ust when you thought the familiar silvery compact disc was all you needed in terms of audio, along comes yet another incompatible recorded music for mat. 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. A small laser that helps erase and record discs, or illuminates 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. Dunng 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 COMING IN 1992 POCKET-SIZE MINI DISC By DENNIS NORMILE said that about two percent of the population, especially musicians 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 Disc system won't quite match that of CDs. Compression has another advantageer 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 a (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 ERASABLE DISCS REVISITED Disc machines designed to record hi-fi audio aren't new. The gi ant Dutch company N. V. Philips showed a prototype compact disc recorder in 1982. And at a European audio-video show two years ago, about 20 varieties of recordable CDs appeared. (Recordable disc formats include permanent recordings that can't be erased.) But the history of erasable-disc announcements and their availability in stores has been dismal. Among the reasons: Representatives of the music industry, fearing a loss of revenue from recordings made with highly accurate digital technology, have successfully blocked or delayed many new recorder entries with threats of copyright infringement lawsuits. But technology can be a problem too. Early in 1988, Tandy Corp. in Fort Worth, Texas, announced an erasable CD called THOR. While Tandy's disc venture is more than a year behind its on-sale schedule, a spokesman says the project is still under way, although he declines to estimate an on-sale date. Tandy isn't giving any details, but if early reports about THOR are accurate. Tandy's erasable disc is based on a rare technology, dye-polymer recording ["Coming: CD Recorder," July '88). In this technique, a record-play laser heats a polymer layer on the disc, causing tiny pits to form. During playback, the pits disperse laser light, producing a blinking pattern needed for digital audio. To erase a THOR disc, another laser heats and softens an adjacent polymer layer, which flattens the pits. Researchers must perfect this flattening stage, because inadequate smoothing of the pits limits how many times a disc can be erased. While the dye-polymer blend gives THOR discs a brilliant blue color, the vast majority of erasable discs have a muddy-brown hue from their thin coating of a magnetic iron-oxide-based powder. The technology for such magneto-optical discs is well established: For recording, laser heating and magnetism from a coil creates microscopic regions of different magnetic polarities. During playback, the magnetic polarity of one of these regions twists the optical polanty of light reflected from it, which identifies the onginal recorded pattem. Last year, the French company Thomson Consumer Electronics showed a laboratory prototype of a magneto-optical disc recorder Electronics Newsfront," Oct '90]. Some of Thomson's technology is strikingly similar to that used in Sony's Mini Disc. But because the two machines are designed for different functions-hi-fi recording at home versus Sony's take-along personal format-there are also major differences in the hardware. First, both recorders can play prerecorded and erasable discs. Thomson's machine handles ordinary compact discs. And because the Thomson recorder is designed for 4.7-inch CDs, it doesn't need the extreme five-to-one data compression Sony employs to squeeze a CD's 74 minutes of music onto its 2.5-inch discs. As a result, Thomson's recorder achieves the full range of fidelity possible with the 16-bit data resolution used for CDs. But to extend the recording time in its machine, Thomson includes a long-play mode based on four-to-one data compression. That compression reduces music fidelity, although Thomson, like Sony, claims few people can detect the missing music data. Few. of course, have had the opportunity. Both the Sony and Thomson machines record by sending data signals to a magnet above the disc while heating tiny points on Its ultra-thin recording layer from below the disc with a laser. The recording materials are also similar, a blend of the rare-earth terbium, cobalt, and iron compounds. Thomson has not been able to agree upon a disc standard with N. V. Philips, which originated both the compact disc and compact cassette. Philips is promoting its own digital compact cassette format [The Second Coming of the Digital Cassette," June). "But later this year," says a Philips spokeswoman, "we will offer a small compact disc recorder for the professional market. Philips hasn't priced the machine yet, but the spokeswoman speculated songwriters and musicians might pay more than $5,000 for a recorder based on the CD format. This machine, however, will probably use write-once disc technology, which permanently pits discs, rather than erasable disc technology. "Once a solution to the copyright problem has been reached," says the spokeswoman, "a compact disc recorder for the consumer market will be introduced."-John Free ice and having it bounce off at a different angle. An analysis of the light with detection circuits registers the magnetic polarities of the spots, reconstructing the recorded ones and zeros. Two technologies were especially important in the development of the portable, battery-powered magneto-optical recorder, says Sony. The first is the magnetic medium of terbium ferrite cobalt used on the erasable Mini Discs. Terbium is a rare-earth element, and ferrite is the iron oxide also used on magnetic tape. For data storage, this material can be magnetically switched with one-third the power needed for the conventional magneto-optical discs used by the computer industry. Second, Sony developed a high-efficiency magnetic recording coil and driving circuit that can reverse polarity within about 100 billionths of a second (see diagram on facing page). Here's why this combination now makes portable disc recording possible: Because the magnetic recording coil needs little power and the terbium ferrite cobalt can be magnetically reoriented with little power, a battery can supply the required energy. Moreover, the rapid reversal rate of the new magnetic head makes it possible to erase old data and record new information simultaneously in one disc rotation. As a result, the Mini Disc recording mechanism is simpler and more compact. This one-rotation erase-record sequence differs from most previous magneto-optical drives, which require a separate step to erase the disc before new data can be written. This separate erasing stage involves either a time-consuming rotation of the disc over the laser combined with magnetic signals to reorient the magnetic layer or separate lasers operating at the same time, one for erasing and one for recording. One final challenge in creating a disc machine that oper ates on the go: overcoming the skips and distortions that result from mistracking. A problem with existing portable CD players is that jarring them throws the optical pickup out of position. Rather than try to prevent mistracking, the |