TAPE GUIDE (Jan. 1984)

Dubbing from CD to dbx

Q. I have a good cassette deck, and a dbx noise-reduction system. I would like to know if I can make successful tape copies of the new digital audio discs, with their dynamic range of more than 90 dB.

-Richard Hermann; Astoria, N.Y.

A. The digital disc, better known as the CD or Compact Disc, has a dynamic range, or signal-to-noise ratio, of about 95 dB or better. A good cassette deck without noise reduction has a signal-to-noise ratio of about 55 dB. Inasmuch as dbx can improve S/N about 30 dB, the deck can achieve S/N of about 85 dB, which is very, very good.

In other words, the tape deck should not be a seriously limiting factor in achieving a good copy. Noise elsewhere in your audio system--preamp, power amp, etc.--may be more significant. Further, the material on the CD may have S/N appreciably less than 95 dB, depending on its origin.

Accidental Magnetization

C. Recently I purchased a head demagnetizer and began the process of using it on my cassette deck. After a few seconds my hand became tired, and my finger slipped momentarily from the on-off switch (spring-return type), turning the unit off and leaving the head with a much stronger magnetic field than the one I was originally trying to remove. Is it possible to remove this magnetic field short of having to replace the head-an expensive thing to do? Why didn't the manufacturer of the demagnetizer install a switch that would lock into place instead of going to the off position when my finger slipped from it?

-Phillip Costello; San Diego, Cal.

A. Try repeated. demagnetization of your heads. Hopefully, this will cause the magnetization to disappear gradually. If this doesn't work satisfactorily, try to borrow or rent from a local audio store the most powerful head demagnetizer that it carries, and go through the demagnetization procedure several times with care. It may be that you can't remove the magnetization completely. However, this can also happen with heads which have not been accidentally subjected to a magnetic surge.

Demagnetizers tend to heat up quickly and thus have a duty cycle which is quite short but adequate such as a minute or two or three.

Hence, a spring-return switch is desirable to prevent overheating.

Noise-Reduction Systems

Q. I'm investing in a new cassette deck and am trying to decide between Dolby C and dbx noise reduction. If Dolby C is better than Dolby B, why do tape decks with Dolby C also provide Dolby B? Why don't they make an NR system that takes a normal signal, runs it through an encoder, and then through a decoder so that one doesn't have to buy encoded records or tapes?

-Mark Townsley; Huntsville, Ala.

A. Decks provide Dolby B as well as C in order to permit proper (flat) playback of the millions of tapes recorded with Dolby B encoding, and to permit proper encoding of tapes intended to be played on the millions of other decks containing only Dolby B. Noise-reduction devices such as dbx and Dolby are intended to reduce noise produced by the tape recording system, that is, tape hiss and hum and other noise produced by the tape deck. They operate by compressing the signal in recording and expanding it in playback; the expansion brings down the noise along with the audio signal. These NR devices cannot reduce noise already present in the audio signal. However, fairly successful reduction of noise already present in the signal can be achieved by means of dynamic NR units, sometimes called single-ended NR units.

In general, the greater the degree of noise reduction that a device provides, the greater the tendency to undesirable side effects such as volume changes and bursts of hiss. However, noise-reduction devices have achieved a stage of development which essentially overcomes these defects, as far as most listeners are concerned.

Dolby B provides about 8 to 10 dB of noise reduction, while Dolby C provides about 16 to 20 dB. In addition.

Dolby C has a special encoding curve that provides increased protection against tape saturation in the upper end of the audio range. The dbx system affords 30 dB of NR. At low signal levels, distortion tends to be less with Dolby than with dbx; the reverse is true at high signal levels. See the article on noise reduction by Howard A. Roberson in the April 1983 issue of Audio for a comparison of Dolby and dbx NR systems.

Accidental Under-biasing

Q. The other night I taped an organ concert, using chrome tape. However, I was dismayed later to find that my recorder had been left on normal bias setting. What can I do to correct this?

-Harry Miller, Toledo, Ohio

A. After the fact there is nothing much you can do about having used the wrong bias setting. The chief result of under-biasing--which is what happened in your case--is to increase distortion and treble response. If the sound is too bright in playback, you can probably correct it fairly satisfactorily with tone controls or a graphic equalizer. If your recording took place at a moderate level, it is possible that the distortion is inoffensive enough to be acceptable. The only way to find out is by playing the tape. If distortion is excessive, nothing can be done.

Speed Problem

Q. Recently I purchased a cassette deck with three motors and a direct drive system, yet I have been greatly disappointed by its speed accuracy.

When I record from an FM broadcast and then play back this tape, the speed on playback is slightly slower than the source sounded; this seems true from the beginning to the end of the tape. I am certain there is nothing wrong with any of my tapes. I must conclude that the playback speed of my deck is not 100% accurate. It sounds as if the music is being played at a slightly slower tempo. What could be the possible cause?

-Jimmy Edwards; Greenville, N.C.

A. Although a deck may not have exact speed--many or most have inaccuracies on the order of about 0.5%, which is generally inaudible to all but the most acute ear--any inaccuracy in recording is compensated by the same inaccuracy in playback, assuming that recording and playback are on the same deck. While the deck is warming up, it is possible that the heat produces changes which cause the drive motor to slow down a bit. Since recording precedes playback, the latter would appear slow. Try letting the deck warm up a while, perhaps half an hour, before recording, and then check whether speed seems slow in playback.

If your problem remains, you will require the services of a competent audio technician-preferably an authorized shop. Before taking your deck into a shop, ascertain how much of a problem there is. That is, time a cassette in recording and time it in playback to find out the exact speed difference between the two modes.

Dolby Mistracking

Q. I recently started using a different brand of CrO2 cassette tape, which yields a level 3 dB higher in playback than the tape I formerly used, as shown by the deck's meters in playback of a 400-Hz tone. If I were to intermix both brands of cassette, wouldn't one of them cause severe Dolby mistracking? A difference of 3 dB between two brands of tape in the same category seems excessive. Are there standards for tapes within the same category?

-Robert Scarborough; Riverside, Cal.

A. The Dolby system permits a mismatch of about 2 dB between recording and playback levels before significant mistracking is apt to occur and cause adverse effects on treble response. Your 3-dB mismatch is a bit outside this range, but it is difficult to guess how serious the consequences are. There is a good chance that, although measurable, the results of mis tracking are not audible. What do your ears tell you? Some tape decks provide a control so that the user can adjust the recording level for tape sensitivity. An internal 400-Hz tone is recorded and played back; if the recording adjustment is correct, the 400-Hz tone will read at Dolby level in playback, as shown by the deck's meter or other indicator.

There are IEC standards for characteristics of each of the four tape types, but this doesn't necessarily mean that manufacturers adhere very closely to them.

(Source: Audio magazine, Jan. 1984, HERMAN BURSTEIN)

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