TAPE GUIDE (Apr. 1983)

When +3 = -2

Q. Tapes recorded at a maximum of-2 VU on my cassette deck register over +3 VU on most other decks.

Could my deck have more headroom than other decks? Are the meters off on my deck?

-William Armistead; Stone Mountain, Ga.

A. If anything, your deck provides less headroom than the others. To provide headroom, a deck's meters are adjusted to provide a higher reading than the actual level of a steady signal.

This more or less compensates for the fact that on brief, sharp signals (transients) the meter cannot fully follow the signal and therefore understates its level. It could be that your deck's meters respond so rapidly (approximating a peak-reading device) that less headroom is necessary.

Further, you may be using a tape with high sensitivity, so that for a given signal input you are obtaining a relatively high signal output, as indicated by the other decks.

Input Distortion

Q. When I record discs from my turntable on my eight-track deck, in playback the drums, cymbals, handclaps, etc. are distorted. The background has a raspy quality, and there is a terrible swooshing sound. However, when I record from my cassette deck onto the eight-track deck, the recording is fine. Therefore, to avoid distortion I first have to record discs on cassette, and then record them from cassette to eight-track. Nobody can seem to figure out what the problem is.

One service technician told me he thought that somehow the output impedance of my turntable doesn't match the input impedance of my eight-track deck.

-Bill Schuh; Scottsdale, Ariz.

A. Others have run into a problem such as yours. The answer seems to be that the record electronics of the offending deck (in your case, the eight-track one) are unable to handle all frequencies presented to them at high or even moderate levels. Most probably, the deck is unable to handle the warp frequencies presented by most discs and therefore by the phono pickup and phono preamp. The warp frequencies are inaudible to the human ear. But they exist, often in considerable strength. Further, they are greatly amplified by the bass boost supplied by the phono preamp, and also by the overall gain of the preamp.

Presented with such a signal, the electronics of a tape deck may block, distort, and/or oscillate. However, it appears that your cassette deck can accept the warp frequencies without going into aberrant behavior. Below 20 Hz or so, the output of your cassette deck drops sharply so that there is very little of the warp frequencies remaining in the signal fed to your eight-track deck. And, therefore, your eight-track deck then works satisfactorily.

You may be able to eliminate the problem by introducing a sharp, low frequency filter between the tape output of your audio system and the eight-track deck.

Taping Backwards

Q. If you are familiar with the later phono discs of The Beatles, you may know that there are a number of backward sections, especially in band 9.

How can I tape these backward passages so that they come out forward? What I did in the past was to tape the backward passage in the normal way on open reel and turn it over so that the shiny side of the tape contacted the heads in playback. However, this gave very poor fidelity and weak sound. Can you suggest a better way of taping the backward passages?

- Gary Teresi; Greece, N.Y.

A. One method that occurs to me is the following. Try to rent or borrow (perhaps from a local audio store) a half-track open-reel recorder. Record the backward phono passages onto tape. Turn the tape over (flip the reels) and play, at the same time dubbing the output onto a tape recorded by your deck.

Hidden Computers

Q. Lately the word microprocessor has been used. What is this?

-Terry Vey; West Bend, Wisc.

A. This is in effect a miniature computer. In a sophisticated tape deck, a microprocessor might be used to automatically adjust bias, equalization, and other parameters in order to obtain optimum performance with a given tape as well as for such functions as programming tape selections, converting tape counters to read in minutes and seconds, and in some decks even acting as built-in clock-timers.

Turnover Frequency

Q. In the November 1981 issue, you stated that 70-uS equalization corresponds to a turnover frequency of 2,274 Hz. Please give the formula for this relationship.

-William Flanagan, New Windsor, N.Y.

A. The simplest equalization network consists of one resistor and one capacitor. The time constant, in microseconds, of such a network is given by the formula T = CR, where time is in seconds, capacitance in Farads, and res stance in ohms. This corresponds to the time it would take the capacitor in the network to charge to 63.2% of its maximum voltage.

A capacitance of even 1 F would, however, be far too large for tape equalization. Hence, the capacitance is usually given in µF, and time, therefore, in µS; the figure would also be in uS where C was in pF and R in megohms. (For a network using an inductor and resistor, incidentally, the formula would be T = L/R, where L is inductance in Henries.) The relationship between the time constant and the turnover frequency (the frequency at which response is 3 dB above or below "flat") is given by the formula:

F_3dB = 1/(2 π T).

Static Pops

Q. Recently I began hearing loud static discharges, about every 10 or 15 seconds, on two of my cassette tapes. All of my other tapes are working well. What could the problem be?

-Adrian Iwachin; Toronto, Ont., Canada

A. Some tapes, depending on their oxide formulation, are more prone to static discharge than are others. As conditions of humidity vary, a tape that was giving no trouble may suddenly act up. If you are having trouble with a particular cassette, a stratagem that might work is to rap it sharply against a hard surface--but, of course, not hard enough to damage the shell.

(Source: Audio magazine, Apr 1983, HERMAN BURSTEIN)

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