Tape Guide (Dec. 1980)

Quarter vs. Half Track

Q. I am considering purchase of one of several high quality tape recorders, all' of which come in either half-track or quarter-track versions. What are the advantages of a half-track system?

-N.P. Ramsden, FPO, San Francisco, Cal.

A. The advantage of half-track operation is a moderate improvement in signal-to-noise ratio--about 3 dB. For home operation, where you usually encounter first-generation recordings, rather than a succession of copies, this is not a great advantage compared with the benefits of quarter-track.

Moreover, use of the Dolby noise-reduction system can improve signal-to noise ratios about 6 to 10 dB, more than making up for the 3 dB loss due to quarter-track operation.

Quarter-track affords twice as much recording time per reel as does half track, assuming stereo operation. If one goes quadraphonic, of course, quarter-track is a necessity--half track wouldn't be feasible then. Quarter-track has the advantage of reducing the consequences of azimuth misalignment. That is, for a given degree of azimuth misalignment, the amount of treble loss is smaller for quarter-track than for half-track. On the other hand, half-track has some what greater freedom from dropouts because these tend to average out (cancel out) to a greater extent for a wider track.

Bias By Us or a Dealer?

Q. Do I have to bring my tape deck to a dealer to have the bias adjusted if I change from one brand of low-noise tape to another?

-Peter Klein, Providence, R.I.

A. I cannot give you a definite answer. Chances are good, however, that in going from one brand to another you will not encounter substantial difference in treble response with the same bias setting. If your machine has more than one bias setting via an external switch (some machines have as many as three settings), you will have flexibility in use of various tapes with out having to make an internal adjustment of bias. The only way to find out if you really need to have bias adjusted internally is to try the new brand and note whether there is an appreciable departure from flat response no matter which bias setting is used. If you decide to have bias adjusted, be careful because an inept technician may do more harm than good. Insist that the adjustment be made when the machine is operating with the tape you plan to use.

Performance of Metal Tape Decks

Q. What happened to the performance of metal tape decks? Manufacturers were predicting that cassette decks capable of handling metal tape would have an increase in signal-to-noise ratio of about 6 dB, plus greater recording headroom of about equal amount. Yet technicians I have talked to who have tested some top-quality decks have found an increase in S/N of only about 3 dB. It is my suspicion that the metal tape manufacturers have agreed to make a product below the full potential of such tape. It is logical to carry this theory one step further: Assuming that makers of metal tapes and decks are capable of an increased dynamic range of about 12 dB, they may have said to themselves, "Why release our best product at once?" By gradually improving the performance of metal tape over time, the conversion can be stretched over many years so that everyone can make more money--including me, for I'm a retail audio sales man. I would appreciate your response.

-Name withheld.

A. In addition to your hypothesis, there are other possible explanations as to why the improvements attributable to metal tape are not quite earth shaking, although the improvements definitely exist. (As you point out, there is something like an improvement of 3 dB in S/N. And reviews of metal tape decks in various periodicals show that at 0 VU recording level the decks provide more extended treble response with metal tape than with other kinds of tape; in other words metal tape does have more head room.) First, in all truth it must be recognized that metal tape has not reached its full potential. In response to the problems that deck manufacturers have had in developing record and erase heads that can cope with the demands imposed on them by metal tape--i.e. sufficient bias, audio, and erase current--the tape makers have reduced these demands. Specifically, instead of metal tape having a coercivity of something like 1500 oersteds, the coercivity at the time this was written was generally being kept down to something like 1000 oersteds. The result is restriction of the amount of signal which the tape is capable of accepting, thus keeping dynamic range below the potential of metal tape.

Second, the history of audio shows that those responsible for its technology have had to fight hard for every 1 or 2 dB of improvement. Accordingly, an improvement of about 3 dB in S/N plus some improvement in headroom is nothing to be scoffed at--at least not by historical standards.

Third, it is rare that the full benefits of any technological advance are reaped immediately. Consider the ex ample of the transistor, where promise greatly preceded fulfillment. It took years and years before transistor products became as good as tube products. (Some still feel that the transistor hasn't caught up; witness tube preamps and power amps being made today.)

Fourth, it must be taken into account that tape recording, particularly at the low speed of the cassette for mat, requires a compromise among three fundamental requirements: Low distortion, low noise, and extended treble response. It is possible to win an improvement in any one respect only by means of a sacrifice in one or both of the others. Accordingly, a tape deck manufacturer who already has a product with a fine S/N ratio may wish to take advantage of metal tape more in terms of low distortion and extended treble rather than in terms of a large increase in S/N.

Finally, it must be recognized that in order to achieve compatibility, the 70 uS playback equalization that has been considered optimum or near-optimum for chrome, ferrichrome, and cobalt-modified tapes is also being used for metal tape. Quite possibly a lower time constant, perhaps around 50 uS, would be optimum for metal tape and allow its benefits to be more fully realized.

Please do not misunderstand. I am not promising you that metal tape will inevitably do all that has been prophesied. I am simply trying to suggest that it is too soon to judge whether or not metal tape represents an unrivaled giant step forward.

Cheap or Inexpensive?

Q. There is a store in my area selling Scotch audio tape, 3,600 feet on 10 1/2-in. reels that have been used once and bulk erased. The ad claims that this tape will perform like new. I wonder if this tape will provide adequate reproduction. It sells for only $1.99, and I am a bit suspicious of tape this cheap.

-Thomas Williams, Baltimore, Md.

A. Offhand, it seems you might have a good buy, except for one possibility: That the tape contains splices. If there are only one or two splices in a 3,600-foot length, this is still a good buy, but not if there are many splices. Ask about this. In any event, you might buy one reel; you can't lose more than $1.99 plus tax!

(Audio magazine, Dec. 1980; Herman Burstein )

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