Mini Disc system compensates with a buffer memory.

To demonstrate this memory feature, Tsurushima picks up a laboratory version of a Mini Disc player and shakes the machine as it plays music. Instead of the rasping sound you would expect as a laser is thrown off track, the music continues uninterrupted. Next he pries open the lid of the player, pulls the disc out, holds it up briefly, and then pops the disc back into the machine. Remarkably, not a beat is missed. Once again he pulls out the disc, but this time he waits several seconds. Finally the music stops. "That was just to show you the sound wasn't coming from somewhere else," he says.

The trick to this playback tenacity is that the optical pickup reads data from discs more than four times faster than is necessary for real-time playback. Data read from the disc flows into a one-megabit buffer memory at the rate of 1.4 megabits per second. But the decoder circuits converting this data into sound only need a 0.3-megabit-per-second flow of data. This enables the onemegabit buffer memory to hold three seconds of music information (see drawing on page 66).

If the optical pickup is jarred out of position, the flow of correct data from the memory to the digital-analog converter continues as long as the pickup resumes proper reading within three seconds. When mistracking occurs in an Mini Disc player-as in conventional CD machines-counting circuits detect the abrupt change in address information recorded periodically with the music data. The laser pickup then quickly repositions itself using the address information registered just before the interruption.

A

FORMAT

MINI DISC

COMPACT DISC

COMPACT CASSETTE

DIGITAL COMPACT CASSETTE

DIGITAL AUDIO TAPE

Ithough the Mini Disc technology has been established, the equipment is still being refined. Sony is also negotiating with music companies and other equipment manufacturers to broaden the use of the new format. One concession to the interests of music companies and recording artists is that Mini Disc machines will include a serial copy management system. This digital encoding scheme, also included on the newest digital tape formats, allows you to make one recording of prerecorded material, but blocks the recording of additional copies. Sony has not disclosed the Mini Disc player or recorder prices. But just as the CD has driven LPs to near extinction, the new 2.5-inch discs may eventually replace standard compact cassettes. Sales of prerecorded cassettes have been declining in industrialized countries, according to data from the International Federation of the Phonographic Industry. Sony's researchers claim that listeners are dissatisfied with the sound quality of cassettes after growing accustomed to CDs. The de

LP RECORD

Beacham Exhibit #8

IT'S BEEN LESS THAN A DECADE sense the compact disc was introduced. In that short time, the CD has brought high-quality audio reproduction to the masses. and taught us to appreciate good sound. We're not exaggerating when we say that the CD has changed the way we listen to music.

It's rare for a new technology and format to catch on so quickly-especially one that threatens to make its predecessors obsolete. CD was a success not only because of consumer acceptance. but because it also offered something to manufacturers, recording companies, and retailers.

It wasn't the CD's "gee whiz" appeal-nor was it the promise of perfect audio reproduction-that caused sales to catch fire. It was convenience. When compared to the LP that it replaced. CD's were a dramatic breakthrough. They can store more audio in a package a fraction of the size. They can be lent to even your most careless friends without getting scratched. They even play back more conveniently, because you can skip tracks that you don't want to listen to, or re-arrange the order in which the songs play back.

It's convenience. also, that makes the venerable compact cassette our music medium of choice. (Cassettes outsell CD's by a ratio of about 1.5:1.) They fit in your shirt pocket, and they stand up reasonably well to abuse. They're ideal for use in a car or in a personal stereo because they're relatively immune to shocks. So what if they can't come close to the audio quality of a CD or even an LP?

How about DAT?

In the belief that consumers had fallen so much in love with the idea of digital audio because of their exposure to CD. Japanese manufacturers reasoned that Digital Audio Tape (DAT) would be to the CD what the compact cassette was to the LP. Unfortunately, it didn't work out that way for a number of reasons. First, the record Industry, spearheaded by the RIAA (Recording Industry Association of America), threatened lawsuits against any Japanese manufacturer who exported the DAT ma

FORMAT

AUDIO

BRIAN C. FENTON

D4

CONFUSION

Two new digital audio formats-Sony's Mini Disc and Philips' Digital Compact Cassette-promise to battle each other as they create consumer confusion.

chines to the U.S. The RIAA was concerned about DAT's potential to make virtually perfect copies of CD's. (They seemingly missed the fact that, for most people, cassettes do the same thing. And despite that, pre-recorded cassettes have outsold both LP's and CD's combined since 1982! They've outsold blank tapes as well.) The threats of lawsuits were enough to stop DAT dead in its tracks, despite considerable accolades for the format in the audio and

general press.

Although some DAT machines were available on the "gray market" of unofficially imported goods. DAT officially arrived in the U.S. market last year-with generally disappointing results. Whether it was the years of delay. the taint of the lawsuits, the expense of the machines, or the lack of pre-recorded software that have killed DAT in the consumer market, we'll never know for sure. Perhaps DAT failed because

enter into the field of digital recording." Tandy Corporation announced that they would be the first U.S. licensee of Philips' technology, and wouid Introduce a home recording deck in late 1992.

The most important feature of DCC is that it doesn't make the familiar cassette obsolete. All DCC players will play back existing analog cassettes. so even when you make the jump to DCC. you can still listen to your existing library of tapes. (You won't. however, be able to record analog cassettes on your DCC machine. or play DCC tapes on your standard cassette deck.) That "backward compatibility" could convince some consumers to upgrade to DCC even though they like what they already have. After all. an upgrade won't just give them better sound, but as well see, more convenience as well.

A DCC deck is essentially a standard cassette recorder that includes some extra digital electronics and a new head design. The dimensions of a DCC cassette are essentially the same as that of a standard cassette, but the digital cassette's sides are flat-the case doesn't get fatter where the head enters the shell. Also, since the DCC standard demands that all DCC players feature auto-reverse, there's never a need to flip the tape over, so you don't need to have holes for the reels on both sides of the cassette. That means that one full side of the cassette can be used for information and graphicssomething the recording compa

nies love.

The spool holes and the tape surface are protected against dust and fingers by a sliding metal cover, which also locks the tape hubs. There's no need for an carrying case, so the digital cassette is easier to use and store, especially in a car.

The key to maintaining compatibility with standard cassettes is a new thin-film semiconductor head, manufactured using a process similar to that used for integrated circuits. The first layer of the head contains one set of 9 magneto-resistive heads for digital playback, and a pair of similar heads for analog playback. On the second head layer is one set of 9 Integrated

PHILIPS' PASC ENCODING Ignores sounds that are below the hearing threshold (a). Of the signals shown in b, only A would be recorded because 8, below the hearing threshold, would not be heard. The hearing threshold, however, varies dynamically depending on what other signals are present. In c, signal B has altered the threshold, making A inaudible.

recording heads for digital recording. We'll see shortly why 9 digital heads are required.

PASC makes it work

The key to the DCC system is the a new digital coding technique called PASC. or precision adaptive sub-band coding. The goal of PASC is to produce a signal equivalent to that of a CD. The results? A dynamic range better than 105 dB. and a total harmonic distortion. including noise, of less than 0.0025%

PASC is based on two important phychoacoustic principles. The first is that we can hear sounds only if they're above a certain level, called the hearing threshold. The second is that loud signals mask soft ones by raising the hearing threshold.

The hearing threshold. as you might expect. varies from person to person. Even a very sensitive ear. however, won't be able to hear a sound if it is masked by a louder sound. (You couldn't. for example. hear an unamplified vtolin at a rock 'n' roll concert!) The theory behind PASC's efficiency can be expressed by the question. "If you can't hear it. why record it?"

During encoding, the PASC processor analyzes the audio sig. nal by splitting it into 32 subband signals. By continuously taking into account the dynamic variations of the hearing threshold, the PASC processor encodes only the sounds that will be audible to the human ear. Each subband is allocated the number of bits that are required to accurately encode the sound within it. If a subband doesn't require .ny bits-because it contains sounds that are masked. for exampleits bits are re-allocated to other subbands so that the sounds within them can be encoded more accurately. On average. the PASC system needs to encode only one quarter the number of bits that a CD or DAT encoder would to reproduce a given audio signal.

The encoded data is multiplexed into an 8-channel data stream. and error-detection and. correction codes are added. The eight channels are recorded on S parallel tracks on the DCC tape. The ninth track can be used to carry auxiliary data. such as song titles. recording times, and the like). The auxiliary track could be used to generate hundreds of characters of text per

PROTECTIVE LAYER THE MINI DISC is composed of 4 layers.

second, so decks could include readouts for song lyrics or other Information about the selection.

DCC. an elegant extension of the most popular music carrier we have. seemed to be a sure-fire hit. It had something for everyone, including hardware man. ufacturers. record companies. retailers, and consumers. It now appears. however, to have run up against a formidable competitor: Sony's Mini Disc.

Sony's Mini Disc

In May of this year. in what seemed to be a deliberate attempt to derail DCC before it got moving. Sony announced a brand new recordable audio format, the Mini Disc or MD. Sony, however. denied that their MD was meant to compete with DCC. In response to the question of what MD replaces. the President of Sony Corporation of America answered "We are replacing nothing. We are Creating new markets."

The Mini Disc format is specifically designed for portable applications (personal stereos. boom boxes, etc.) and is slated for introduction. conveniently, in late 1992-the same time that DCC decks are due. The disc. about 21⁄2 inches in diameter. looks-and acts-like a cross between a compact disc and a micro noppy computer disk. Like a compact disc. the Mini Disc is an optical medium-it is read by a laser and can store up to 74 minutes of digital audio. Like a floppy disk, the mini disc can be mag. netically recorded again and

again.

How did they manage to get the same capacity as a CD on a disc that has about the surface area? Interestingly, by treating audio in much the same way as DCC does. Sony's encoding scheme, which is called ATRAC. or adaptive transform acoustic coding, is also based on the psychoacoustic principles regarding the threshold of hearing and the masking effect.

Because the ATRAC encoder 1gnores sounds that fall below the threshold of hearing (which varles dynamically because of signal masking) it can encode data five times more efficiently than CD or DAT systems. That's even better than DCC's 4:1 advantage!

Can a recording that "leaves out 80% of the bits" sound as good as a CD? In theory, if all you're leaving out is things you can't hear. then yes. In practice. we don't know yet. At Sony's announcement, they demonstrated a prototype by playing some pop rock for a half minute or so. It sounded OK, we guess. considering that the listening environment was a crowded hotel meeting room. No A/B comparisons were provided between CD and MD. Sony claims that "only 2% of the population will be able to hear the difference."

The Mini Disc is constructed of four layers. Including a newly developed magnetic layer of terbium ferrite cobalt. Since magneto-optical discs can't come in contact with the recording heads. It's important that the magnetic material be able to

change polarity when subject to a very small magnetic field. The new material fills the bill.

The Mini Disc requires both a laser and a magnetic head for recording. When the magnetic layer is heated by the laser (to a temperature of about 400°F). it loses its coercive force—that is, it becomes very easy to magnetize. The head then supplies a magnetic field to set the material's magnetic polarity. When the heated spot cools, the new polarity is "locked in" and, thus, the digital data are recorded.

Sony's Mini Disc has a couple of advantages over other optical recording methods. The structure of the head is much simpler because the laser can be on continuously during recording and playback. And the low-coercivity of the magnetic material greatly reduces the power required. making portable operation feastble.

One feature of Mini Disc touted by Sony is that the portable Walkman players will have "shockproof memory." One of the problems with current portable CD players is that they don't work too well unless they're standing still. Any sharp jarring causes the laser to mistrack. Mini Disc players shouldn't suffer from that problem because data is read off the disc at a rate far faster than required by the ATRAC decoder, creating a data buffer of

nological reason why portable CD players couldn't offer their own shock-proof memory buffer. But since the buffer would have to be 5 times the size, it would add greatly to the cost.

Who wins?

Ever since we forecast that DAT would be a sure-fire success. we've been reluctant to make predictions. But let's look at some of the issues involved, and how DCC and MD stack up.

For consumers-assuming that both formats offer highquality audio-DCC has the decided advantage in that existing libraries of cassettes won't be obsolete. Both formats have the potential to supply such convenience features as song title and lyric readouts, but MD offers much faster random access of tracks Although it's too early to say for sure. prices for home DCC decks should be under $500 when introduced, while a porta. ble MD player is expected to cost around $400. For consumers, we give DCC a slight edge.

The recording companies will have a hard time taking sides. Both technologies will use the serial copy management system or SCMS, an anti-piracy system. Manufacturers will be able to duplicate DCC at 64 times normal speed on equipment similar to what is now used for standard cassettes. Min! Disc players will be able to play back not only magneto-optical discs. but pre-recorded optical discs as welldiscs manufactured using the same process as is used for CD's. Various recording companies have expressed support for each format. Which way will the record companies go? For us, it's too close to call.

Hardware manufacturers should prefer DCC because standard tape transports can be used. Retailers, always reluctant to have to stock the same titles in

various formats, are dreading the thought of re-vamping their stores to accommodate either DCC or MD.

What about you? In the long run-since both formats seem destined to compete with each other for your money-It's you who will decide whether DCC or MD is the personal recording format of the 90's and beyond. A-E