TAPE GUIDE (Nov. 1984)

Is the Price Right?

Q. I purchased a high-quality cassette deck, costing over $500, about 15 months ago. Recently, I took it back to where I bought it to have the deck cleaned and checked. I was told it would cost me $60 in labor plus the cost of parts to correct the following: Azimuth misalignment of the playback head, excessively fast speed, erratic motion of the take-up reel, and bias out of adjustment. Is this an excessive price? If I am going to have to pay this much each year for service, is it really worth having a deck like this?

-Richard Butia; New Wilmington, Pa.

A. For a high-quality deck, the price doesn't seem out of line. After all, it costs more to service a Cadillac than a Chevrolet. On the other hand, you could choose to live with your deck as it is if you hear nothing wrong, and wait until something goes obviously wrong before you seek repairs. But I don't think this is the way to go. As with a car that you value, you should be prepared to periodically pay repair and maintenance costs for your tape deck.

Taping AM

Reader Harrison Pierce Reed III has some cautions about taping AM broadcasts in the Dolby mode:

When taping AM radio, unless one is practically on top of the transmitter, Dolby noise reduction should never be used. The 19-kHz filter in the deck will be practically worthless, because the problem is caused by other frequencies. The problem frequencies include heterodynes at 10 and 20 kHz. Additional heterodynes are caused by the many Caribbean stations that operate on half-channels-for example, 1,035 kHz-and European stations that are spaced at 9-kHz intervals. Further, noise frequencies are generated by cross-modulation of sidebands. This mash may be acceptable to the ear, but the Dolby circuit sees it as legitimate treble energy and tries to follow it. The Dolby will go nuts.

If one insists on Dolby encoding when recording AM, one must settle only for clear-channel, 50,000-watt stations that are nearby, and tape only from two hours after sunrise until two hours before sunset. And one cannot tape during the colder months, when skip signals of 1,000 miles or more are common during midday. Even if taping during the time of year when skip signals are virtually nonexistent, one had better make sure there is no adjacent channel station within the remotest ground-wave distance, and had better hope there isn't a thunderstorm anywhere within 800 miles.

In sum, no matter what AM tuner is used, one is hardly going to tape AM satisfactorily with Dolby on unless the signal is so strong that it overloads the tuner's front-end, resulting in gross distortion! Anyways, considering how noisy and low-fi AM typically is, it is not going to benefit much from Dolby. I am aware that some AM broadcasters, especially in high-budget major markets, do an excellent job of producing audio signals worthy of the best on FM. However, the AM medium itself adds too much distortion unless one lives a nose-distance from the station. Something as critical as Dolby tracking requires at least the reliability of FM reception quality to work properly.

Dubbing-Deck Quality

Q. High-speed dubbing cassette decks, such as those made by Radio Shack and Technics, may save time when dubbing, but will the quality of sound on the dubbed tape match the original's? I've been told these systems introduce distortion and reduce sound quality, but I thought higher tape speeds meant less information per tape area, thus improving sound quality.

-Theodore Floyd; Tampa, Fla.

A. Try the model of your choice, connected to a good demo system in the store, and see if you can hear a difference when you play back the dubbed tape.

In general, any copy will add some THD and noise. Also, with Dolby B and C NR, there may well be tracking problems with high-speed duping, since the normal procedure (decode the tape you're playing and re-encode for the tape you're recording) will be thrown off because the frequencies on the tapes will temporarily be transposed up. The only way to prevent this would be to copy Dolby tapes without any Dolby circuits switched on-and to make that work, you'll have to set your recording levels so that a recording made on the dubber would have precisely the same level as the tape it was copied from. You might try dubbing a Dolby-level test tape to check and adjust the level match.

Dubbing at high speed does not change the information per tape area.

It the tape speeds are doubled, then the frequencies are also doubled, which neutralizes the effect. For example, a 1-kHz tone on a normal-speed cassette will occupy about 0.05 mm of tape. Double the playback speed, and that tone's frequency becomes 2 kHz-just long enough to record onto 0.05 mm of a tape being recorded at double speed. The tone's recorded wavelength stays the same, even though its frequency has been temporarily doubled.

Magnetic Field Danger

Q. I have bought an old tube amp (it was cheap and has a nice sound) which comes in a wood cabinet and has several large transformers inside.

Should I go to great lengths to keep my tapes away from this amp? Or is it okay to place my cassette deck next to the amp?

-Paul Rubin; Berkeley, Cal.

A. Ordinarily, tapes are safe if they are kept at least 3 inches distant from magnetic fields which may be emitted by the transformers in audio equipment. Therefore, it is probably safe to place your cassette deck alongside your amplifier.

Warp Frequency Problem

Q. I hope you can help me with a problem involving my cassette and 8 track decks; I have written to the manufacturers but have never received an answer. Whenever I copy a phono record onto 8-track tape, the drums, cymbals, and handclaps are distorted in playback. This makes the background very raspy and produces a terrible swooshing sound. When I record from the cassette deck to the 8-track deck, this does not happen, and the recording is perfect. I have had the 8 track deck checked, and it is fine. I use only the best 8-track cartridges and am not overloading the tape, as I have tried recording at various levels. Phono records play fine; and my cassette tapes of these records are also fine.

Prerecorded 8-track cartridges play excellently on the 8-track deck. It is only when I try to record phono discs onto 8-track cartridges that I experience distortion.

-Bill Schuh; Scottsdale, Ariz.

A. Others have run into a problem such as yours. The answer appears to be that the electronics of the offending deck (in your case, the 8-track unit) are unable to handle all frequencies presented to them at high or even moderate levels. Specifically, the deck is unable to handle the warp frequency generated by many discs and therefore presented by the phono pickup and phono preamp. The warp frequency is too low to be audible to the human ear, but it exists and is amplified by the bass boost supplied by the phono preamp. Given such a frequency, the electronics of a tape deck may block, distort, and/or oscillate.

On the other hand, it appears that your cassette deck, apparently of high quality, can accept the warp frequency without trouble. However, its output drops sharply below about 20 or 30 Hz, so very little of the warp frequency remains in the signal presented to your 8-track deck. Therefore, the 8-track works satisfactorily when dubbing from cassette.

Flat Spots

Q. My car cassette deck does not automatically eject the tape when the motor is turned off, and tapes are therefore forgotten at times and remain in play position. Can this damage any of the parts and cause a deterioration in playback response?

-Samuel J. Neiditch; Redlands, Cal.

A. Yes. If the tape remains in operating position, this may cause the pressure roller to develop a flat spot at the place of contact with the tape and capstan, resulting in wow. If the mechanism contains rubber drive wheels, these may also tend to develop flats.

The flat is not necessarily permanent, because the rubber tends to have a "memory" of its original shape, but it may tend to become so if the deck is in play position long enough. It's difficult to define long enough; a few minutes of engagement is unlikely to do harm, but a few hours very well may.

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

= = = =