Jack of All Ohms?
Q. My tape machine has a head phone jack to monitor recordings, and a small earphone of the type used with transistor radios is supplied for this purpose. I have found that when I plug a good-quality headphone, rated at 8 ohms, into this jack, I get excellent sound quality. However, the instruction manual of my tape machine states that a load impedance of 10,000 ohms is required at the monitor jack. Could I hurt my machine using 8-ohm head phones?
-Tom Nicholson, Las Vegas, Nev.
A. I am surprised that you get sound of good quality when plugging a low-impedance headphone into a source requiring a load impedance of 10,000 ohms. Usually one would expect a very low sound level and distorted sound, with possible loss of high or low frequencies. Apparently the source impedance is much lower than 10,000 ohms, making such a connection feasible. It is unlikely that you are doing any harm to the headphones or to the tape machine as the result of the mismatch. If the sound is acceptable to you, I think you can continue on. If you do desire to obtain a proper match, you would have to convert to high-impedance headphones or em ploy a transformer which converts from high to low impedance.
S/N Improvement With Dolby NR
Q. By using a Dolby noise-reduction unit, how much can I expect to improve signal-to-noise ratio?
- Don Summers; APO San Francisco, Cal.
A. A Dolby B noise-reduction system can reduce apparent noise by as much as 6 to 10 db.
S/N Defined
Q. I would like to know what the signal-to-noise ratio of a tape deck actually is.
-C. Odgers, Mackenzie, B.C., Canada
A. Signal-to-noise ratio refers to the ratio between the audio signal processed by a tape system and the noise generated by the tape system. The measure is most usually based on a signal between 400 and 1,000 Hz that is recorded at a level which results in 3 percent total harmonic distortion (or 3 percent third harmonic distortion, which is the chief component of the total) on the tape. If 1 percent harmonic distortion is used as the reference level, this means that the signal is recorded at a lower level, so that the output signal is correspondingly less; then the rated S/N tends to be about 6 to 8 db lower.
Let us assume that a 400-Hz signal is recorded at a level resulting in 3 per cent distortion, and that the playback signal has a level of one volt. Let us also assume that the tape is again put through the recording process but without a signal input, that all gain controls are at the same setting as be fore, and that the output voltage is again measured. Now the output volt age consists solely of the noise generated by the tape system-by the record electronics, playback electronics, and tape. The S/N is the ratio be tween the first and second voltages, namely between the output of 1 volt (audio signal) and the noise-output voltage. (Very strictly speaking, the 1-volt output also includes noise, but the effect of this on the S/N ratio is very trivial, so that we may generally forget about the matter.) If the noise-output voltage is 0.001 volt, then the S/N is 1/0.001, or 1,000 to 1. S/N is ordinarily expressed in decibels (dB). A ratio of 1,000:1 between two voltages, if you consult a dB conversion table, is 60 dB.
Often the noise measurement is weighted to allow for the fact that our hearing sensitivity decreases at lower frequencies. The noise output is put through a filter which reduces the lower frequencies, tending to result in a lower noise measurement. Accordingly, the rated S/N goes up.
Which Is the Real Tape Playback Curve?
In Audio magazine, tape playback equalization at 7 1/2 ips has been pictured in the manner of Fig. 1, with pronounced bass boost. Elsewhere it has sometimes been presented in the manner of Fig. 2, which shows quite the opposite--bass cut and treble boost. The uninitiated reader may well wonder which is correct.
They are both correct but approach the subject from different viewpoints.
Before going on, it should be noted that the following explanation applies in principle not only to playback equalization for 7 1/2 ips, but also to equalization for other tape speeds and for various cassette tape formulations.
Conventionally we think of equalization as a change in frequency response performed by an electronic circuit. For example, an FM tuner pro vides treble cut in order to compensate for the treble boost applied by the broadcast station; this strategy helps reduce noise. A preamplifier provides both bass boost and treble cut when a magnetic pickup is employed to play a phono disc; this compensates for the bass cut and treble boost employed in disc recording to minimize distortion at low frequencies and noise at hit, frequencies.
Turning to tape, we find that in the complete absence of equalization, record-playback response would take the shape of an inverted U: A combi nation of severe bass loss and severe treble loss. Bass loss is due to the intrinsic nature of a magnetic playback head, which responds to the rate of change of the signal, so that output varies with frequency. Given a flat signal (constant flux in its core), the head produces an output signal that changes at the rate of six dB per octave as frequency changes. The change in output with frequency may be viewed as either bass loss or treble rise. Here we refer to it as bass loss.
Fig. 1--Standard tape playback equalization at 7 1/2 ips for an ideal magnetic
playback head.
Further, owing to the contour effect-where the playback head as a whole and not only its gap responds to the recorded signal--a practical head may produce somewhat greater out put than an ideal head in the low bass region. Thus, Curve A in Fig. 1 might be modified slightly, as shown by Curve b. All in all, however, the play back equalization called for in order to achieve flat response is quite close to that of Curve A in Fig. 1.
In 1965, to emphasize the fact that playback equalization must reflect the irregularities of a practical head, and to get away from the notion that a play back head must inevitably have a six-dB-per-octave characteristic, the NAB (National Association of Broadcasters) decided to present standard playback equalization in a different manner, that of Curve B in Fig. 2. This practice was also adopted by the RIAA (Re cording Industry Assoc. of America).
Fig. 2--Standard reproducing characteristic: Reproducing amplifier output for
constant flux in the core of an ideal reproducing head.
Figure 2 considers the playback head, as well as the playback electronics, part of the playback equalization system. Given a flat signal (constant flux in the core of the head), it indicates that the playback head and the amplifier electronics should in combi nation produce the output of Curve B in Fig. 2--treble boost and bass cut.
But the output of a magnetic play back head tends to rise six dB per octave with frequency, as shown by Curve C in Fig. 2. Therefore, the difference between Curves B and C must be the equalization supplied by the play back electronics. Specifically, the difference between Curves B and C in Fig. 2 is equal to Curve A in Fig. 1.
One way to see this is to plot the difference between Curves B and C on audio graph paper. A second method is to turn Fig. 2 about 34 degrees clock wise until the six-dB-per-octave line (Curve C) is horizontal. In reference to this line, it may easily be seen that equalization supplied by the playback electronics consists of bass boost.
Another way of describing Curve B is that it shows the electronic equalization that would be required if the output of the playback head were flat instead of rising six dB per octave. A Hall-effect head, which has long been in the offing, would have flat output.
But up to the time of this writing, play back heads have universally had a six-dB-per-octave characteristic. Therefore, if you measured the playback equalization of a 7 1/2-ips tape deck, you would obtain something quite close to Curve A in Fig. 1.
Similarly, at other tape speeds and for the various cassette tape formulations, you would find that playback equalization supplied by the deck's electronics consists primarily of bass boost.
(Audio magazine, May 1980; Herman Burstein )
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