Digital audio tape recorders use the same helical-scan recording principles as video recorders do. During playback or recording, the tape winds around a large drum at a slight angle. Inside the drum, one or more read/write heads spin horizontally; each head writes a stripe across the tape as it swings by. Because the tape is at an angle to the drum, the stripe runs diagonally across the tape.

However, unlike videotape, DAT information is organized into discrete sectors, much like a floppy disk. Some of the sectors hold synchronizing and control information, including the SCMS copy protection bits, but most of them hold digitally coded music.

Even though the general principle is the same as videotape, DAT is not particularly compatible with any other existing format. The width of the tape, the size of the case, the speed past the recording drum, the tape-to-drum angle and the sector layout combine to make DAT unique.

DCC is completely different. One of Philips’s goals in developing DCC was compatibility with existing tapes. That requirement dictated fixed-position read/write heads. Cramming in all the bits needed for CD-quality sound, while keeping the same size tape and running it at the same speed as an analog cassette, is possible by virtue of two innovations.

First, digital information is recorded in nine tiny subtracks (eight for music and one for control) side by side down the length of the tape. DCC uses integrated thin-film heads, a semiconductor technology adapted from hard disks. Both the electronic and magnetic components are etched onto a silicon wafer in much the same way that computer chips are made. There are actually two complete sets of heads, one for analog playback (but not for recording) and another for digital recording and playback, integrated onto the chip.

Second, the data is compressed and decompressed on the fly. Philips calls its scheme Precision Adaptive Subband Coding and claims that it offers a 4:1 compression ratio. This compression method is fine-tuned to the characteristics of the human ear; although the actual data rate coming off the tape is much less than a CD provides, Philips says the perceived quality of sound is every bit as good.

Will the Computer Industry Buy In?
Thinking about the Mini Disc, we can’t help wondering whether the computer industry will adapt it for laptop and notebook computers the way CD-ROM was adapted from the audio CD format. As with CD-ROM, the computer industry will probably wait to see if MD is a success in the consumer market, which allows low prices in the computer industry.

On the plus side:
• MD holds about 100 megabytes. For some applications, that’s plenty; lots of today’s CD-ROMs do not use anywhere near the 550 MB that is available on a 5″ disc.
• Mini Disc could be a suitable substitute for the 44-MB removable Winchester disks that make portable many of today’s multimedia presentations.
• Existing CD-ROM mastering and stamping systems (which are derived from audio CD) could easily be adapted.
• A disc weighs about half an ounce; drives would probably weigh less than today’s 3.5″ floppy drives.
• Data transfer rate is similar to a CD-ROM’s; with suitable compression devices, even motion video could be played from the disc.

On the minus side:
• Slow access times, comparable to a CD, mean the Mini Disc will be unsatisfying as the computer’s primary storage device.
• IBM is aggressively marketing a new, computer-ready 3.5″ magneto-optical disc drive to computer makers and retailers. (See news story, page 12.) Sony will have its own entry in this arena.
• A Mini Disc can hold 100 or so MB, which means that very little video, and not all that many stills, could fit on a disc.

Will the computer industry buy in? For one view, see the letter on page 2.