Tape Guide (June 1978)

Speed Measurement

Q. I have a tape recorder with an adjustable capstan motor capable of exact speed, but the problem is how to measure it.

Ed McElroy, Brookline, Mass.

A. Dubbings Electronics Co., 1305 S. Strong Ave., Copiague, N.Y. 11726 used to make stroboscopic wheels for measuring tape speed. You might write to them and find out if this device is still available. Some test tapes have timing signals included in them for measuring speed accuracy, so you might inquire about these at your local audio dealer.

Also you might construct your own measuring device by very carefully measuring a given length of tape, say 750 inches, and then measuring how long it takes the machine to run through this prescribed length. According to whether your machine runs through the tape in the exact time, fast, or slow, you can figure its accuracy. For example, the 750-in. tape should take 100 seconds at 7 1/2 ips, but if the tape runs through in 95 seconds, then the speed is a little over 5 percent fast; while if it runs through in 105 seconds, then it is a little over 5 percent slow.

Performance Contradiction

Q. After much reading about tape recorder performance a basic contradiction in the available literature has become apparent. The conflict is one of "controlled bandwidth" vs. "extended response." Many recorders have flat response to 15 kHz at 3 3/4 ips, yet some manufacturers claim that about 12 kHz is the maximum usable response at 3 3/4 ips. Can you resolve this apparent contradiction?

-Craig Chambers, Scott AFB.

A. Tape recorder performance involves juggling three basic factors, low distortion, extended treble response, and low noise. To maximize any one of these performance aspects requires sacrifices in one or both of the remaining aspects. Furthermore, there is the factor of time which can cause deterioration of the signal on tape with respect to both noise (specifically, print-through) and treble response.

Taking all these factors into account, some of those who have studied the situation carefully have come to the conclusion that in the current state of the art, a reasonably conservative statement is that at 3 3/4 ips optimum performance should not seek flat treble response beyond about 13 or 14 kHz-about the limit of hearing for most adults.

One may get out to 15 kHz or higher at 3 3/4 ips, but this may not represent the optimum combination of low noise, low distortion, and wide treble response. And, it does not mean that the very highest frequencies recorded on the tape will retain their full magnitude some months from now. On the other hand, there has been constant progress in tape formulations so that we find the limits of treble response, without appreciable sacrifice in terms of noise and distortion, are steadily being expanded.

Deck Switching

Q. I would like to purchase a switching mechanism that would allow me to use either of two tape decks while leaving the monitoring function intact and permit dubbing from one tape recorder to the other. Can you advise me if such a unit is available and from whom?

-Emil Aftandilian, Marion, Ill.

A. Inquire at Russound/FMP, Inc., P.O. Box 204, Stratham, N.H. 03885, USA.

Tape Whistle

Q. When recording off FM I have noticed a very weak high-pitched whistle which is audible at 1 7/8, and 3 3/4 ips, but not at 7 1/2 ips. It sounds like some kind of feedback for if I lower the recording level it disappears.

-Stan Schwartz, Storrs, Conn.

A. Most likely the whistle is due to overloading the tape which results in severe harmonic distortion, with the whistle being one of the harmonic products. At 7 1/2 ips you are less likely to obtain such distortion because of less treble emphasis in recording.

If you have a problem or question on tape recording, write to Mr. Herman Burstein at AUDIO, 401 North Broad Street, Philadelphia, Pa. 19108. All letters are answered. Please enclose a stamped, self-addressed envelope.

(Source: Audio magazine, June 1978; by Herman Burstein)

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