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. ">
MAGNETIC TAPE RECORDING SPECIFICATIONS
After completing this chapter, you'll be able to do the following:
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 (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.
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 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.
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:
FREQUENCY-RESPONSE LIMITING FACTORS
Four factors that can limit or degrade the Frequency response of magnetic tape recorders are:
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