enter into the field of digital recording." Tandy Corporation announced that they would be the first U.S. licensee of Philips technology, and would 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 exist. Ing analog cassettes, so even when you make the jump to DCC. you can still listen to your exist. ing 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 companies 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 B. 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 sig. nal equivalent to that of a CĎ. 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 v1. olin 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-ailocated 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 Icould be used to generate hundreds of characters of text per

PROTECTIVE LAVER 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 specifi. cally 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 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 psy choacoustic principles regarding the threshold of hearing and the masking effect.

Because the ATRAC encoder ignores sounds that fail below the threshold of hearing (which var les 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 environ. ment was a crowded hotel meeting room. No A/B com. parisons 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 ter blum 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 feasible.

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. Mini 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. R-E

By DAVID E. SANGER

Special to The New York Times

TOKYO, June 18-When the com

pact disk emerged from the labora tory as a consumer product in the mid-1980's, recording companies

hated it. It would confuse consumers

and ruin the recording business, they said. Today, records are indeed near extinction, but the recording business has doubled since CD's, with their scratch-free, hiss-free digital clarity, went on sale eight years ago.

Now the battle is about to be fought again-this time over compact disks that record. The industry is choosing sides over a new technology called MD, for mini disk, a variant of the compact disk that the Sony Corpora tion is betting will make its own Walk man obsolete.

The MD, or mini

disk, can

record as

well as play..

Only two and a half inches in diameter, about the size of soda can tops, the disk is not only made for portables but is also "rewritable," meaning that data stored on it, whether music or digits, can be changed. With that innovation, the one great advantage of tapes over compact disks is about to be wiped away. Sony is not saying yet, but when production of the player-record

ers begins next year, they are expected to cost about $400.

For a decade, the CD that can record has been one of the Holy Grails of the electronics industry, and Sony is hardly the only entrant. foshiba, Philips N.V. of the Netherlands, 1.B.M., and many others have been building prototypes, and there are already some specialty systems on the market as disk drives for computers -taking advantage of the huge stor age capacity of what the industry calls "optical disks."

But Sony is attempting a classic Japanese strategy: It is quickly foreIng new, cutting-edge technolgy into a relatively inexpensive consumer. product in hopes that big manufactur ing volume will cut production costs

A variant of the new mini disk, with

its small size, would have obvious ap

plications to laptop computing.

For now, Sony says its only im

mediate interest is the audio market

"To expand the market for the compact disk, we needed a much smaller disk that could be used outdoors,"

said Terusaki Aoki, who heads Sony's tape and disk products division and until recently ran its research and

development programs. And, of

course, we needed recording capability."

So far, small size and recording capability have been available only with floppy disks and audio and video tape. These rely on thin layers of particles that are magnetically read or altered to play or record. In compact disk technology, lasers pick up reflected light from a disk's finely pitted surface, and these optical signals are converted it into a stream of digital 0's and 1's. The compact disks can store far more information.

Now, the race between magnetic and optical technologies is on. The first problem for the optical researchers was to shrink the disks,

The one great

advantage of tapes over CD's is about to be

wiped away.

and players, to no more than the size of cassette tapes and Walkmans. Or dinarily, a mini disk the size of the one Sony developed would store far less data than a standard-size, fiveInch compact disk, which can play about 74 minutes of music. But Sony's new compression technology can jam the same amount of music into a fifu. the space, partly by cutting out frequencies that cannot be detected by the human ear. The price: audio qual ity that is a bit lower than on ordinary compact disks.

In the future, similar technology may be used to compress the data.

The days of the traditional laboratory notebook may be almost over. As scientists and engineers do more and more of their work on computers, the task of keeping data in a handwritten notebook has become cumbersome and impractical. How can a scientist enter a complex, three-dimensional color model into a notebook?

Researchers at the Baylor College of Medicine in Houston have come up with an electronic alternative, the Virtual Notebook System, or VNS, a software package that turns a computer work station into a multimedia lab notebook that can accept not only text but also sound, electronic mail, photographs and still video images. The software can also receive faxes, allowing data from them to be incorporated

into the lab notes.

More Important, VNS easily ties into a computer network, which makes the lab notebook mobile. A scientist who is traveling can call up the notebook on any work station, regardless of brand. It also allows scientists to share their notebooks with selected colleagues anywhere in the world using any type of computer running the popular X Windows operating system that I.B.M., Apple, Digital Equipment and others use to control their computers' basic functions.

The Virtual Notebook System borrows a key concept from airline reservations systems: a change made by one user is seen immediately by all According to Kevin Long, a Baylor reseacher and one of the developers of VNS, program users can amend the notes in Texas and colleagues running the program in California, New York or Hong Kong will

-intely can then plannen their own erronne,