We recently got our hands on a "half-speed" cassette recorder. Judging from test results, the world of tape recording may be in for another big change -- one that may be to cassette tape what the LP was to disc recording.

The particular product that suggests this idea is the new Nakamichi 680ZX, which not only runs at the slow speed of 15/16 inches-per-second in addition to the standard speed of 1 7/8 i.p.s., but also handles the new metal tape (better, I might add, than any other recent "metal-tape capable" cassette deck I have yet encounntered).

It is, actually, the metal tape feature that lends the slow speed its unprecedented high level of audio performance, which is, overall, equal to the performance considered very good for cassette decks running at the standard speed only a few years ago. Frequency response, with metal tape at 15/16-i.p.s. speed, was measured within 3 dB from 10 to 15,500 Hz. Distortion was a mere 0.8 percent. Signal-to-noise, with the built-in Dolby switch on, came to an impressive 66 dB. Ample signal recording headroom was available, and wow-and-flutter came to only 0.07 percent.

Naturally, at the normal speed of 1 7/8 i.p.s., things were even better: response extending to beyond 20,000 Hz, distortion down to 0.6 percent, signal-to-noise making the 70-dB mark, even more signal head room, and wow-and-flutter a mere 0.04 percent.

But it is the slow-speed, metal-tape results that demand attention. They demonstrate, finally, that it is possible to double the running time of a given length of cassette tape and still get performance that is very much "in the ball park" for home hi-fi sound. The added cost of metal tape is thus very nicely offset by the fact that a C-45 size becomes, in effect, a C-90 size in half-speed use. FEEDBACK

Q. According to what I have read, a listening room should be balanced acoustically between being overly "live" and "dead" in terms of sound absorption. If so, why are speakers tested in anechoic chambers that are very "dead"?

A. The anechoic chamber eliminates the normal environment as a factor in determining a speaker's response. Such a room has no resonances or reverberation. In a sense it "penalizes" all speakers in the same way. It does not, however, give a true picture of how a speaker will sound in a normal listening environment. A major failing, for instance, is the anechoic room's poor loading for base response, which always seems worse than when the speaker is installed in a normal room. At best, the anechoic chamber is most applicable to a direct radiator driver in a sealed box, which hardly describes all the many types of speaker systems made. The fact is, many labs and manufacturers are getting away from what once was a heavy reliance on the anechoic chamber for speaker testing.

Q. Is it true that the new super-discs should be played only with a moving-coil pickup?

A. This is another of those debatable questions in audio. My own feeling is that the tracking requirements of these discs are not the exclusive achievement of the moving-coil pickup. A top-quality moving-magnet pickup, installed in a low-mass, well-balanced tone arm, should do just as well. Besides, there are all kinds of moving-coil pickups -- some are great and others are just so-so. I still believe that each model should be judged on its own merits rather than by the generic class of design.