Tape Guide (March 1985)

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Tape "Glitches"

Q. When playing prerecorded cassettes, "glitches" or "blurbs" sometimes develop on the tape. The more the tape is played, the more these occur, and they are permanent; they appear thereafter with any deck used to play these tapes. This only happens with a few tapes. Is there something wrong with the tapes or with the deck?

-T. P. Johnston, Jr., Pulaski, Va.

A. Inasmuch as the problem occurs only with a few tapes, and the tapes that offend continue to do so in other decks, it is quite likely that the fault is in the tapes rather than in your deck. It may be that the magnetic coating is flaking off in spots or that, perhaps because of static attraction, the tape is picking up foreign particles. If the problem really occurs only with a few tapes, return them. If the brand that has given you trouble continues to do so, you had best avoid this label in the future. Or, it may be that you had the bad luck to get tapes from a bad batch of a normally good company's output, a problem unlikely to recur.


Q. I have heard conflicting information as to the need to demagnetize tape heads on a fairly frequent basis.

I've been told that, in the past, tape heads tended to become magnetized rather quickly, and hence there was a need to demagnetize them frequently.

I have read that the heads in recent decks don't need demagnetizing very often. Can you shed some light on this matter?

-Freeman Matthews, Columbus, Ohio

A. Opinion remains divided on the subject. The need depends on the material from which the heads are made and on any special circuitry in the deck to demagnetize the heads or to prevent current surges that might magnetize them. Demagnetizing the heads does no harm, so, to be on the safe side, many persons continue to do it.

Moreover, not only the heads but the guides and other metal parts contacted by the tape are subject to magnetization, while only the heads benefit from any demagnetization circuits in the deck. It pays, therefore, to demagnetize those other parts-and while doing so, it is only a matter of another few seconds to demagnetize the heads as well. Use a demagnetizer with a plasticor rubber-covered tip to keep from scratching the heads.

For more on this, I suggest my articles, "Focus on Head Demagnetization" and "Refocus on Demagnetization," which appeared in the April 1981 and September 1982 issues of Audio.

Single-Ended Dolby NR

Q. I have a cassette that I recorded without Dolby NR from a phono disc.

When played back without Dolby, scratches are heard; when the cassette is played with Dolby C, all scratches are eliminated. Can you explain this phenomenon? Will playing the tape with Dolby C in any way hurt my tape deck?

-Stephen A. Leslie, Philadelphia, Pa.

A. Dolby C applies substantial treble cut in playback, which tends to eliminate scratch sounds since they are predominantly in the treble range.

If you had recorded with Dolby C, substantial treble boost would have been applied in recording, thus cancelling the playback cut and leaving the scratches audible. (However, the Dolby system would have greatly reduced noise generated in the tape record-playback process.) Playing the deck with Dolby C, regardless of how you made your recording, will in no way hurt your deck or tape.

Automatic Level Control

Q. My cassette deck has an automatic level control for recording. Is this reliable? How does it work? Does it perform as well as a conventional system in the hands of an attentive recordist?

-Marc Claessens, Toronto, Ont., Canada

A. Automatic level control (ALC) rectifies the audio signal and employs the resulting d.c. voltage to control gain, seeking input to the tape high enough to achieve a good signal-to-noise ratio but not so high as to cause excessive distortion. Generally it works reliably.

But it is intended chiefly for speech or other applications where high fidelity is not important, since it compresses the signal's dynamic range too much for high-quality music reproduction. For music, it's better to set gain manually so that signal peaks never drive the recording meter above the maximum permissible level. That level would usually be about 0 or a few dB higher, depending on the individual deck and on the tape used.

When you refer to "an attentive recordist," I hope you do not have in mind the practice of riding gain, that is, changing the record level as signal level changes. This is undesirable if you wish to preserve the original dynamic characteristics of the signal source.

Need for Bias Adjustment

Q. I just bought a new cassette deck and requested the shop where I bought it to adjust the bias for the brand of tapes that I use. When a tape manufacturer upgrades his tapes, as has happened with my brand, is a new bias adjustment needed?

-Stan Davis, Buena Park, Cal.

A. Theoretically, bias requirements of upgraded tapes should not change but should conform to standards promulgated by the IEC. As a practical matter, bias requirements may change slightly, but usually not enough to make a large difference in performance. However, the perception of what is "large" may differ from one person to another.

Use FM interstation noise to check how your deck performs with the old and with the upgraded tapes. If the upgraded tapes reproduce this noise about as faithfully as before (or possibly better), do nothing. If there is a noticeable change in pitch, a bias touch-up seems in order.

Multiplex Filter On or Off?

Q. An article that I recently read stated that the multiplex filter in a tape deck should be switched on at all times. But the instruction manual of my tape deck says that the multiplex filter should be used when recording a Dolby FM broadcast to remove the subcarrier signal from a regular FM broadcast. I have a large collection of records on cassettes which have been recorded without the multiplex filter.

Am I missing something?

-Paul T. Spyrison, Schaumburg, Ill.

A. When it comes to taping off the air, I agree that it is safest to keep the multiplex filter of your cassette deck on at all times. The 19-kHz pilot tone and/ or the generated 38-kHz subcarrier may interfere with proper operation of your deck's Dolby circuitry when recording with Dolby NR. Also, these tones may beat with the tape deck's bias frequency, causing audible noises. Most FM tuners incorporate multiplex filters which sharply cut off signals beyond about 17 kHz or so, but further attenuating these frequencies can't hurt. It might not be a bad idea to use the filter when taping AM broadcasts too, as anything above 15 kHz will only be noise.

When taping from high-quality LPs, CDs or cassettes, the filter should not be switched in, as it can attenuate high frequencies in the music. How audible this will be depends on the steepness of your filter, your own high-frequency hearing ability, and the amount of treble energy in the recording. When taping from noisy recordings, though, the MPX filter may help attenuate some of the noise.

Digital Component Availability

Q. Recently I have been exposed to literature extolling the virtues of digital recordings, specifically the digital audio processors and the digital audio cassette recorders which are presently available. Should I go digital?

-Peter F. Tague, Brooklyn, N.Y.

A. There seems to be a misunderstanding. To my knowledge, as this is written there are no digital cassette machines yet available to consumers.

The industry is presently working on standards for such a unit, with the expectation that such machines will be available some time in 1986. What is available now in the way of digital equipment for consumers is all based on videocassettes. Most of this takes the form of PCM processors to be used with VCRs, but there are also one or two recorders with built-in VCR transports, which tape in the same format as the PCM-processor/VCR combinations (but do not record video). In the meantime, if you are looking for top quality, not much behind the capabilities of digital, you might investigate the Beta and VHS Hi-Fi videocassette recorders.

If you have a problem or question on tape recording, write to Mr. Herman Burstein at AUDIO, 1515 Broadway, New York, N.Y. 10036. All letters are answered. Please enclose a stamped, self-addressed envelope.

(adapted from Audio magazine, Mar. 1985; HERMAN BURSTEIN)

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