The Comparable Cassette
Development one, perhaps the most significant factor in the changing picture, is the ready availability of B-type Dolby devices (which are single-band Dolbys, acting only on hiss frequencies). Advent makes two that can be used with any tape machine, cassette or otherwise, while Fisher, Advent, and Harman-Kardon (as of this moment) are producing cassette recorders with built-in Dolby-B. No doubt there will be others by the time this gets in print.
We have tried the Advent 100 universal-type Dolby and Harman-Kardon's CAD-5 cassette machine with a built-in Dolby, and can vouch for the fact that tape hiss is no longer an inherent limitation of cassettes. With the CAD-5 in the Dolby mode (you can switch it in or out), and using a regular (non-low-noise) TDK cassette, background hiss was judged to be about the same as that from our 4-track Ampes (also with regular) tape) at 7½ips. Which is to say, hiss was audible at high listening levels but was not at all obtrusive. Most of it was midrange energy, and thus rather easier to tolerate than higher-frequency hiss.
Development two is the servo-controlled tape drive, which seems to have pretty much licked the other big bugaboo of cassettes: the painfully bad speed regulation. This gain does not usually show up in published spec sheets; some makers have been claiming speed regulation of 0.2% all along. The difference now is that a lot of them are actually meeting their specs. And recent refinements in cassette manufacture have also helped to reduce speed variations from that source.
The promise is DuPont's chromium dioxide "Crolyn" tape, which is soon to be marketed in cassette format by Advent Corporation. Crolyn oxide particles are extremely small, and result in a tape with better resolution and a smoother surface than any ferrous oxide produced to date. The result is significant improve ments in both treble response and noise characteristics, relative to currently-available tapes. Having never tried Crolyn tape, we can only speculate that it will probably make cassettes even more hi-fi, but cannot guess how much more.
Exit hiss, exit wow. What's left to keep cassettes out of the hi-fi camp? Well, let's see...
At 17/8ips, it is self-evident that no existing tape can do as well at the high end as at 3¾ or 7½ips. But how much difference is there in actual practice? With currently available cassette tapes and a certain amount of tradeoff in the distortion department (due to slight underbiasing, to reduce self-erasure at high frequencies), a cassette can be made respectably flat out to around 10 or 12kHz, which is most respectable.
The low end can be made to extend as low as 20Hz if the designer so desires, but the bass range usually exhibits the same mild humps and dips that are observed from 4-track open-reel playbacks (although careful design can minimize these).
For open-reel tape recorders, there is an industry-standard "Normal Maximum" or Zero recording level, which allows for at least 6dB of "headroom" above the Zero level before overload sets in. The hiss from cassette machines made it necessary to set their "Zero" level at just a dB or two under the overload point, in order to squeeze out as much S/N as possible. The Dolby system now specifies a standard recording level, yet two Dolby cassette machines we used produced what we felt to be excessive distortion at "conservative overload" levels,
Modulation noise is not really distortion, but we're including it under this heading because it produces the same haziness and loss of transparency as a good case of intermodulation. Like hiss and distortion, this has always been fairly noticeable on cassette playbacks. The Dolby treatment helps significantly, but then it helps just as much in the 4-track higher-speed formats. Crolyn tape may help even more, but how much it will help remains to be seen.
Since these momentary losses of tape-to-head contact are more often caused by rough handling of the tape than by actual surface defects, they tend to be less frequent from the relatively inaccessible cassette tape than from open-reel tapes. On the other hand, the extremely delicate cassette tape is much more sus ceptible to stretching and buckling due to high-speed shuttling.
On a well-designed cassette machine, crosstalk between adjacent channels is slightly worse than from a good 4-channel, open-reel machine, which is to say, it is audible at moderately high listening levels. The Dolby noise reduction helps here, but not significantly, because most crosstalk interference is below the Dolby's operating range.
What it all adds up to is that the cassette format can now be compared with open-reel formats without provoking snickers of contempt. Prerecorded cassettes are, by and large, still pretty miserable-sounding, and even with Dolbyization, cassette sound is still no match for the best disc reproduction, but the chasm between them has narrowed to a gulch, and shows signs of narrowing even more.
In short, the cassette field bears watching. We shall watch it with interest.J. Gordon Holt