ust when you thought the familiar silvery compact disc was all you needed in terms of audio, along comes yet another incompatible recorded music format. The latest format, Sony's take-along Mini Disc music system, combines features of CDs and Walkman-type portable cassette machines. Both the Mini Discs and another new format scheduled to appear next year, digital compact cassettes, bring the advantages and disadvantages of computer technology to music recording and playback. The growing variety of audio hardware promises a confusing battle for market domination. Miniaturization has been the key goal in designing the Mini Disc system. If Sony engineers succeed in cramming all the components into the mock-ups shown recently, you will have a choice of two exceptionally compact machines: a recorder about the size of today's portable cassette recorders or a tiny playback-only machine that fits into your shirt pocket with room to spare. In addition to extreme compactness, the machines give you one-second access to any music selection on the 2.5-inch discs, plus the advantages of digital audio technology compared with standard cassettes (see A Growing Menu of Incompatible Audio). The development of prerecorded and erasable Mini Discs involves the refinement of four technologies: •Digital-audio compression that uses five times less data than standard compact discs for 74 minutes of audio-with some loss of music fidelity. A technique for erasing and recording Mini Discs at the same time, using magnetism and laser heating. COMING IN 199-2 POCKET-SIZE "A small laser that helps erase and record discs, or illumi- PAYS MUSIC nates both prerecorded and erasable discs for playback. A memory feature that enables you to handle the machines roughly-even jog with them-without causing audible interruptions. If Sony markets its Mini Disc system next year as scheduled, it will be a first for most of these technologies in audio products. Except for the memory feature, however, similar technologies have already appeared in other prototype disc recorders not yet sold (see Erasable Discs Revisited). The new Mini Discs are mounted in plastic cases with metal shutters, much like 3.5-inch diskettes used in personal computers. This protects the discs and makes them easier to handle, an important advantage for a portable audio system. To achieve their goal of storing the same amount of music -74 minutes-on Mini Discs as conventional compact discs, Sony engineers had several options. "One possibility," said Katsuaki Tsurushima, "was to develop some completely new recording mechanism. But another option was to use digital technology to manipulate and compress electronic signals." Sony settled on a compression scheme that takes advantage of two particular limitations of human hearing: the threshold of hearing, referring to the decibel level below which humans can no longer detect sound vibrations; and the masking effect that occurs when loud and soft sounds with similar frequencies strike the ears simultaneously and the soft sound isn't recognized. During Mini Disc recording, the incoming analog signal is sampled and digitized much like it is in existing CD technology. But then the compression encoder analyzes the data and selects only those digital signals representing sounds the human ear is likely to hear. Address information, which helps the laser find its place on the disc when there's an interruption, and error correction data are added and the digital signals are recorded onto the disc. Sony's compression scheme squeezes the same amount of data into one-fifth the space of conventional digital recordings with only a slight loss in sound quality after it's decompressed, the company claims. Demonstrations of Mini Disc audio have so far been too restrictive to allow for comparisons with other audio media. However, one Sony engineer ON A MINI DISC By DENNIS NORMILE said that about two percent of the population, especially mu. sicians and audiophiles, might be able to hear the differences between full-range CD recordings and the uncompressed audio from Mini Discs. The Mini Disc system, though, is designed for listening anywhere-with headphones, in a boom box, or in a car audio system-where there's a potential for background noise. This format is not earmarked for audiophile hi-fi equipment you would savor in a quiet listening room. Sony executives adimit the sound quality of their Mini Dise system won't quite match that of CDs. Compression has another as intageer developing a special recording technology to store CD-quality on a 2.5-inch disc: Music publishers wil be able to use current CD-recording equipment to produce prerecorded Mini Discs, making it easier to put a variety of titles on store shelves. Although the same laser can play back music from both prerecorded and erasable Mini Discs, the record-playback technologies for the two discs are completely different. The new prerecorded discs use the same optical technology as present CDs in which pits are formed on a metallic disc surface at the factory. These pits disrupt a laser beam during playback, making its reflection strong or weak to correspond with digital ones and zeros, respectively. By contrast, the recordable discs use magneto-optical technology. "If you look closely, you can tell the difference," says Tsurushima, holding up both types of Mini Discs. From the back the two discs appear the same. Along one edge is a sliding metal shutter that gives the laser access to the disc from below. But while the front of the prerecorded disc is smooth, the recordable disc has another shutter. For magneto-optical recording, it's necessary to have (magnetic) head above the disc. Tsurushima explains. With the magneto-optical technology used for erasable Mini Discs, a laser briefly heats a microscopic spot on the disc's magnetic layer. The high temperature (about 400 degrees F) makes it easier to reorient the magnetic polarity at the spot with a magnetic recording head. After the spot cools, its polarity is difficult to change unless it is reheated. The magnetic polarity of the spots encircling the disc corresponds to the ones and zeros of digital music data. When magneto-optical recordings are played, the laser's power is reduced and its light is polarized and trained on the magnetized spots. When the polarized light interacts with the magnetic field of the spots, a phenomenon called the Kerr effect, the polarization plane of reflected light is twisted slightly. It's analogous to throwing a stick at ene angie onto a sheet of |