AS CABLE TV expands, it merits attention from those who are interested in music and sound. The number of TV programs with significant musical content is increasing, and FM programs -- including those in stereo -- are becoming a staple offering of more and more cable systems. Thanks to such advances as improved lines and the widening use of satellite relay, the second quality of most TV programs is noticeably better than in the past.

Some genuine improvements in set design happily coincide with this new largesse of programming. Today's better models include a "comb filter," which effectively adds horizontal scanning lines. The result is a smoother, less gainy picture, especially on larger screens, which tend to show up video flaws more glaringly than smaller screens. The comb filter also is credited with stabilizing the picture, overcoming the jitters caused by outside interference.

The better sets also offer electronic tuning instead of the older knob with its click-stops. This makes tuning easier and more accurate; and with no mechanical parts to go bad, electronic tuning lasts longer and obviates at least one potential repair job.

Electronic tuning may be handled by a "varactor" system or by a micro-processor. Either way, the channel is selected by touching a button, or by punching in the channel number on a small keyboard. Some varactor systems let you "defeat" the set's automatic fine-tuning and then "detune" to another channel. This way, you can -- if you wish -- match nominal channel numbers with the actual channels received. You also can go hunting in the spectrum between channels 6 and 7, and somewhere beyond channel 13 too, where -- depending on your locale -- you might just pick up some signals you didn't know about, such as telecasts received experimentally via satellite.

The microprocessor-tuned set can be programmed to receive up to 105 channels directly. Such a set is said to be "cable ready." That is to say, it will receive all 12 channels (2 to 13) minimally supplied by a cable system, plus additional channels, without the need for (and added cost of) an external converter. This does not apply to such offerings as Home Box Office, which are "scrambled" and do require an adapter with any TV set. "Cable ready" also denotes that a TV set accepts coaxial cable without the need for a little "balun" transformer that converts 75 ohms to 300 ohms at the antenna-input terminals. (Some sets advertised as "cable ready" have only this minor feature and not the all-channel readiness possible only with electronic tuning.)

Sound is still the weakest aspect of most TV sets, but some improvement is discernible. If you buy a set larger than one with a 15- or 17-inch screen, you might get one with a speaker larger than the customary 2-inch cone. A larger speaker can make for stronger bass and clearer treble. Look for the audio output jack that lets you pipe the TV sound into your own stereo system. Try also to determine whether the TV set you are considering has separate intermediatt-frequency (IF) stages for sound and for video. Such a set will produce cleaner sound than one in which the video and audio share a common IF circuit. if there's no information on this feature, simply turn up the volume control.