Mix-and-match has long been the name of the game in stereo hi-fi, but increasingly manufacturers are offering complete systems in which the mixing and matching of components have been done for you. Usually such systems come fitted into a vertical housing called a "rack mount."

So many well-known names in audio components are doing this that it may become a major new trend -- spilling over from the traditional audio specialist outlets into department stores. Already, many audio shops are turning for business to TV and other burgeoning video interests: video cassettes, video discs, games, home computers, survey-monitor systems and so on.

For the all-out sound buff, the completely separate component approach probably will remain dominant. The most critical, and experimentally minded, stereo enthusiast will continue to be concerned over ultimate performance values such as the lowest possible distortion, the least amount of system noise, the highest-powered amplifiers and the most efficient speakers. This is especially true in light of the rising number and availability of so-called audiophile discs and cassettes whose expanded dynamic range makes greater power demands on a playback system.

But for "everyone else" the one-brand system -- as implemented by the well-known names in audio components -- becomes a very attractive alternative. Such a system is, after all, made up of actual separate components and is not in the same performance class as the old radio-phonograph "console," which was an amalgram of audio compromise, from pickup to speakers, and which could never supply full stereo if only because of the restricted spacing between its built-in speakers. Today's system consists of legitimate hi-fi components stacked neatly, with the stereo speakers separately housed and capable of being positioned anywhere in the room.

The firms producing a variety of components in different categories are the ones that have both the products and the marketing clout to foster the "systems" trend. Many of these systems are as "hi-fi" as one could wish, and a number of them include features never before thought suitable for the systems buyer. For instance, Sansui's top system uses a computerized direct-drive turntable, a 60-watts-per-channel DC servo amplifier, a digital-synthesizer tuner and a pair of three-way speaker systems. The power amplifier boasts an LED display and a spectrum analyzer. Hotly competing in this area are such bands as Panasonic, JVC, Pioneer, Akai, Kenwood, Technics, Hitachi, Sharp, Fisher, Yamaha, Sony, Philips, Mitsubishi, Toshiba, Onkyo. Teac is expected to join the trend, and B&O has introduced the only horizontally laid-out matched sytem, which looks like (and is, in great measure, controlled by) a small computer.

As a class, these one-brand systems are well-engineered, good-sounding and fairly priced. For many potential stereo buyers they may well represent a new kind of "rack" -- one without torture -- since questions need never come up of which component goes best with another.