A FAIR amount of time has passed since the introduction of metal-particle tape and the appearance on new cassette recorders of the legend "MET" near the tape selector switch. During this time, more and more manufacturers have begun to issue metal-particle tapes while also announcing improvements in their existing tape formulations.

As a result, the consumer about to purchase blank cassettes for home recording now faces a choice among no less than six designations of recording tape:

To begin with, there is the garden variety of ferric-oxide tapes. They require what is called "normal" or "standard" bias, and an equalization designated as "120 microsecond time-constant."

One step up are the "premium" grade ferric-oxide tapes, whose magnetic coating has been made with greater precision and density. These also take the same standard bias and the 120-microsecond equalization. Results are likely to be a little better than with using the lower-grade ferric-oxides.

The next major class of tapes contains formulations known collectively as "high-bias." The equalization required for these is 70 microseconds. The first of these tapes was chromium-dioxide, followed by a competing category known as "chrome-equivalent." Included here are the cobalt-doped ferric-oxides, and the twin-layered type known as ferri-chrome. Partisans of any of the three high-bias tapes will argue for their choice at the drop of a capstan, but lab tests indicate that any of them is capable of superior results when properly matched, in terms of bias and equalization, to the particular deck being used. In general, these tapes can be credited with a little more headroom, and little more response in the extreme high frequencies. They also cost more.

Then, of course, there is "metal." Although this tape works with the same equalization (70 microseconds) as do the tapes in the previous group, its bias requirements are higher yet. Metal-particle tape also is harder to erase, and the erase head on a typical deck made before the advent of metal tape will not erase it completely. For this reason, metal tapes should be used for recording only on a deck that is specified as being "metal-tape capable." However, it is perfectly all right to play a recorded metal tape on any deck, including those in car stereo systems.

Is metal tape worth the added cost per cassette, not to mention the changes required in the deck itself? The answer is far from certain. In the lab, metal tape can be shown to produce a cleaner and stronger response at the extreme high end of the musical range. But in practical terms, this advantage may have meaning only for those who are recording live performances of program material that has unusually strong signals in the extreme highs -- a typical example would be a rock band. But for most other recording the home enthusiast is likely to do -- such as dubbing records or broadcasts, or an occasional live taping of a small ensemble or choir -- the use of metal tape has only marginal benefits (if at all) over the other, less expensive tapes.

Incidentally, if you record with a high-bias tape on a cassette deck that does not have a high-bias switch position, you may get improved highs. However you also may get more distortion, depending on the deck. Your best bet in this instance would be to record at relatively modest signal levels. If you record with a normal-bias tape on a deck that is adjusted for high-bias, you will gain nothing. There is a good chance that distortion will rise, and signal-to-noise will be degraded.