by CRAIG STARK
Theoretical and practical tape problems solved.
The question-and-answer format of Larry Klein's "Audio Q. and A." column has proved to be so popular that he is beginning to need a little help, so this monthly tape column will be devoted, while the need lasts, to readers' theoretical and practical problems with re cording tape and machines that use it.
Noise-Reduction Ratio
Q . Most cassette decks claim that their Dolby system improves the signal-to noise ratio up to 10 dB, but one specification I saw recently said the machine had a 50-dB S/N without Dolby and 60 dB with Dolby "above 5,000 Hz." What does that mean?
STEVE GOLDBERG; New York, N.Y.
A. The full noise-reduction potential of the Dolby B system is available only at frequencies of approximately 5,000 Hz and up.
Lesser amounts of quieting are available at lower frequencies, down to about 1 or 2 dB at 500 Hz, this latter being a typical frequency where non-Dolby S/N measurements are made. A cassette deck with a 50-dB non-Dolby specification at 500 Hz will not have any where near that good a rating at 5,000 Hz, however, since the maximum undistorted signal that can be obtained from the cassette de clines very rapidly in the high-frequency area.
So, while the Dolby B system does give an additional 10 dB or so of usable high-frequency dynamic range by lowering the hiss level, you can't just add that amount to a specification based on a low- or middle-frequency signal.
Head Demagnetization
My dealer told me not to try to demagnetize the heads on my machine, since if I did it wrong I'd do more harm than good. What do you say?
JANICE SMETHURST; San Francisco, Calif.
A. Unwanted magnetism does build up on tape heads (and guides and capstans), and if it isn't removed by proper demagnetizing it will tend to erase the highest frequencies from every tape you play on the machine. In the time it took to scare you, your dealer could have shown you how to degauss the heads safely.
1. Obtain a standard inexpensive "head" demagnetizer such as is sold in most audio stores. If the demagnetizer tip is hard and sharp, cover it with electrical tape so it can't scratch anything.
2. Remove all tapes from the immediate vicinity and turn off the recorder. Hold the de magnetizer several feet away from the heads and turn it on. Then slowly bring it up to the heads, guides, and capstan(s) and slowly move it up, down, and around the exposed surfaces. Then slowly withdraw the demagnetizer, being sure to al feet (arm's length) away before turning the demagnetizer off. This is important, since the turn-on and turn-off surges in the degausser could magnetize the head if it is near them. That may be what worried your dealer.
Conspiracy of Silence
Q. I notice that you and the other hi-fi magazines talk about cassettes and open-reel tapes; but all of you seem to ignore the eight-track format. What's wrong with cartridges?
SIDNEY GOLUB; Anaheim, Calif.
A. Several years ago, 3M made a last, valiant attempt to revive interest in high-quality eight-track cartridges by showing a technically advanced deck at the Consumer Electronics Show. But it never achieved the hoped-for popularity. My files are full of reader letters asking me how to salvage jammed cartridges, and in my replies I've cited the words of an industry spokesman who once described the eight-track cartridge as "a self destruct mechanism." Sooner or later the delicate balance between the front-side and the back-side friction of endless-loop tape is bound to be upset, and every playing then tightens up the tape pack in the center until the cartridge jams.
Further, because of the higher tape speed, while cartridges should have a wider frequency response and dynamic range than cassettes, in practice they don't. For whatever reasons, the recorded quality seems to come out more suitable for the automotive and "never before offered on television" markets than for critical hi-fi listening. So, most of us tape pundits do ignore the cartridge. For my self, when I make or buy a recording I want to keep it--not put it into a mechanical time bomb.
CrO2 Tape and Head Wear
Q. I have a large number of chromium-dioxide. cassettes, but I've hesitated to use them on my new recorder because I've been told they'll wear out the tape heads faster. Is this true?
AL GROSS; Pontiac, Mich.
A. The relative abrasiveness of CrO2 and M. ferric-oxide cassettes has been debated back and forth for years, and when a group of dealers recently asked me for the latest re search findings on the subject I countered with a question of my own: "When was the last time your service technician had to re place a worn-out cassette tape head, regard less of which type of cassette was used?" The silence was eloquent.
All tape is abrasive, of course, and at open-reel and duplicating speeds head wear can be and is a problem. No doubt someone, some where, has managed to wear out a cassette head in more or less normal use, but, in my experience, the motor bearings are likely to go first, making any possible difference in the oxide formulations academic. And the latest findings I've seen (no doubt to be contradict ed, then re-confirmed, then contradicted again) place the wear factor between good chrome and ferric cassettes about even.
On the Dull Side
Q. A number of my prerecorded cassettes sound okay on one side, but dull (no highs) on the other side. Since the cassettes I record myself sound fine on both sides, I don't think there's anything wrong with my deck. Is it just another case of sloppy duplicating?
MAURICE POWELL; New Orleans, La.
A. The problem you describe is quite common. I once measured a 15-dB difference in the 10,000-Hz playback level between the two sides of a full-track test cassette in tended for consumer use, and a 4- to 5-dB difference at that frequency is not unusual even with professional test cassettes.
The explanation is that sometimes minute imperfections within the plastic cassette shell can cause the tape to skew, so that it contacts the playback head at slightly varying angles, depending on which side is playing. The resulting azimuth error, even if only a fraction of a degree away from the proper 90 degrees, will cause significant treble losses.
Since your prerecorded tapes play well in at least one direction, it is probably safe to say that your tape head is properly aligned; you'll just have to live with the occasional second-side vagaries. The reason your own tapes don't show the problem is that even if there is a slight skewing in one of your cassettes, you're using the same head on the same ma chine for both the record and playback, so there is no relative azimuth error. And, of course, the shells of good quality consumer cassettes are generally better than those used for mass duplication.
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