Magnetic Tape Recording Specifications

6-1

Chapter 6

Magnetic Tape Recording Specifications

Learning Objectives

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.

Introduction

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

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.

Signal-To-Noise Ratio Definition

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.

Signal-To-Noise Ratio Measurement

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: Figure 6-1.—Test equipment setup for measuring signal-to-noise ratio. 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.

Figure 6-2.—Frequency response of a reproduce head. 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 Figure 6-3.—Effects of poor tape-to-head contact. 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.

Harmonic Distortion

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 Definition

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.

Figure 6-4 A & B.—Effect of signal bias level and record level on harmonic-distortion level. 6-7

Harmonic-Distortion Measurement

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. Figure 6-5.—Test equipment setup for measuring harmonic distortion. 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.

Phase Response

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 Definition

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 Figure 6-6.—Pictures showing the effect of good and bad phase response on square-wave reproduction.

Phase-Response Measurement

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. Figure 6-7.—An example of good linear 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? FLUTTER 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 Definition

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.

Flutter Measurement

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 Figure 6-8.—Test equipment setup for measuring flutter.

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.

Time-Base Error

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.

Time-Base Error Definition

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.

Time-Base Error Measurement

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 Figure 6-9.—Test equipment setup for measuring time-base error. 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.) SKEW This magnetic tape recording specification only applies to multi-tracked magnetic tape recorders.

Skew Definition

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.

Fixed skew can be caused by • magnetic tape recorder electronics, 6-12 • gap scatter in the magnetic head stack, • azimuth alignment of the magnetic head stack, or • fixed difference in tension along the tape path 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 Measurement

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: Figure 6-10.—Test equipment setup for measuring skew. 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? SUMMARY 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. Factors that can degrade a recorder’ s frequency response are an improper bias level setting, reproduce head gap, or tape transport speed. 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.