MAGNETIC TAPE RECORDING SPECIFICATIONS

LEARNING OBJECTIVES

After completing this chapter, you'll be able to do the following:

• Define the seven most common magnetic tape recording specifications.
• Describe a magnetic tape recorder's signal-to-noise ratio (SNR) specification, how it's measured, and why a high SNR is important.
• 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.
• Describe a tape recorder's harmonic-distortion specification, how it's measured, and how a recorder produces harmonic distortion.
• Describe a recorder's phase-response specification, how it's measured, and why good phase response is important.
• Describe a recorder's flutter specification, how it's measured, and why minimal flutter is important.
• Describe a recorder's time-base error (TBE) specification, how it's measured, and why minimal TBE is important.
• 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:

• Signal-to-noise ratio
• Frequency response
• Harmonic distortion
• Phase response
• Flutter
• Time-base error
• 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.

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.

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.

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.

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:

• Set the signal generator to output a test signal. The technical manual for the tape recorder will tell you how.
• Set the recorder's reproduce electronics output level to a 0-dB reference on the VTVM.
• 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.
• 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.

FREQUENCY-RESPONSE LIMITING FACTORS

Four factors that can limit or degrade the Frequency response of magnetic tape recorders are:

• A too-high or too-low bias signal level setting for the record head.
• An improper reproduce head.
• An improper tape transport speed.
• 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.

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.