Tape Guide (Jun. 1979)

Dolby Calibration

Q. How do I go about setting the playback calibration on my Dolby noise reduction unit for operation at 7 1/2 ips without a 7 1/2-ips playback level tape?

-Hugh Reaves, New Smyrna Beach, Fla.

A. To the best of my knowledge, ac curate playback calibration of a Dolby noise reduction unit requires a playback calibration tape.

TV Taping

Q. I have a tube TV connected to my receiver by means of shielded cable coming from the TV volume control. I make tape recordings of the TV sound via the receiver. However, these tape recordings have a very audible attenuation of the higher frequencies, even though I get very satisfactory results when recording from other sources. Do you have any suggestions?

-Frederick Berlingen, Chicago, Ill.

A. The problem may be due to excessive treble de-emphasis employed after the sound discriminator circuit of the TV set, quite possibly to balance out the sound (to compensate for the bass deficiency of the small speaker).

Capacitance of the cable leading from the TV to the receiver may be excessive; you should use as short a lead of microphone cable as possible with a low capacitance (about 25 pF per foot).

To remedy this situation, the de-emphasis circuit could be changed to produce less treble cut, but, unless you are technically oriented, you will need a service technician to make the change. Another remedy might be to make judicious use of the treble control when playing tapes recorded from TV.

Head Comparison

Q. Is there a significant difference between ferrite tape heads and hyperbolic heads?

- Richard Wahl, APO, San Francisco

A. Ferrite refers to the material from which the head is made. Such a head is claimed to have a longer life than one made of permalloy by a factor of 10:1 or more. Hyperbolic refers to the shape of the head. Its purpose is to have the tape approach and leave the head in such a fashion as to ensure very close tape-to-head contact for good treble response and to minimize the peaks and dips in the bass response owing to the contour effect, where the entire head tends to respond to the signals on the tape, in stead of only the gap responding.

Response Differences

Q. What is the difference between record/playback response and play back response. Why does the record/ playback response always cover a wider frequency range in all reports I have read on open-reel decks?

-Jeffrey Pratter, Brooklyn, N.Y.

A. Playback response is based on a test tape and such tapes, ordinarily, do not go beyond 10 or 15 kHz, even though the particular deck may be capable of a wider response. The record/ playback response is measured with out reference to a test tape; signals of equal amplitude covering the audio range are recorded on the deck being reviewed, and their relative amplitudes are measured in playback. Consequently, one may measure record/ playback response out to whatever frequencies the deck is capable of reproducing, 20 kHz or more.

Playback Parameters

Q. I have a collection of cassette tapes made on a high-quality deck, and I would like to know if there would be any detrimental effect if they were played on a relatively inexpensive portable cassette deck?

- Martin Herbstman, New York, N.Y.

A. You will probably not damage your cassettes by playing them on an inexpensive deck, assuming that it is not a toy. A very cheap deck might put undue tension on the tape and possibly deform it or it might cause the tape to jam and snarl. If the deck you have in mind treats other cassettes carefully, it should do the same to yours.

One thing you might watch out for is magnetized heads and tape guides.

Such magnetization would increase noise and reduce the treble response.

It would be a wise precaution to de magnetize the heads and guides of the portable deck before playing your valuable cassettes on it.

Taping Interference

Q. When recording an FM broadcast a burst of static is audible through the monitors as well as on the tape play back. All interference stops when the tape deck is off, also, no interference occurs when recording through the phono circuit of my receiver. What could be wrong?

-Patrick Grealy; Stratford, Conn.

A. The problem seems to be interference between the 19-kHz pilot signal of stereo broadcasts and the tape oscillator frequency, or between the tuner's oscillator and the tape machine's oscillator. If the latter is the cause, then moving the tuner and tape deck further apart might help. If the former is the cause, you need a 19-kHz filter at the output of your tuner, or at the input of your tape deck. Inquire at your local audio store about a suitable filter.

Distortion Figures

Q. Can you tell me why the inter-modulation distortion of even the best tape decks ranges from 5 to 15 percent?

-Anson Reynolds, Sierra Vista, Ariz.

A. The subject of IM distortion in tape decks has been given much less public attention than that of harmonic distortion (THD), quite possibly because most audiophiles would be alarmed by specifications in the range of something like 5 to 15 percent. But they are less apt to be unnerved by the THD figures of 1 to 3 percent generally published. On the other hand, listening tests correspond more nearly with the THD, rather than the IM figures in the case of most tape decks.

I can only speculate as to why IM of, say, 5 to 10 percent is more acceptable to the ear in the case of a tape deck than in the case of an amplifier. Part of the reason may be the limited frequency response of tape machines so that not all the distortion products are audible. Furthermore (and this is true of all components), it has been ob served on mixed tones, such as we generally listen to, that it takes much greater amount of distortion to be discernible to the ear than in the case of pure tones. IM of 5 percent or more is often indiscernible on mixed tones, whereas as little as about 0.5 percent or less, may be discernible on single tones.

Finally, IM of 5 percent or more is experienced in the vicinity of the peak recording level, while most of the audio signal is 10 to 20 dB below the peak signal. Therefore, most material has IM well below the 5 to 10 percent range.

Setting Bias

Q. What is the proper procedure for setting bias in an open-reel tape deck?

-B.E. Herring, Goldsberg, N.C.

A. There are various ways to set bias and the recommendations vary among different manufacturers. Some advise setting bias so that a prescribed volt age (given in the service manual) is obtained at a certain point, for example across the record head. Others recommend that the bias be set low, then gradually increased until maximum audio output is obtained for both record and playback at a specified frequency, usually in the range of 500 to 2000 Hz. Some manufacturers add that you should further increase bias until the audio output drops 0.5 dB to make the treble less susceptible to variations owing to slight changes in bias. Other manufacturers advise touching up bias to obtain the best compromise be tween low distortion and extended treble response. However, it is always wisest to follow the bias adjustment instructions, given in the service manual, for your specific tape deck.

(Source: Audio magazine, Jun. 1979; Herman Burstein )

= = = =