tape due to worn tape or defective manufacture.
A-12. Tape’s performance degrades to a point where it’s no longer usable. A-13. Normal wear, accidental damage, environmental damage, and winding errors.
A-14. Repeated contact with a recorder’s fixed surfaces such as magnetic heads, tape rollers, and tape guides.
A-15.
a. Improperly adjusted tape transport mechanism. b. Dropping a reel of tape. c. Improperly threading tape.
2-17
A-16. Ideally, use and store tape at 60 to 80º F and at 40 to 60% relative humidity.
A-17. Tape deformation, oxide shedding, head-to-tape sticking, layer-to-layer sticking, dirt build-up, and excessive tape and head wear.
A-18. Oxide shedding. At temperatures above 130º F, oxide coating becomes soft and sheds. A-19. Head-to-tape sticking and layer-to-layer adhesion.
A-20. Dirt build-up caused by static electricity.
A-21. High humidity causes increased friction as the tape passes over the heads. A-22. Cinching, pack slip, spoking, and windowing.
A-23. The tape is stopped too quickly when winding or rewinding.
A-24. Pack slip. It's caused by loosely wound tape on a reel that is exposed to excessive vibration or heat. The vibration or heat causes the tape to shift, causing steps in the tape pack. The uneven tape will then rub against the reel's sides and the recorder's tape guides.
A-25.
a. Reel has a distorted hub,
b. tape wound over small particle deposited on hub, and
c. tape wound on reel with tension increasing toward end of winding. A-26. Tape is loosely wound on reel.
A-27. No. The reel is designed to hold the tape on its hub without letting the tape touch the sides of the flanges.
A-28. Using an erase head is slow, and it may not completely erase the tape.
A-29. Using a magnetic tape degausser.
A-30. Poor handling, improper storage, or shipping damage.
A-31. Always hold reel by the hub, never by the flanges. Never touch the working tape surface. A-32. Always replace a warped reel.
A-33. Oxide shedding, layer-to-layer sticking, and tape deformation. A-34.
a. Make sure the tape is wound properly on the reel hub,
b. store tapes vertically,
c. keep storage area at right temperature and humidity,
d. store away from equipment that generates stray magnetic fields.
2-18
A-35.
a. Package reels so they’re supported by their hub,
b. use reel bands,
c. package reels in vertical position,
d. package tape cartridges in their shipping cases.
3-1
After completing this chapter, you'll be able to do the following:
1. Describe the construction, function, and placement of magnetic tape recorder record, reproduce, and erase heads.
2. Describe the preventive maintenance requirements for magnetic tape recorder heads.
Magnetic tape recorder heads are the heart of magnetic tape recording, because it's the magnetic heads (as we'll call them in this chapter) that actually:
1. Record signal or data information onto magnetic tape
2. Reproduce (play back) signal or data information from magnetic tape
3. Erase any signal or data off of magnetic tape
To do these things, a magnetic tape recorder can have up to three different heads installed: one head for recording, one for reproducing, and one for erasing. Some magnetic tape recorders will use the same head for both recording and reproducing.  They're all made up of a magnetic core wrapped with a coil of very thin wire. But, there's where the similarity ends. From here on, each magnetic head is built to perform a specific job. Will the head be used on a single track recorder? Will it be used on a multitrack recorder? Will it be a record head or a reproduce head? Or, will it be an erase head? What frequency will it be recording and/or reproducing? The answers to these questions will determine the final construction of a magnetic head.
Figure 3-2 shows the construction of a typical multitrack magnetic head. Magnetic cores are wound with very thin wire, cemented together, and placed inside a half-bracket. A tip piece is then placed on top of the ferrite core, and the two half-brackets are assembled together. It's during this final assembly process that the headgap and the resulting frequency response of the magnetic head are determined. After some final contouring to give the magnetic head its curved face, it's ready for use.
3-3
Record and reproduce heads convert and transfer electrical signals onto and off of magnetic tape. The maximum frequency these heads can transfer depends on the size of the headgap and the speed of the magnetic tape (we'll discuss speed in the next chapter). Most record and reproduce heads are in one of these three general bandwidth categories:
1. Narrowband—100 Hz to 100 kHz
2. Intermediate band—100 Hz to 500 kHz
3. Wideband—400 Hz to 2 mHz
The only physical difference between a record head and a reproduce head is in the number of turns of wire on the core. A reproduce head will have more turns than a record head. This is because reproduce heads must be able to recover low-level signals from magnetic tape. The extra turns of wire allow the reproduce head to output the highest level possible and at a good signal-to-noise level.
Record heads are always placed before reproduce heads on magnetic tape recorders. This allows you to monitor signals that you're recording. Figure 3-3 shows the placement of record and reproduce heads. Figure 3-4 shows some of the typical track arrangements used.
3-4
Erase heads transfer a signal to the magnetic tape that causes the magnetic particles to assume a neutralized or erased state. To do this, a high current signal that is 3 to 5 times higher in frequency than the maximum frequency response of the record and reproduce heads is used. In some audio recorders, a simple direct current (dc) voltage is used.
Erase heads are always placed before the record and the reproduce heads on tape recorders. This allows you to erase the magnetic tape before it's recorded on. Figure 3-3 shows the placement of erase heads.
Q-1. Magnetic tape recorders can have up to three different heads installed. What are the three functions performed by a recorder's heads?
Q-2. The way a magnetic head will be used determines how it is constructed. Name three factors that determine the final construction of a magnetic head.
Q-3. What two specifications determine the maximum frequency that a recorder's record and reproduce heads will be able to transfer?
Q-4. Most record and reproduce heads are in one of what three bandwidth categories?
Q-5. Why are record heads always placed before reproduce heads on recorders?
Q-6. A recorder's erase head is always placed in what sequence on the record/reproduce track?
3-5
It's very important to regularly maintain magnetic heads. If you do, you'll greatly reduce the chance of getting a poor recording or playback. Regular preventive maintenance will also increase the life of the magnetic heads. There are two things you must do to properly maintain magnetic heads: (1) keep them clean, and (2) keep them demagnetized.
Through use, magnetic heads pick up dirt, dust, lint, and oxide particles from the magnetic tape. These particles collect on the magnetic head and, if left unchecked, could cause signal dropout errors that degrade the quality of recording and playback. To keep magnetic heads clean, regularly clean them with a cotton-tipped applicator soaked in either isopropyl alcohol or in a magnetic head cleaner recommended by the recorder's manufacturer. A good rule of thumb is to clean the heads each time you change a tape reel or cartridge.
Magnetic heads can become magnetized from many sources. It could happen
• from normal use.
No matter the cause, magnetized magnetic heads degrade the quality of the magnetic recording or playback.
To demagnetize magnetic heads, you'll use a hand-held degausser. It could be like the one shown in figure 3-5, or like the manual degausser shown in the previous chapter. No matter how they look, they all generate an ac magnetic field that demagnetizes the metal parts of a magnetic head.
3-6
The procedure for demagnetizing a magnetic head is similar to the procedure for degaussing a magnetic tape. Here are the basic steps:
1. Remove the tape (reel or cartridge) from the magnetic recorder.
2. Holding the degausser an arm's length away from the magnetic head, energize the degausser.
3. Slowly bring the degausser closer and closer to the magnetic head. Don't touch the head with the degausser.
4. Move the degausser back and forth across the head for 15 to 30 seconds. Figure 3-6 shows how this looks.
5. Slowly move the degausser away from the magnetic head. When the degausser is an arm's length away, de-energize it.
3-7
That's all there is to it. It's hard to determine exactly how often magnetic heads should be de-magnetized. Manufacturer's recommendations vary from every 8 to 25 hours of operation. To be safe, check the equipment's technical manual.
Q-7. What two preventive maintenance actions must you do regularly to increase magnetic head life and to ensure good tape recording and playback?
Q-8. How should you clean your recorder's magnetic heads?
Q-9. What are four sources that can cause magnetic heads to become magnetized?
Q-10. What type of equipment should you use to demagnetize your recorder's magnetic heads?
Q-11. How often should you demagnetize a recorder's magnetic heads?
Now that you've finished chapter 3, you should be able to (1) describe the construction of magnetic tape recorder heads; (2) describe the purpose and placement of record, reproduce, and erase heads; and (3) describe the preventive maintenance requirements for tape recorder heads. The following is a summary of important points in this chapter:
Magnetic tape recorders have up to THREE MAGNETIC HEADS to perform the erase, record, or reproduce function.
Three factors that determine the CONSTRUCTION OF A MAGNETIC HEAD are the (1) type of head, (2) frequencies it will record, reproduce, or erase, and (3) use on a single or multitrack recorder.
3-8
Most tape recorder heads are designed for ONE OF THREE BANDWIDTHS: (1) narrowband, (2) intermediate band, or (3) wideband.
A recorder's magnetic heads are in the following SEQUENCE on its record/reproduce track: (1) erase, (2) record, and (3) reproduce.
Two important PREVENTIVE MAINTENANCE requirements for magnetic heads are cleaning and demagnetizing.
ANSWER TO QUESTIONS Q1. THROUGH Q11.
A-1. Record, reproduce, and erase.
A-2.
a. Type of head (record, reproduce, or erase).
b. Frequencies it will record or reproduce.
c. Whether it will be used on a single or multitrack recorder.
A-3.
a. Size of the headgap.
b. Speed of the magnetic tape.
A-4.
a. Narrowband—100 Hz to 100 kHz.
b. Intermediate band—100 Hz to 500 kHz.
c. Wideband—400 Hz to 2 mHz.
A-5. Allow you to monitor the signals you're recording.
A-6. First, before the record and reproduce heads.
A-7.
a. Keep the heads clean.
b. Keep the heads demagnetized.
A-8. With a cotton-tipped applicator soaked in either isopropyl alcohol or a head cleaner recommended by the recorder's manufacturer.
3-9
A-9.
a. During ac power losses.
b. During testing.
c. Because of stray magnetic fields.
d. From normal use.
A-10. A hand-held degausser like the manual degaussers used for degaussing magnetic tape.
A-11. Every 8 to 25 hours depending on the manufacturer's recommendations.
4-1
After completing this chapter, you'll be able to do the following:
1. Describe the function and components of a basic magnetic tape transport system.
2. Describe the operating characteristics and parts of the three most common tape reeling systems.
3. Describe the physical characteristics of the two basic tape reeling configurations, co-planar and co-axial.
4. Describe the characteristics of open-loop drive and closed-loop drive tape transport configurations and the three most common closed-loop designs.
5. Describe the capstan speed control function of a tape transport system and the relationship of the six basic parts of a typical capstan speed control unit.
6. Explain why, and describe how, magnetic tape transports must be cleaned and degaussed.
Magnetic tape recorder transports are precisely built assemblies that move the magnetic tape across the magnetic heads and hold and protect the tape. Figure 4-1 shows a basic tape transport assembly. Tape transports have four basic parts:
4-2
1. A tape reeling system that, with the aid of tape guides, physically moves the tape across the magnetic heads.
2. A tape speed control system that monitors and controls the movement of the magnetic tape.
3. An electronic subsystem that activates the reeling device to move the magnetic tape.
4. A basic enclosure that holds and protects the reels or cartridges of magnetic tape.
This chapter describes these basic parts, tells how they work, and shows diagrams of the more common ones.
A basic magnetic recorder tape reeling system (figure 4-1) has one supply reel and one take-up reel. Its job is to move the magnetic tape from one reel to the other. When this happens, four things occur:
1. The supply reel feeds out magnetic tape at a constant tension.
2. The tape passes the magnetic heads in a straight line.
3. The take-up reel accepts the magnetic tape at a constant tension.
4. Both the supply and take-up reels start and stop smoothly while maintaining the proper tape tension.
These four things must happen, or the magnetic tape could be damaged. Three of the most commonly used tape reeling systems are (1) take-up control, (2) two-motor reeling, and (3) tape buffering.
This system uses a motorized take-up reel which pulls the magnetic tape off of a free-spooling supply reel. It maintains tape tension by using mechanical drag on the supply reel. As you might guess, this method has its disadvantages. It only works in one direction, and the tape tension doesn't remain constant throughout the reel. As the supply reel gives out tape, the tape tension varies. Uneven tape tension can cause stretched tape, poorly wound tape reels, and tape damage during starts and stops.
To overcome the problems of take-up control reeling systems, designers added a motor to the supply reel. By using two motors, the magnetic tape direction can be forward or reverse.
Two-motor reeling configurations usually use dc (direct current) motors, instead of ac (alternating current) motors, because dc motors run smoother and are easier to control. To help control tape tension, a small hold-back voltage is added to the motor for the supply reel.
Unfortunately, two-motor reeling systems do not properly control tape tension during starts and stops. Something called tape buffering must be added.
4-3
Controlling a recorder's tape tension during starts and stops is a big problem. Tape buffering overcomes this problem by regulating the tape reel speed and by protecting against changes in tape tension.
Every manufacturer of high-quality, high performance magnetic tape recorders uses some sort of tape buffering. It's especially important in magnetic recorders that operate at many different speeds, where precise tape tension must be maintained.
Figure 4-2 shows the relationship between the tape reeling system and the tape buffering system. As you can see, the speed at which a tape reel will give up or take up magnetic tape is controlled by its respective speed control servo. Feedback from the supply and take-up buffers tells the servo to speed up or slow down.
There are two basic types of reeling system buffers: (1) spring-tension, and (2) vacuum-column.
1. Spring-tension buffering systems use an electro-mechanical device to sense changes in tape tension. These changes are feedback that the speed control servo needs to adjust the speed of the tape reels. Figures 4-3 and 4-4 show two of the more common arrangements for spring-tension buffers.
4-4
2. Vacuum-column buffering systems operate like the spring-tension systems. They also regulate the speed control servos that control tape reel speed. But, as shown in figure 4-5, the vacuum-column buffer system uses a vacuum chamber instead of a spring to hold a length of magnetic tape as slack during tape recorder starts and stops. An electronic sensor in the vacuum chamber helps to control how much tape is in the buffer.
4-5
Another job of a tape reeling system is to make sure the magnetic tape is protected from damage during operation. To do this, tape reeling systems use tape guides. Tape guides come in two designs, fixed and rotary. Both of these are shown in figure 4-6. Each type of tape guide has its drawbacks. Fixed tape guides produce a lot more friction, and rotary tape guides are more likely to cause errors because of their moving parts.
4-6
Tape guides are strategically placed in a tape reeling system to make sure the magnetic tape is kept straight with respect to the supply and take-up reels and the magnetic heads. Some magnetic recorders use only fixed tape guides, some use rotary tape guides, and some use a combination of the two.
There are two basic tape reeling configurations: (1) co-planar, and (2) co-axial. Both of these describe the physical relationship between the supply reel and the take-up reel. The co-planar, which is used more often than the co-axial, has the supply reel and the take-up reel side by side. Figure 4-7 shows this configuration.
The co-axial configuration is used when physical space is limited. It places the supply and take-up reels on top of each other. Figure 4-8 shows this configuration.
4-7
Q-1. What are the four basic parts of a magnetic tape recorder's tape transport system?
Q-2. What are the three most commonly used tape reeling systems?
Q-3. What are two disadvantages of the take-up control reeling system?
Q-4. What are two advantages of a two-motor reeling system over a take-up control reeling system?
Q-5. What type of reeling system best controls a tape recorder's tape tension during starts and stops?
Q-6. What are the two basic types of tape buffering reeling systems?
Q-7. How do the tape guides on a tape reeling system protect the tape from damage during operation?
There are two types of tape transport configurations: (1) open-loop capstan drive, and (2) closed- loop capstan drive. The following paragraphs describe each of these.
This is probably the simplest tape transport configuration. Figure 4-9 shows how the magnetic tape is pulled off of the supply reel, taken across the magnetic heads, and wound onto the take-up reel. The tape is pulled by sandwiching it between a single capstan and a pinch roller. As the capstan turns, the friction between it and the pinch roller pulls the tape across the magnetic heads. The magnetic tape is held against the magnetic heads by using tape guides.
4-8
The open-loop drive transport configuration has two major drawbacks:
1. It can only work in one direction. It can pull the tape, but it can't push it across the magnetic heads.
2. Tape tension and head-to-tape contact can vary. If the capstan motor hesitates or speeds up, the tape tension will vary.
Closed-loop capstan drive tape transports were designed to overcome the drawbacks of the open- loop drive design. They use more than one capstan and/or pinch roller to clamp the magnetic tape in the area around the magnetic heads. This keeps tape tension constant and improves the quality of the recording or the playback. Figure 4-10 shows the basic arrangement of the closed-loop capstan drive.
4-9
The three most common closed-loop capstan drive designs are (1) differential velocity capstans, (2) dual-motors dual capstans, and (3) peripheral drive capstans.
Figure 4-11 shows a differential velocity capstan. In this design, the take-up capstan is made a little larger than the supply capstan. This causes the take-up capstan to pull the tape away from the heads slightly faster than the supply capstan feeds the tape to the heads. The result is a constant tape tension in the area around the magnetic heads.
Both capstans are turned by a single motor which is coupled to the capstan pulleys by a belt. This arrangement is very efficient in one direction, but, unfortunately, differential velocity capstans don't work in reverse. If you reversed the tape direction, a negative tension would occur, and the tape would bunch up in the area around the magnetic heads.
Figure 4-12 shows a dual-motor dual capstan drive. In this design, each capstan is driven by its own motor. Tape tension is maintained by slowing down one of the motors. When reverse tape motion is needed, the opposite motor is slowed down.
4-10
In this design, the magnetic tape is moved by a capstan placed directly against the tape reel or tape pack. Figure 4-13 shows two different peripheral drive capstan arrangements.
The first arrangement, figure 4-13A, shows a single capstan design. In this method, two tightly wound tape reels, without flanges, are pushed against the capstan. As the capstan turns, it forces the tape reels to turn in the appropriate direction. Magnetic tape tension is maintained by using either spring loading or servo control.
The second arrangement, figure 4-13B, uses two capstans. In this method, the two tightly wound tape reels, without flanges, are pressed directly against the capstans. Tension in the magnetic head area is maintained by controlling the speed of the individual capstans.
4-11
Capstan speed control is an important part of the magnetic tape transport system. It makes sure the capstan is turning (1) at the right speed and (2) at a constant speed. This is important because errors in speed control can cause poor recordings and playbacks.
Capstans are turned either by a motor only, or by a motor, belt, and pulley arrangement. In either case, it's the motor that the capstan speed control function acts upon to do its job. A capstan speed control function typically consists of six basic parts. Figure 4-14 shows these six parts and how they're related. Each of the parts is described below.
This part of the capstan speed control provides a reference frequency that the speed select network and the comparison network use to drive the capstan motor. The precision frequency source is usually a very-high-frequency crystal with an accuracy of at least .001 percent.
4-12
This network selects the desired operating tape speed. It takes the reference frequency from the precision frequency source and (depending on the desired operating tape speed) generates another specific reference signal that the comparison network uses to control the speed of the capstan. Table 4-1 is a list of the speed control reference signal frequencies for the various operating tape speeds.
Table 4-1.—Typical speed control reference signal frequencies
15/16 1.5625 1 7/8 3.125 3 3/4 6.25 7 1/2 12.5
This circuit monitors the true capstan motor speed. It sends the true speed to the comparison network circuit. Most capstan speed monitor circuits are made using a photo-optical tachometer that's directly attached to the shaft of the capstan motor. Figure 4-15 shows this.
4-13
This network takes the input signals from the speed select network and the capstan speed monitor, compares the two signals, and decides if the capstan is at the right speed. If not, it tells the speed correction circuit.
Sometimes, a third input signal, which comes from the magnetic tape itself, is supplied to the comparison network. It's called a servo control from tape signal. Tape recordings made on a specific recorder are sometimes shipped off for further analysis and played back on a different recorder. To help compensate for speed errors in the tape transport systems of the two recorders, the precision reference frequency of the originating recorder is recorded onto a track of the magnetic tape. During playback, this reference signal is also fed to the recorder's comparison network and is used to correct speed errors.
This circuit takes speed correction signals from the comparison network and tells the capstan motor drive circuit to either speed up or slow down the capstan motor.
This circuit takes the speed-up or slow-down signals from the speed correction circuit and actually speeds up or slows down the capstan motor.
If you want good recordings and playbacks, you must keep magnetic tape transports clean and demagnetized. The following paragraphs describe preventive maintenance procedures for magnetic tape transport systems.
You can clean most magnetic tape transports with isopropyl alcohol, cotton swabs, and lint-free cloths. (Caution: Cotton swabs are not lint free, so use them only in places you can't get to with the lint- free cloths.) Figure 4-16 shows a technician cleaning a capstan. Here are some other points to remember:
4-14
• DO always remove the magnetic tape from the transport before cleaning it.
• DO apply the cleaner onto the lint free cloth or cotton swab; DON'T apply it directly onto the tape transport.
• DO pay extra attention to the flanged parts of tape guides. It's here that oxide particles collect the most.
• DON'T use the same lint-free cloth or cotton swab to clean many parts of the tape transport. Switch cloths and swabs often. If you don't, you may transfer dirt and oxide particles from one part of the tape transport to another.
With use, tape transport parts become magnetized. It's hard to say exactly what will happen if the magnetic tape passes a magnetized part of the tape transport before the tape is recorded on. The effects can range from just a little more noise on the tape to a complete tape saturation. Either way, magnetized tape transport parts can ruin magnetic recordings.
To prevent this, you must periodically demagnetize the tape transport. The procedures for doing this are identical to those listed in chapter 2 for demagnetizing magnetic heads. You'll even use the same manual hand-held degausser you saw in figure 2-8 of chapter 2. Figure 4-17 shows a technician demagnetizing a tape guide.
4-15
Q-8. There are two types of tape transport configurations, open-loop capstan drive and closed-loop capstan drive. What are two major disadvantages of open-loop capstan drive tape transports?
Q-9. How do closed-loop capstan drive tape transports overcome the disadvantages of the open-loop drive design?
Q-10. What are the three most common closed-loop capstan drive designs?
Q-11. How do tape transports with differential velocity capstans maintain a constant tape tension in the area around the magnetic heads?
Q-12. How do dual-motor dual capstan drives maintain a constant tape tension while operating in either a forward or reverse direction?
Q-13. What are the two critical functions of the capstan speed control part of a magnetic tape transport system?
Q-14. Which part of the capstan speed control function monitors the true capstan motor speed?
Q-15. Sometimes it's necessary, but why should you avoid using cotton swabs when cleaning a magnetic tape transport?
Q-16. When cleaning the parts of a tape transport, why should you switch lint-free cloths and swabs often?
Q-17. What equipment should you use to de-magnetize a magnetic tape transport?
4-16
Now that you've finished chapter 4, you should be able to describe magnetic tape transport systems in terms of their operating characteristics, parts, and preventive maintenance requirements. The following is a summary of the important points in this chapter.
A MAGNETIC TAPE RECORDER TRANSPORT has four basic parts: (1) tape reeling system, (2) tape speed control system, (3) electronic subsystem, and (4) basic enclosure.
The TAPE REELING SYSTEM must move the tape in a straight line at a constant tension, and it must start and stop smoothly while maintaining the proper tension.
Three of the MOST COMMON REELING SYSTEMS are (1) take-up control, (2) two-motor reeling, and (3) tape buffering.
The two types of TAPE TRANSPORT CONFIGURATIONS are (1) open-loop capstan drive and (2) closed-loop capstan drive. The open-loop type works in only one direction, and the tape tension can vary. The closed-loop type keeps the tape tension constant.
Three types of CLOSED-LOOP CAPSTAN DRIVES are (1) differential velocity capstans, (2) dual-motors dual capstans, and (3) peripheral drive capstans.
The CAPSTAN SPEED CONTROL component of a tape transport keeps the capstan turning at the correct operating speed and at a constant speed. It has these six parts: (1) precision frequency source, (2) speed select network, (3) capstan speed motor, (4) comparison network, (5) speed correction circuit, and (6) capstan motor drive circuit.
You should CLEAN magnetic tape transports with isopropyl alcohol, cotton swabs, and lint free cloths and DEMAGNETIZE them using a hand-held degausser.
ANSWERS TO QUESTIONS Q1. THROUGH Q17.
A1.
a. Tape reeling system.
b. Tape speed control system.
c. Electronic subsystem.
d. Basic enclosure.
A2.
a. Take-up control.
b. Two-motor reeling.
c. Tape buffering.
4-17
A3.
a. It only works in one direction.
b. The tape tension varies as the supply reel unwinds, which can cause damage during starts and stops.
A4. The two-motor configuration runs in both directions and a holdback voltage helps control tape tension, but it does not properly control tape tension during starts and stops.
A5. A tape buffering reeling system.
A6.
a. Spring-tension buffering systems.
b. Vacuum-column buffering systems.
A7. They keep the tape straight with respect to both the supply and take-up reels and the magnetic heads.
A8.
a. Only operates in one direction.
b. The tape tension and head-to-tape contact can vary.
A9. Closed loop capstan drive transports use more than one capstan to clamp the tape in the area around the magnetic head.
A10.
a. Differential velocity capstans.
b. Dual motors dual capstans.
c. Peripheral drive capstans.
A11. The supply capstan is slightly larger than the take-up capstan. This causes the take-up capstan to pull the tape slightly faster than the supply capstan feeds the tape.
A12. Each capstan is driven by its own motor. It maintains tape tension by slowing down one of the motors. When the tape motion is reversed, the opposite motor is slowed down.
A13. Makes sure the capstan turns at the right speed and at a constant speed.
A14. Capstan speed monitor.
A15. Cotton swabs are not lint free.
A16. You may transfer dirt or oxide particles from one part of the tape transport to another.
A17. A hand-held degausser.
5-1
After completing this chapter, you’ll be able to do the following:
1. State the two types of record and reproduce electronics used on magnetic tape recorders.
2. Describe the purpose and function of direct record electronics and the four main parts of a recorder’s direct record component.
3. Describe the purpose and function of direct reproduce electronics and the three main parts of a recorder’s direct reproduce component.
4. Describe the purpose and function of frequency modulation (FM) record electronics and the three main parts of a recorder’s FM record component.
5. Describe the purpose and function of FM reproduce electronics and the four main parts of a recorder’s FM record component.
There are two ways to record and reproduce analog signals. The first way is direct record. It’s also called amplitude modulation (AM) electronics. The second way is frequency modulation (FM). Even though direct record and reproduce circuits are much different from FM record and reproduce electronics, they both share the same two very important jobs. They both must:
1. Take an input signal, process it as needed, and then send it to the record magnetic head for reproduction.
2. Take the reproduced signal from the reproduce magnetic head, process it as needed, and output it for listening or analysis.
Direct record electronics record input signals onto magnetic media just as the signals appeared at the recorder’s input. The only processing an input signal receives is the adding of a bias signal. The added bias signal makes sure the signal stays away from the steps of the magnetism curve. Figure 5-1 shows a basic block diagram of a recorder’s direct record electronics.
5-2
Direct record electronics has four main parts:
1. Input pre-amplifier circuit. This circuit takes the input signal, amplifies it, and sends it to the summing network. It also matches the impedance between the source of the input signal and the magnetic tape recorder.
2. Bias source. This circuit generates the high-frequency bias signal and sends it to the summing network. Normally, the frequency of the bias signal will be five to ten times higher than the highest frequency the tape recorder can record.
3. Summing network. This network takes the input signal and the bias signal, mixes them, and sends the resulting signal to the head driver circuit.
4. Head driver circuit. This circuit takes the signal from the summing network, amplifies it, and sends it to the record head for recording.
Direct reproduce electronics amplify the very weak input signals from the reproduce head, and send them out for listening or analysis, as needed. Figure 5-2 shows a basic block diagram of direct reproduce electronics.
Direct reproduce electronics consists of three main parts:
5-3
1. Pre-amplifier circuit. This circuit takes the very weak reproduced signal from the reproduce head and (a) amplifies the signal, (b) removes any bias signal that was used during the recording process, and (c) sends the signal to the equalization and phase correction circuit.
2. Equalization and phase correction circuit. This circuit takes the pre-amplified signal and fixes any frequency response problems that the reproduce magnetic head may have caused. To better understand this, look at the voltage versus frequency response graph in figure 5-3. The top of the graph shows the input signal that comes from the pre-amplifier and the bottom shows the equalization signal generated by the equalization circuit. In the top part of the graph, note how the output voltage level varies as the frequency of the signal varies. This isn’t good. A good output voltage level is one that remains constant as the frequency changes. The equalization signal corrects this problem. Notice that when the input signal and the equalization signal are combined they cancel each other out. This allows a nice flat (voltage versus frequency) output signal to go to the output amplifier circuit.
3. Output amplifier circuit. This circuit takes the signal from the equalization and phase correction circuit and amplifies it for output. It also matches the magnetic recorder’s impedance to the output device that is used for listening or recording.
FM record electronics process signals to be recorded differently than direct record electronics. Instead of recording the input signal just as it appears at the recorder’s input, FM record electronics use the input signal to vary (modulate) the carrier frequency of a record oscillator. The frequency modulated output signal of the record oscillator then becomes the signal that’s actually recorded onto the magnetic media. Figure 5-4 shows a block diagram of the FM record electronics.
5-4
FM record electronics consist of three main parts:
1. Input pre-amplifier circuit. This circuit does two things: (a) it serves as an impedance matcher between the signal source and the magnetic recorder, and (b) it pre-amplifies the input signal.
2. Record oscillator circuit. This circuit generates a carrier signal onto which the input signal will be modulated. The input signal is used to vary (frequency modulate) the carrier signal. This is how the input signal gets frequency modulated onto the carrier signal. The output of this circuit is the frequency-modulated carrier signal. The center frequency of the carrier depends on two things: (a) the bandwidth of the signal you’re recording, and (b) the media onto which you’re recording. For magnetic tape, the carrier frequency can be as low as 1.688 kHz for an operating tape speed of 1-7/8 inches per second, and as high as 900 kHz for 120 inches per second.
3. Head driver circuit. This circuit takes the frequency-modulated output from the record oscillator circuit, amplifies it, and sends it to the magnetic head for recording. The output level of this circuit is set to be just below the magnetic saturation point of the magnetic media.
The FM reproduce electronics work just like direct reproduce electronics, with one exception. FM reproduce electronics must first demodulate the original input signal from the carrier frequency before the intelligence can be sent to the output device for listening or analysis. Figure 5-5 shows a block diagram of the FM reproduce electronics.
FM reproduce electronics consist of four main parts:
1. Pre-amplifier circuit. This circuit takes the frequency modulated carrier frequency from the reproduce head and amplifies it.
2. Limiter/demodulator circuit. This circuit takes the output of the preamplifier, stabilizes the amplitude level, and demodulates the signal intelligence from the carrier frequency.
5-5
3. Low-pass filter circuit. This circuit takes the signal intelligence from the limiter/demodulator circuit and cleans up any noise or left over carrier signal.
4. Output amplifier circuit. This circuit takes the output from the low-pass filter and amplifies it for output. It also matches the impedance of the magnetic recorder to the output device.
Q-1. What two types of record and reproduce electronics are used by magnetic tape recorders? Q-2. The head driver circuit in a tape recorder’s direct record electronics component (figure 5-1)
performs what function? Q-3. The equalization and phase correction circuit in a tape recorder’s direct reproduce electronics
(figure 5-2) performs what function? Q-4. How do FM record electronics differ from AM (direct record) electronics? Q-5. The head driver circuit of a tape recorder’s FM record electronics (figure 5-4) performs what
function? Q-6. What is the major difference between direct reproduce electronics and FM reproduce
electronics?
Now that you’ve finished chapter 5, you should be able to (1) state the two types of record and reproduce electronics used on magnetic tape recorders and (2) describe the function and main parts of direct record and reproduce electronics and FM record and reproduce electronics. The following is a summary of important points in this chapter:
DIRECT RECORD (AM) and FREQUENCY MODULATION (FM) are the two types of record and reproduce electronics used by magnetic tape recorders.
The four main parts of DIRECT RECORD ELECTRONICS are the (1) input pre-amplifier circuit, (2) bias source, (3) summing network, and (4) head driver circuit.
The three main parts of DIRECT REPRODUCE ELECTRONICS are the (1) pre-amplifier circuit, (2) equalization and phase correction circuit, and (3) output amplifier circuit.
FM RECORD ELECTRONICS record a frequency modulated signal onto the magnetic tape. It has three main parts: (1) input pre-amplifier circuit, (2) record oscillator circuit, and (3) head driver circuit.
FM REPRODUCE ELECTRONICS must demodulate the original input signal from the carrier signal. It has four main parts: (1) preamplifier circuit, (2) limiter and demodulator circuit, (3) low-pass filter circuit, and (4) output amplifier circuit.
5-6
ANSWERS TO QUESTIONS Q1. THROUGH Q6.
A1.
a. Direct record (AM). b. Frequency modulation (FM).
A2. It takes the signal from the summing network, amplifies it, and sends it to the record head for recording.
A3. It generates an equalization signal which corrects any frequency response problems in the input signal from the pre-amplifier circuit.
A4. Instead of recording the signal just as it appears at the recorder’s input, FM record electronics records a frequency-modulated carrier signal from a record oscillator (figure 5-4) onto the magnetic tape.
A5. It amplifies the frequency-modulated output from the record oscillator and sends it to the record head.
A6. FM record electronics must use a limiter and demodulator circuit (figure 5-5) to demodulate the signal intelligence from the carrier frequency.
6-1
After completing this chapter, you’ll be able to do the following:
1. Define the seven most common magnetic tape recording specifications.
2. Describe a magnetic tape recorder’s signal-to-noise ratio (SNR) specification, how it’s measured, and why a high SNR is important.
3. Describe a tape recorder/reproducer’s frequency-response specification, how it’s measured, and the three factors that can limit or degrade a recorder’s frequency response.
4. Describe a tape recorder’s harmonic-distortion specification, how it’s measured, and how a recorder produces harmonic distortion.
5. Describe a recorder’s phase-response specification, how it’s measured, and why good phase response is important.
6. Describe a recorder’s flutter specification, how it’s measured, and why minimal flutter is important.
7. Describe a recorder’s time-base error (TBE) specification, how it’s measured, and why minimal TBE is important.
8. Describe a multi-track magnetic tape recorder’s skew specification, how it’s measured, and why minimal skew is important.
Have you ever gone to a store to buy a magnetic tape recorder? Were you able to decide which of the displayed models was the good one to buy? Or, did you instead end up confused when the salesperson started spouting words like SNR, flutter, and bandwidth. If so, you weren’t alone.
This chapter (1) defines the seven most common magnetic tape recording specifications, (2) describes their effect on the magnetic recording process, and (3) tells how to measure each specification. The remaining paragraphs in this chapter describe each of the following magnetic tape recorder specifications:
1. Signal-to-noise ratio
2. Frequency response
3. Harmonic distortion
4. Phase response
6-2
5. Flutter
6. Time-base error
7. Skew
Signal-to-noise ratio (SNR) is the first magnetic tape recorder specification we’ll describe. It’s one of the most important specifications of a magnetic tape recorder.
The SNR is the ratio of the normal signal level to the magnetic tape recorder’s own noise level. It’s measured in decibels (dB). In other words, the higher the SNR of a magnetic tape recorder, the wider the range of input signals it can properly record and reproduce.
The noise part of the signal-to-noise ratio is generated in the magnetic tape recorder itself. Although noise can be generated by almost any part of the magnetic tape recorder, it’s usually generated by either the magnetic heads or the magnetic tape.
You can measure the SNR with a vacuum tube voltmeter (VTVM) and a signal generator. The equipment set up for measuring the SNR is shown in figure 6-1. After equipment setup, measure the SNR as follows:
1. Set the signal generator to inject a test signal into the tape recorder. The technical manual for the tape recorder you’re testing will tell you how to set up the signal generator.
2. While recording and reproducing, set the output level of the tape recorder’s reproduce electronics to a level that displays 0-dB reference on the VTVM.
3. Disconnect the signal generator. The voltage displayed on the VTVM will drop from 0-dB to some negative dB level. This level is the magnetic tape recorder’s SNR.
6-3
There are two things you should know when reading SNR specifications in technical manuals, equipment brochures, etc.
First, the SNR is stated in three ways. You’ll see it as (1) root-mean-square (RMS) signal-to-RMS noise, (2) peak-to-peak signal-to-RMS noise, or (3) peak signal-to-RMS noise. If the SNR specification doesn’t state which way it was measured, you could be mislead. For example, a 25-dB RMS SNR is equal to a 34-dB peak-to-peak signal-to-RMS noise ratio, or a 28-dB peak signal-to-RMS noise ratio.
Second, all SNR specifications should include the record level that was used. Since the SNR varies directly to the record level, you could be mislead by a SNR that doesn’t include the record level of the test signal used when the SNR was measured.
FREQUENCY RESPONSE The frequency-response specification of a magnetic tape recorder is sometimes called the bandwidth.
A typical frequency-response specification might read within + / - 3 db from 100 Hz to 100 kHz at 60 ips. This means the magnetic tape recorder is capable of recording all frequencies between 100 Hz and 100 kHz at 60 inches per second (ips) without varying the output amplitude more than 3 dB. FREQUENCY-RESPONSE DEFINITION
Frequency response is the amplitude variation with frequency over a specified bandwidth. Let’s convert this to plain English. The frequency-response specification of a magnetic tape recorder tells you the range of frequencies the recorder can effectively record and reproduce. What exactly does the word effectively mean? That’s hard to say because frequency response varies from recorder to recorder, and from manufacturer to manufacturer. But a good rule of thumb is that an effective frequency-response specification tells the lowest and highest frequencies that the recorder can record and reproduce with no more than + / - 3-dB difference in output amplitude. FREQUENCY-RESPONSE MEASUREMENT
The equipment setup for measuring the frequency response of a magnetic tape recorder is the same as for measuring the signal-to-noise ratio. It’s shown in figure 6-1. After equipment setup, measure a recorder’s frequency response as follows:
1. Set the signal generator to output a test signal. The technical manual for the tape recorder will tell you how.
2. Set the recorder’s reproduce electronics output level to a 0-dB reference on the VTVM.
3. While recording at a set speed, vary the frequency of the signal generator from the lowest to highest frequency you’re checking. Make sure that the output level of the signal generator stays the same.
4. As you sweep through the frequencies, look at the VTVM. You’ll see the amplitude rise and fall as you vary the output frequency of the signal generator. As you approach the lowest and the highest frequencies that the magnetic tape recorder can effectively record, you’ll see the VTVM drop to less than - 3 dB. This determines the lower and upper limits of the frequency-response specification for that magnetic tape recorder.
6-4
FREQUENCY-RESPONSE LIMITING FACTORS Four factors that can limit or degrade the frequency response of magnetic tape recorders are:
1. A too-high or too-low bias signal level setting for the record head.
2. An improper reproduce head.
3. An improper tape transport speed.
4. A poor magnetic tape-to-head contact.
The magnetic record head transforms the electrical signal into a magnetic field for recording onto magnetic tape. If the bias signal level is set to high, you might erase the higher frequencies. If it’s too low, you’ll get excessive tape distortion.
The reproduce head transforms the magnetic field from the magnetic tape back into an electrical signal. As explained in chapters 3 and 5, the head gap of a recorder’s reproduce head and the operating speed of the magnetic tape transport determine the wavelength of the reproduce head. The wavelength determines the center frequency of a recorder’s frequency-response specification. Once you pass this center frequency, both below and above, the output voltage level of the recorder’s reproduce head will decrease. Figure 6-2 shows this. This is why the equalization circuits described in chapter 5, figure 5-3, are used.
Poor tape-to-head contact can seriously degrade the record and reproduce process. Magnetic heads are designed to reduce tape-to-head gap as much as possible. A tape-to-head gap is extremely degrading at the higher frequencies. Figure 6-3 shows this. Note how a .1-mil gap causes only a small loss at 10 kHz. But, at 1 MHz, it causes a 46-dB loss! You must maintain tape-to-head contact. Keeping the magnetic tape recorder heads and tape transport clean is the best way to do this.
6-5
Q-1. Two tape recorders have signal-to-noise ratios (SNRs) of 25-dB RMS and 35-dB RMS respectively. Which of the SNRs can record and reproduce the widest range of input signals and why?
Q-2. You plan to measure your tape recorder’s SNR. What test equipment will you need? Q-3. Technical manuals for tape recorders can state the SNR in what three different ways? Q-4. The frequency-response specification of your tape recorder reads within +/- 3 dB from 150 Hz to
150 kHz at 60 ips. What does this mean?
Q-5. While measuring frequency response, as the signal generator approaches the lowest and highest frequency the recorder can effectively record, the VTVM reading drops to less than - 3 dB. What does this indicate?
Q-6. List four factors that can degrade the frequency response of magnetic tape recorders.
A magnetic tape recorder’s harmonic-distortion specification is very important. It usually determines where the record level of a recorder’s electronics should be set. The record level is also used to determine the signal-to-noise ratio and frequency-response specifications. A typical harmonic-distortion specification might read "1% third harmonic of a 100-kHz signal at 60 ips." This means that the magnetic tape recorder has 1% third-harmonic distortion of a 100-kHz signal at 60 ips.
6-6
Harmonic distortion is the production of harmonic frequencies by an electronic system when a signal is applied at the input. When an input signal goes through nonlinear electronic circuitry, the output signal will include some harmonic distortion (or unwanted frequencies). If you analyzed this distortion, you’d see that a pattern exists. A pattern, whereby the frequency of each unwanted frequency is a multiple ( · 1, · 2, · 3, etc.) of the center frequency of the input signal.
There are two types of harmonic distortion: even-order and odd-order. If the frequencies of the distortion are 2, 4, 6, etc., times the center frequency, it’s even-order harmonics. If the frequencies of the distortion are 3, 5, 7, etc., times the center frequency, it’s odd-order harmonics.
Odd-order harmonics are normally caused by the magnetic tape itself. Even-order harmonics are normally caused by (1) permanently magnetized magnetic heads, (2) faulty circuits, or (3) asymmetrical or unbalanced bias signals. As you might guess, even-order harmonics can be reduced by doing the right maintenance and periodic performance tests.
The primary harmonic distortion in magnetic tape recorder systems is third-order harmonics. If the level of third-order harmonics in a recorder increases, the level of distortion will also increase (figures 6-4A and B show this relationship). Two things that determine the level of third-order harmonics in a recorder are (1) the signal bias level, and (2) the record level. Figure 6-4A shows how third-order harmonic distortion decreases as the signal bias level increases. Figure 6-4B shows how the third harmonic increases gradually at first and then abruptly as the record level increases. That’s why the third harmonic is used to determine the normal record level.
6-7
Figure 6-5 shows a typical test equipment setup for measuring harmonic distortion. With this setup, the test signal from the signal generator is recorded and reproduced by the magnetic tape recorder at a normal record level. The amount of harmonic distortion is measured at the recorder’s output on the wave analyzer.
The technical manual for the magnetic recorder you’re testing will tell you how to set up the test equipment. It’ll tell you to set up the wave analyzer to measure a specific frequency. This frequency will be one of the multiples (· 1, · 2, · 3, etc.) of the frequency the signal generator is outputting.
For example, let’s say the technical manual told you to set up the signal generator to input a 10-kHz test signal into the magnetic tape recorder. Since you want to measure third-order harmonics, the technical manual will tell you to set the wave analyzer to measure the amount of harmonic distortion at 30-kHz.
It used to be thought that the only important specifications of magnetic tape recorders were signal-to-noise ratio and frequency response. But now, with the need to record and reproduce more complex waveforms, such as telemetry and computer data, the phase-response specification becomes as important as frequency response.
Phase response is the expression of the variation of the phase shift with respect to frequency. A good magnetic tape recorder will have linearly increasing phase response as frequency increases. In simpler terms, good phase response shows that a magnetic recorder can reproduce a complex waveform (such as a square wave which has an infinite number of sine waves) without distorting it. Figure 6-6 shows both good and bad phase response.
6-8
You cannot directly measure phase response. The best way to check the phase response of a magnetic tape recorder is to record and reproduce a square wave and watch the output on an oscilloscope. If the output signal is symmetrical, like in figure 6-7, the recorder has good phase response.
Q-7. A recorder’s harmonic-distortion specification reads 2% third harmonic of a 100-kHz signal at 60 ips. What does this mean?
6-9
Q-8. What are three possible causes of even-order harmonics? Q-9. What number harmonic is the primary harmonic distortion in magnetic tape recorders? Q-10. When measuring harmonic distortion, you set the signal generator to input a 15-kHz test signal.
To what frequency should you set the wave analyzer? Q-11. How should a tape recorder with good phase response reproduce a complex waveform, such as a
square wave?
Q-12. How could you check the phase response of a tape recorder?
The general audio and broadcast field coined the term flutter to describe what you’ll actually hear from the bad effects of this specification.
Flutter is the result of non-uniform tape motion caused by variations in tape speed that produces frequency modulation of signals recorded onto magnetic tape.
Flutter is usually expressed as a percent peak or a peak-to-peak value for instrumentation recorders and as a root-mean-square (RMS) value for audio recorders. It’s caused by magnetic tape transports. Low-frequency flutter (below 1000 Hz) is caused by the rotating parts of a tape transport such as:
• Irregular magnetic tape supply or take-up reels.
• Uneven or sticking guide rollers and pinch rollers.
• Capstans.
High-frequency flutter (above 1000 Hz) is caused by the fixed parts of a tape transport, such as fixed tape guides and magnetic heads. When the magnetic tape passes over a fixed tape guide or magnetic head, the transition from static to dynamic friction causes something called stiction. It’s this stiction that causes the variations in tape speed which, in turn, cause the flutter.
As you might guess, it’s hard to prevent flutter. The only way to lessen flutter is through skilled engineering, machining, and design of magnetic tape recorders.
There are many ways to measure flutter. Most are based on the fact that tape speed variations cause frequency modulation of a recorded tone. Figure 6-8 shows a typical setup for measuring the peak-to-peak value of flutter with a frequency-modulation (FM) demodulator and an oscilloscope. The technical manual for the magnetic tape recorder you’re testing will tell you how to set up the signal generator to output the test signal. After setting up the test equipment, follow these procedures:
6-10
1. Record the test signal onto magnetic tape; then rewind the magnetic tape. This is necessary because you can’t measure flutter as you’re recording. Since the tape-speed variation past the record head is almost the same as past the reproduce head, the flutter level is too small to see.
2. After you rewind the tape, play it back. During playback, the output signal from the tape recorder goes through the FM demodulator to remove the original test signal. The waveform you now see on the oscilloscope is the actual flutter signal that was modulated onto the test signal.
3. Using the oscilloscope display, measure the peak-to-peak value of the flutter signal.
The time-base error (TBE) specification of magnetic tape recorders is closely related to the flutter specification. In fact, the TBE is a direct measure of the effects of flutter on the stability of recorded data.
The TBE is the time-relationship error between two or more events recorded and reproduced from the same magnetic tape. It’s also defined as the displacement of a point on the magnetic tape from where it should have been, during a specific time interval.
A typical TBE specification might read "+ / - 100 microseconds over a 10-millisecond time interval at a tape speed of 60 inches per second, referenced to a control tone." This means that the time-base error could cause a signal to jitter +/ - 100 microseconds over a 10-millisecond period at a tape speed of 60 inches per second.
TBE jitter introduces noise or unwanted frequency modulation (when using FM recording techniques) into the magnetic tape recording process. It can also cause a loss of accuracy in pulse- duration modulation (PDM), pulse-coded modulation (PCM), or other magnetic recordings where precise timing relationships exist between two or more signals.
The simplest way to measure the TBE is with an oscilloscope. Figure 6-9 shows a typical test equipment setup for measuring TBE. After you set up the test equipment, measure the TBE as follows:
6-11
1. Set the signal generator to generate a test signal. The technical manual for the magnetic tape recorder you’re testing will tell you how.
2. Connect the test signal output from the signal generator to both the recorder’s input and the oscilloscope’s trigger (sync) input.
3. Connect the output of the tape recorder to the oscilloscope’s signal (vertical) input. 4. Record and reproduce the test signal.
5. Adjust the oscilloscope’s intensity control until you can see the TBE on the oscilloscope’s display. (Limit glare by using a hood on the oscilloscope’s display.)
This magnetic tape recording specification only applies to multi-tracked magnetic tape recorders.
Skew is the inter-track fixed and dynamic displacement, or change in azimuth, encountered by different tracks across the width of the magnetic tape as it passes the magnetic heads. In other words, it’s the time difference between the tracks on a multi-tracked magnetic head.
A typical skew specification might read "+/ - 0.15 microseconds between adjacent tracks on the same head stack at 120 inches per second." This means that one of the tracks on a magnetic head could lead, or lag, the track next to it by as much as 0.15 microseconds at 120 ips. This specification applies to both fixed and dynamic skew.
6-12
You can minimize most fixed skew by adjusting the magnetic recorder’s electronics or by realigning the magnetic heads.
Fixed skew errors usually do not show up when magnetic tapes are recorded and reproduced on the same tape recorder. Since fixed skew errors are additive, they’ll usually show up when you record on one magnetic tape recorder and then reproduce on another.
Dynamic skew errors are caused by either the magnetic tape transport or the magnetic tape itself. If the tape transport guides are worn or sticking, the magnetic tape won’t properly pass over the magnetic heads. It’ll drift and pass the magnetic head at an angle (like a car skidding on an icy road). If the magnetic tape itself is warped or isn’t uniform across its width it, too, will cause dynamic skew.
Skew is best measured with an oscilloscope. Figure 6-10 shows a typical test equipment setup for measuring skew. The technical manual for the magnetic tape recorder you’re testing will tell you how to set up the signal generator. After test equipment setup, measure the skew as follows:
6-13
1. Inject the test signal into a reference track and one other track of the multi-track magnetic tape recorder. (The reference track should be one of the two outside tracks of the magnetic head.)
2. Connect the output from the reference track to the sync input of the oscilloscope to trigger the horizontal sweep.
3. Connect the output from the other track to the vertical input of the oscilloscope.
4. While recording and reproducing the test signal, measure the fixed and dynamic skews which are displayed on the oscilloscope. Figure 6-10 shows how this looks.
Q-13. What causes flutter in a tape recorder’s output? Q-14. What causes low-frequency flutter (below 1000 Hz)? Q-15. What causes high-frequency flutter (above 1000 Hz)? Q-16. Your recorder’s TBE specification reads " +/ - 80 microseconds over a 10 millisecond time
interval at a tape speed of 60 ips, referenced to a control tone." What does this mean? Q-17. Why is it important to minimize TBE jitter in magnetic tape recordings where precise timing
relationships exist between two or more signals?
Q-18. The skew specification of your multi-tracked tape recorder reads " +/ - 0.20 microseconds between adjacent tracks on the same head stack at 120 ips." What does this mean?
Q-19. How can you minimize fixed skew? Q-20. When are fixed skew errors most likely to show up? Q-21. How do worn or sticking tape transport guides cause dynamic skew on a multi-track recorder?
Now that you’ve finished chapter 6, you should be able to describe the seven most common magnetic tape recording specifications and how to measure each specification. The following is a summary of important points in this chapter:
The SIGNAL-TO-NOISE RATIO (SNR) is the ratio of the normal signal level to the tape recorder’s own noise level measured in dB. The higher a recorder’s SNR, the wider the range of signals it can record and reproduce.
SNR IS STATED IN ONE OF THREE WAYS based on how it was measured. If you don’t know the way it was measured, you could be misled.
A recorder’s FREQUENCY-RESPONSE specification is sometimes called its bandwidth. It tells the range of frequencies a recorder can effectively record and reproduce.  Also, failure to clean the heads and the tape transport can cause poor tape-to-head contact.
6-14
HARMONIC DISTORTION is the production of unwanted harmonic frequencies when a signal is applied at the recorder’s input. The primary harmonic distortion in tape recorders is third order harmonics. It’s measured with a wave analyzer. You can reduce this distortion with proper preventive maintenance and periodic performance tests.
Good PHASE RESPONSE means the recorder can reproduce complex waveforms such as square waves without distortion. The best way to check a recorder’s phase response is by recording and reproducing a square wave and checking the output on an oscilloscope.
FLUTTER results from non-uniform tape motion caused by variations in tape speed. The tape speed variations are caused by design and machining deficiencies in the rotating and fixed parts of the tape transport.
TIME-BASE ERROR (TBE) is the time-relationship error between two or more events recorded on and reproduced from the same magnetic tape. It causes TBE jitter, which introduces noise or loss of accuracy where precise timing relationships exist between two or more signals.
SKEW is the time difference in microseconds between the tracks on a multi-tracked tape recorder. Fixed or dynamic skew can happen when one of the tracks on the multi-track head leads or lags the track next to it. Fixed skew errors only show up when you record on one recorder and reproduce on a different recorder. You can minimize fixed skew by adjusting the recorder’s electronics and aligning the heads. Dynamic skew errors are caused by worn or sticking tape transport guides or by warped magnetic tape.
ANSWERS TO QUESTIONS Q1. THROUGH Q21. A1. 35-dB RMS because the highest SNR can always record and reproduce the widest range of input
signals.
A2. A VTVM and a signal generator. (See figure 6-1.) A3.
a. Root-mean-square (RMS) signal-to-RMS noise. b. Peak-to-peak signal-to-RMS noise.
c. Peak signal-to-RMS noise.
A4. The recorder can record all frequencies between 150 Hz and 150 kHz at 60 ips without varying the output amplitude more than 3 dB.
A5. The upper and lower limits of the frequency response specification for that tape recorder. A6.
a. A too-high or too-low bias signal level setting for the record head. b. An improper reproduce head gap.
c. An improper tape transport speed.
d. Poor tape-to-head contact.
6-15
A7. The recorder has 2% third-harmonic distortion of a 100-kHz signal at 60 ips. A8.
a. Permanently magnetized heads.
b. Faulty circuitry.
c. Asymmetrical bias signal.
A9. Third-order harmonic.
A10. 45 kHz.
A11. With no distortion.
A12. Record and reproduce a square wave and see if the output on an oscilloscope is symmetrical. A13. Non-uniform tape motion caused by variations in tape speed. A14. Rotating parts of a tape transport, such as irregular tape reels, sticking guides and pinch rollers,
and capstans.
A15. Fixed parts of a tape transport, such as fixed tape guides and magnetic heads. A16. The TBE could cause a signal to jitter +/ - 80 microseconds over a 10-millisecond period at a
tape speed of 60 ips. A17. The jitter could cause noise and a loss of accuracy. A18. One of the tracks on a magnetic head could lead or lag the track next to it by as much as 0.20
microseconds at 120 ips.
A19. Adjust the recorder’s electronics or realign the magnetic heads. A20. When you record on one tape recorder and then reproduce on a different recorder. A21. The tape drifts past the multi-track head at an angle.
7-1
After completing this chapter, you’ll be able to do the following:
1. Describe the characteristics of digital magnetic tape recording and the difference between analog and digital recording.
2. Describe each of the three formats for digital magnetic tape recording (serial, parallel, and serial- parallel).
3. Define the following terms as they apply to digital magnetic tape recording: mark, space, bit-cell period, packing density, and bit-error rate (BER).
4. Describe the eight most common methods for encoding digital data onto magnetic tape.
5. Describe the characteristics and use of the following categories of digital magnetic tape recorders: (1) computer-compatible, (2) telemetry, and (3) instrumentation.
This chapter introduces you to digital magnetic tape recording. It describes (1) the three formats for digital magnetic tape recording, (2) the eight methods of encoding digital data onto magnetic tape, and (3) the configuration differences between the three types of digital tape recorders.
Until now, you’ve learned about magnetic tape recording from an analog point-of-view. That is, the signal you record and reproduce is the actual analog input signal waveform. In digital magnetic tape recording, the signal you record and reproduce is, instead, a series of digital pulses. These pulses are called binary ones and zeros. These ones and zeros can represent one of three types of data: (1) data used by digital computers, (2) pulsed square-wave signals, or (3) digitized analog waveforms.
The digital magnetic tape recording process stores data onto tape by magnetizing the tape to its saturation point in one of two possible polarities: positive (+) or negative ( - ). The saturation point of magnetic tape is the point where the magnetic tape is magnetized as much as it can be.
There are three digital magnetic tape recording formats: serial, parallel, and serial-parallel. Each of these is described below. Figure 7-1 shows each of the three formats as they apply to recording an eight-bit binary data stream.
7-2
This is the simplest of the three digital magnetic tape recording formats. It’s usually used when recording instrumentation or telemetry data. In this format, the incoming data pulses are recorded onto a single recorder track of the magnetic tape in a single, continuous stream. Figure 7-1A shows how this looks.
In this format, the incoming data pulses come in on more than one input channel and are recorded side-by-side onto more than one tape track. The data pulses across the width of the magnetic tape are related to each other. Figure 7-1B shows how this looks. This format is usually used to store computer data.
7-3
This format is more complex. It takes a serial input stream of data pulses, breaks them up, and records them on more than one recorder track. When the tape is reproduced, the recorder recombines the broken-apart data into its original form. Figure 7-1C shows how this looks. The serial-parallel format is usually used in instrumentation recording when the input data rate is high.
Before we describe the methods for encoding digital data onto magnetic tape, let’s define the following terms:
Mark: The voltage state of a digital one (1) data bit. It’s also sometimes called true. Space: The voltage state of a digital zero (0) data bit. It’s also sometimes called false. Bit-cell period: The time occupied by a single digital bit.
Packing density: The number of bits per fixed length of magnetic tape per track. There are three categories of packing density:
1. Low density—200 to 1,000 bits per inch (bpi). 2. Medium density—1,000 to 8,000 bpi.
3. High density—8,000 to 33,000 bpi.
Bit-error rate: The number of bits within a finite series of bits that will be reproduced incorrectly.
Q-1. In digital magnetic tape recording, the series of recorded digital pulses can represent what three types of data?
Q-2. What three formats are used for digital magnetic tape recording? Q-3. What format of digital tape recording is normally used to store computer data?
7-4
Q-4. What format of digital tape recording takes a serial input stream of data pulses, breaks them up, and records them on more than one data track?
Q-5. What format of digital tape recording is normally used to record instrumentation or telemetry data?
This section describes how digital data is electrically encoded onto the magnetic tape. The following paragraphs describe the eight most common digital data encoding methods.
1. Return to bias (RB) 2. Return to zero (RZ) 3. Non-return to zero (NRZ) and these four variations of the NRZ method:
a. Non-return-to-zero level (NRZ-L) b. Enhanced non-return-to-zero level (E-NRZ-L) c. Non-return-to-zero mark (NRZ-M) d. Non-return-to-zero space (NRZ-S)
4. Bi-phase level
RETURN-TO-BIAS (RB) ENCODING The RB encoding method uses magnetic tape that is pre-set to one of the two polarities (+ or - ). This
pre-sets the magnetic tape to all zeros. Digital ones are then recorded onto the magnetic tape by magnetizing the tape in the opposite polarity. After each one pulse, the tape returns to its original bias condition. Figure 7-2 shows the magnetic tape preset to a negative bias condition. It also shows how the digital data word<PHONE_NUMBER> is stored onto the magnetic tape using the RB encoding method.
This method has a serious drawback: It requires an external clocking signal to read the zeros stored on the tape.
7-5
biased positively or negatively). A digital one is recorded as a positive-going pulse: a digital zero is recorded as a negative-going pulse. The magnetic tape returns to its neutral state in between pulses. Figure 7-3 shows the magnetic tape in its neutral state. It also shows how the digital data word<PHONE_NUMBER> is stored onto the magnetic tape using return-to-zero encoding.
NON-RETURN-TO-ZERO (NRZ) ENCODING The NRZ encoding method is, by far, the most widely used. It’s accurate, simple, and reliable. It
does not return the magnetic tape to its neutral state in between pulses. The magnetic tape is always in saturation, either positively or negatively. The polarity of the saturating signal only changes when incoming data changes from a zero to a one and vice versa. Figure 7-4 shows how the digital data word<PHONE_NUMBER>10 is stored onto the magnetic tape using the NRZ encoding method.
7-6
There are four widely used variations to the basic NRZ encoding method. Each of these is described in the following paragraphs.
changes from a one to a zero or from a zero to a one. Figure 7-4 also shows how the digital data word<PHONE_NUMBER>10 is stored onto the magnetic tape using the NRZ-L encoding method. Note that the NRZ-L method looks just like the NRZ method, except for the first input one data bit. This is because NRZ does not consider the first data bit to be a polarity change, where NRZ-L does.
The NRZ-L encoding method isn’t normally used in higher density (over 20,000 bpi) digital magnetic recording. This encoding method is sometimes called the non-return-to-zero-change (NRZ-C) encoding method.
incoming data bits. The polarity of the parity bit is such that the total number of ones in the eight-bit data word will be an odd count. Figure 7-5 shows how the digital data word 0100010 is stored onto the magnetic tape using the E-NRZ-L encoding method.
Before the parity bit is added, the original incoming data is compressed in time. This is done so that when the parity bit is added, the eight-bit data word takes up the same amount of time as the original- seven bit data word. When the tape is reproduced, the parity bit is taken out.
This encoding method works very well in high density (up to 33,000 bpi) magnetic tape recording. And, it offers an extremely good bit-error rate of 1 error per 1 million bits.
magnetic tape recording. In this method, the polarity of the saturating signal changes when the incoming signal is a one. An incoming zero would not change the polarity of the saturating signal.