TAPE GUIDE (Sept. 1989)

Mufflement

Q. I have a baffling problem with my cassette deck. At times, the sound suddenly becomes muffled but then returns to its wonderful, crisp nature. I have tried demagnetizing and cleaning the heads. Cleaning helps a little, but nothing seems to completely clear up the problem. What's wrong?

-Paul Mendelowitz, Redwood City, Cal.

A. The muffled sound may be due to temporary azimuth misalignment be tween the playback head and the tape.

That is, the tape may not be consistently moving at a right angle to the head gap. This could be due to a defective cassette that allows the tape to skew appreciably within the shell or to a defective mechanism in the deck.

Another possibility is that the Dolby circuit is not working properly, that something recorded on the tape-or something else-is causing it to go erratically into sharp treble cut. A third possibility is that the heads have not been cleaned well enough, thus allowing dirt or dislodged particles of magnetic coating to cause a slight space between tape and heads. A final possibility is an intermittent fault in the play back electronics.

Are Three Heads Better Than Two?

Q. I have a two-head cassette deck, but someone told me that a three-head deck is better. Why?

-Michael Clark, Wichita, Kans.

A. A three-head deck, which has separate record and playback heads, plus an erase head, ordinarily has several advantages over a two-head deck, which has a single head used for both record and playback, plus an erase head. It permits monitoring the tape (immediately listening to it during re cording) to assure that the recording is satisfactory and to facilitate tests and adjustments such as bias adjustment.

Use of a separate head for playback permits it to have an extra-narrow gap, which is essential for extended treble response; such a gap is unsuitable for recording because it tends to short circuit the audio signal. A separate head for recording permits a relatively wide gap-roughly five times the width of an ideal playback head gap-which is conducive to accurate, distortion-free recording. A separate playback head may be designed for maximum signal output, thus improving the S/N ratio a bit. This is achieved by a relatively large number of turns in the head's coil, but the resulting high inductance would make it difficult to drive high-frequency bias current through the head for recording purposes.

On the other hand, a two-head deck has the advantage of not presenting an azimuth problem, provided the same deck is used for recording and play back. An azimuth error in recording is compensated for by an equal error 'n playback when the same head is used for both purposes. With a three-head deck, it is quite possible for the azimuth alignment to differ in the record and playback heads, with a resultant loss in treble response.

Even though a three-head deck tends to be superior in performance and convenience, with good engineering and construction, a two-head deck can be very satisfactory. If your two-head deck enables you to make recordings that sound like faithful replicas to your ears, you should not feel something is seriously lacking.

Which Type of NR?

Q. My cassette deck includes both Dolby and dbx noise reduction. For taping voices, / wonder if one is better than the other.-Juan K. Ferry, Wey mouth, Mass.

A. Whether taping voice or music, dbx NR achieves the greatest noise reduction-typically about 30 dB, compared with about 20 dB for Dolby C NR and about 10 dB for Dolby B NR.

Generally, dbx NR doesn't do as good a job of maintaining frequency response as either Dolby NR, particularly at the low end. Also, the greater the noise reduction, the greater the tendency toward undesirable side effects such as sudden changes in level and bursts of hiss. Hence, some may prefer Dolby NR, even though dbx NR is quieter. On the other hand, side effects tend to be a very minor problem with present-day circuitry.

More on Head Demise

In the June issue, a reader arced how to tell when the heads in his tape deck are shot ("Head Demise"). I recently received a valuable commentary from Bob Ursdevenicz of Omaha, Nebr., who is manager of a company called National Sound. Ursdevenicz writes:

A method I have found to easily detect a worn tape head is to check it for "grooving." Most inexpensive tape decks use the softer Permalloy tape heads. With heavy use, these heads develop a groove where the tape contacts the head. These grooves can be felt on a severely worn tape head, using a cotton swab stick or finger nail (careful!). When the tape travels over a grooved head, one channel will tend to lose treble response more than the other. This is due to the tape edge riding the groove and pulling the tape away from the head gap closest to the edge of the tape. When a deck gets into this condition, it is time to replace the head. If the deck is an inexpensive model, it is probably a good time to upgrade to a new machine, since most head changes cost $60 to $120.

Playback Discrepancy

Q. I own two high-quality decks and a CD player. Whenever I record, whether from CD to tape or from deck to deck, the meter readings are slightly lower in playback than in recording.

For example, a level that occasionally reaches +3 dB in recording just reaches 0 dB in playback. Why? Is this normal? Is there something I can do or something I should have done?

-David Marx, Putney, Vt.

A. Your situation is within normal bounds. The lower playback readings can be due to tape sensitivity or to the way the playback metering circuit has been adjusted. Sensitivity refers to the amount of tape output, in playback, produced by a given amount of signal input to the tape during recording.

Some tapes have higher sensitivity than others, although this does not necessarily mean the former are better overall. The deck manufacturer may have calibrated the playback reading on the basis of a tape with higher sensitivity than the tapes you use. I think you should leave things alone.

(Source: Audio magazine, Sept. 1989, HERMAN BURSTEIN)

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