TAPE GUIDE (Dec. 1988)

Test-Tone Generator

Q. I have a Numark TC4100 tape deck with a built-in tone calibrator/mixer which generates 400-Hz and 8-kHz test tones and has adjustable bias. The instruction manual is not very good, however, and I have a couple of questions. Can the test tones be used to adjust bias for a particular brand and type of tape, and, if so, what is the procedure? Can the test tones be used to check the azimuth alignment of the tape heads?

-Mike Morena, Camarillo, Cal.

A. The usual procedure for adjusting bias in a tape deck by means of a two-tone calibration system is to re cord both tones and adjust bias so that the levels of the two tones are equal in playback. If you find the resultant very high treble response unsatisfactory, you can touch up the bias by reducing it slightly. Contrariwise, if treble proves excessive, you can increase bias somewhat (which has the advantage of also reducing distortion).

The test tones cannot be used for adjusting playback azimuth. For that you need a test tape (quite expensive) containing a high-frequency tone, typically about 12 or 15 kHz. The procedure is to play the test tape and adjust playback-head azimuth for maximum output. If the deck has a separate re cord head, an audio generator is required to produce the same tone, and record-head azimuth is adjusted to produce maximum output in playback.

Picking a Reliable Deck

Q. Have there been any studies concerning the reliability of various brands and models of tape decks? Or can you point out any guidelines for buying a reliable deck? My family and I have had a lot of problems with every type of cassette deck we've had-home, car, or portable.

-Joseph A. Arlt II, Philadelphia, Pa.

A. I do not know of any studies that have sought to rate tape decks, or other audio components, in terms of reliability. This would be a very time-consuming study, and by the time it was completed, there would be a whole new assortment of cassette decks on the market.

A reasonable guide to reliability is the deck's price and the reputation of the manufacturer. Two decks may outwardly look very much alike and may offer pretty much the same features, yet they may differ substantially in price. A key reason for this difference is usually the quality of parts and construction, which substantially deter mines reliability.

Another factor in reliability is complexity. The more features a deck boasts, the more things there are that can go wrong; so the simpler the better, all else being equal.

Equipment reviewers sometimes look inside a deck to evaluate the quality of components and assembly. Their remarks in this respect can be an index to reliability. Finally, a model that has been on the market for a substantial period and has sold successfully may have an edge in reliability be cause the manufacturer will have had time to refine the product and eliminate the bugs. Similarly, a firm which has made cassette decks for many years will usually have an edge.

"Hot" Tape

Q. I have just changed to a different brand of tape; its output is 3 dB greater at the high end. My deck automatically optimizes itself for any tape, and tests have shown that its frequency response is ± 1 dB from 20 Hz to 18 kHz.

Will my deck compensate for the 3-dB gain at the high end and still give flat response, or will there be a 3-dB rise at the high end? If the deck compensates for the high-end gain and gives flat response, what would be the purpose of having a tape with such a characteristic?

-Michael P. Conner, St. Louis, Mo.

A. If your deck does a proper job of automatically adjusting bias, EQ, and record sensitivity for the tape in use, response should remain flat with the new tape.

The purpose of a tape with an extra few dB of response at the high end is to reduce the chances of distortion owing to insufficient bias. Reduced bias may be needed to maintain treble response of a poor tape, but if a tape has good high-end response, more bias may be used with less chance of distortion.

If your deck automatically adjusts record EQ, the tape with superior treble response requires less treble boost in recording, which reduces the chance of tape saturation. (Tape saturation causes distortion and loss of response at the very high end.) All in all, if everything else remains equal, the extra gain at the high end can prove useful.

Tape Type Choice

Q. After reading Howard A. Roberson's article in the November 1987 is sue, in which he reviewed 35 cassette tapes, I am more confused than ever about the pros and cons of low- and high-bias tapes. In the article, it seems that Type I (low-bias) tapes are rated better than Type II (high-bias) tapes. If that's the case, why do tape companies push high-bias tapes, and why do they cost more?

-Greg Stefano, Seattle, Wash.

A. Originally, Type II tapes were introduced because they provided better performance than Type I, particularly with respect to high-frequency response. However, continued techno logical advances have enabled Type I tapes to stay not much behind (and in some cases to equal or surpass) Type II, as was brought out in Mr. Roberson's article and in other articles. In general, the chief advantage of Type II lies in its somewhat higher S/N ratio, which is due to the fact that this tape type's playback EQ entails greater bass boost (or, depending on how you look at it, greater treble cut). Still, some companies that employ Type II for pre recorded cassettes eschew this ad vantage in order to achieve more headroom, and designate their cassettes for Type I playback EQ. They do so because less treble cut in playback entails less treble boost in recording and, therefore, less risk of tape saturation at high frequencies.

The generally higher price of Type II tape owes principally to the higher cost of making its magnetic coating, which is either chromium dioxide or cobalt-doped ferric oxide.

Type IV tapes (which require by far the highest bias of all) are particularly advantageous in providing headroom.

That is, they permit more extended treble response at high recording levels.

Still, by providing a safety margin of a few dB when setting record level, you can achieve very good treble response with Types I and II cassettes and a good tape deck. As for price, Type IV tape is the most difficult and costly to make.

New Tape for an Old Deck

Q. I have an open-reel deck and am shifting to a new tape. However, with the present bias and record EQ, the new tape has excessive highs from about 4 kHz upward, reaching 6 dB at 16 kHz. This indicates under-biasing and/or excessive treble boost in the record circuitry of the deck. How can I optimize bias for the new tape? The instruction manual says that I should adjust bias so a 1-kHz tone drops 0.2 dB past maximum response. Should I do this with the old tape and then adjust the record EQ on the basis of the new tape?

-R. Wendell Butz; Columbus, Ohio

A. Bias should on the tape you plan to use: Your new tape. Therefore, following the procedure recommended in your manual, you would record a 1-kHz tone on the new tape, gradually increase bias until you get maximum output of this tone in playback, and further increase bias until output drops about 0.2 dB.

If this results in nearly flat response, you can stop there. You might consider adjusting bias slightly to get even flatter response; if you don't change the bias, you'll have to adjust the record EQ. As you already recognize, increasing bias causes high-end response to drop, and vice versa. If the manual's bias adjustment procedure produces a slight drop in treble response, I would be inclined to accept this; otherwise, if you reduce bias, you will probably be increasing distortion.

Bias for Metal Tape

Q. In a recent review of cassette apes, it was noted that a particular metal tape will not perform at its best unless one's deck is adjusted to match he tape's high bias requirement. My deck has a system for adjusting bias in order to match various tapes, but, just as the owner's manual predicts, it does not work with metal tapes. How do I adjust bias so I can use metal tape to its best advantage? Should the adjustment vary from side to side, tape to tape, or batch to batch?

-Ruhl Heffner, Hunt Valley, Md.

A. From what you say, it may well be that your deck cannot produce sufficient bias for metal tape. However, if you simply mean that your deck's automatic bias-adjustment system does not work well with metal tapes, you can (if your deck permits) try adjusting bias by ear. Record FM interstation noise at a low level, play it back, and compare it with the original. Experiment to find the bias setting which provides the playback response most similar to the FM noise.

Metal tape's bias requirements (and sensitivity) tend to vary a fair amount from one brand to another. However, they do tend to be fairly stable from one cassette to another of the same brand and grade, and from one direction of tape travel to the other. Still, you might find that optimum bias varies from one batch to another of the same tape, making it advisable to buy your metal tapes in volume rather than a few at a time.

Dulled Sound

Q. I presently own a cassette deck which is connected to a dbx noise-reduction unit. I wish to purchase a higher-quality deck, but when I played one of my tapes on this new deck through another dbx unit, I found that the playback readings were about 5 dB below those obtained in playback on my current deck. The music quality on the proposed new deck was good, except that it seemed as if the speakers were underneath a blanket. If I have a service technician recalibrate the meters on the new deck to match the meter readings on my present deck, would the music then sound as good on the proposed deck? Would this change in calibration affect the new deck's frequency response?

-Andy Ross; Beloit, Wisc.

A. Recalibrating the meters of the new deck in playback would not affect the quality of the sound in any way.

The veiled sound that you describe seems to stem from an azimuth mis match between your present deck and the contemplated new one. One or the other, maybe even both, seems to be out of correct azimuth alignment.

Another possibility is some discrepancy between the dbx unit employed in recording and the one employed in playback. This can be checked by re cording a tape on your present deck without dbx, and playing it on the new one without dbx. If the sound is still veiled, this indicates an azimuth mis match.

Adjusting Azimuth

Q. I own a Nakamichi ZX-9 that pro vides for easy azimuth alignment in the record mode. Unfortunately, as far as I know, the azimuth alignment control can only be operated when recording.

I would like to find out how to adjust the deck's playback head.

-Alex Ortiz; El Monte, Cal.

A. Playback-head azimuth is adjusted at the Nakamichi factory to conform to a standard test tape-that is, to pro vide maximum output at a high frequency on this tape (typically 15 kHz or so). Record-head azimuth is made variable in order to achieve maximum response when simultaneously recording and playing a high-frequency test tone (such as 15 kHz) or, if this tone is unavailable, to achieve best high-frequency response when reproducing FM interstation noise.

Adjusting the azimuth of the play back head is not advised, unless you have reason to believe that it is out of correct alignment. Even then, this adjustment is best left to a competent audio technician. He will make the adjustment on the basis of a standard test tape which is quite expensive. If you are determined to adjust play back-head azimuth yourself, you will have to refer to the service manual; it typically costs about $10.

MPX Filter

Q. My cassette deck has something called an MPX filter. I am not sure what it does. Can you please explain it to me?

-Mitchell Dodo; Hilo, Hawaii

A. The multiplex (MPX) filter is present in order to exclude any of the 19-kHz pilot tone that may remain in the signal which the tuner presents to the cassette deck in recording. Some tuners have adequate filtering; others do not. This 19-kHz tone can interfere with correct operation of Dolby circuitry, so it is probably best to leave the MPX filter on when recording from FM.

(Source: Audio magazine, Dec. 1988, HERMAN BURSTEIN)

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