Dutch company's double-duty digital tape recorder shrouded in mystery

December 23, 1990|By Andrew Pollack | Andrew Pollack,New York Times News Service

Philips N.V., a large Dutch electronics company, created a stir in October when it announced that it was developing a digital tape recorder that also will be able to play conventional analog cassettes.

The new system, called digital compact cassette, could pose a strong challenge to the digital audio tape recorders that went on sale this year. DAT machines cannot play the analog cassettes that consumers have bought by the millions for use as portable stereos and in home and automobile sound systems.

But after making its announcement, Philips -- which is developing the new system with the Tandy Corp. -- has gone mum, shrouding in mystery how its machine works and sounds. Those details are crucial to determining whether the new machines, which are expected to reach the market in 1992, will be successful.

Still, information about the design of the system can be pieced together from interviews with industry experts, from Philips' own disclosures to international standard-setting bodies, and from an interview a Philips executive gave in October to Audio Week, a New York industry newsletter.

The key is the use of mathematical data-compression technology, combined with advanced recording heads and sturdier cassette construction.

The gist of the new technology is this: Analog cassette systems store the music on linear tracks that run along the length of the tape. The tape moves past a stationary head, which writes information onto the tape or reads information off it.

The DAT players, by contrast, use a system, borrowed from videocassette recorders, in which both the tape and the heads move and in which data are written on the tape in a helical pattern, like the diagonal stripes on a candy cane. Thus, they are totally incompatible with analog cassettes.

For the new system, Philips has developed a digital machine that uses a stationary head and linear tracks on a tape the same size as an analog cassette.

Such a machine will also be able to play analog tapes. However, digital tapes made on the Philips machine could not be played on analog-only machines.

Analog systems, like records and cassette tapes, store music as an electromagnetic wave form that mimics the sound wave. Digital systems, such as DAT and compact discs, store music as a series of numbers that are written in the zeros and ones of computer code.

Digital music can be much crisper, free of the hisses and pops that mar analog recordings. The drawback is that it requires a lot more tape to store music digitally.

In a DAT machine, for instance, the music is recorded by sampling it 48,000 times each second. Each of those samples is represented by a number that is written as a 16-digit string of zeros and ones.

There are two such signals, one for each stereo channel, meaning that storing a single second of music requires 1.5 million zeros and ones.

On top of that, extra bits are added to allow the system to mathematically correct errors and to store information that helps the machine automatically find a particular song on the tape.

A single second of music on a digital audio tape requires 2.8 million zeros and ones, or 2.8 megabits. The most powerful floppy disks now used in personal computers would be able

to hold only four seconds of such music.

Japanese consumer electronics companies that developed digital audio tape several years ago contemplated how they could pack that much information onto a tape.

It would be best, they decided, if the tape moved slowly. Otherwise, an extremely long tape would be needed. But the tape would also have to move fairly quickly past the head if the head was going to be able to read or write 2.8 million bits each second.

Fortunately, the Japanese companies, in developing videocassette recorders, had already devised a way of making a tape that moves slowly yet can be read quickly and carries a lot of data. That solution was adopted for digital audio tape. In this system, known as helical scan, the tape moves slowly past a drum that contains two heads. The drum rotates rapidly in a direction at an angle to the moving tape; data are laid down in adjacent diagonal stripes on the tape.

Philips uses mathematical techniques known as data compression to reduce the amount of zeros and ones needed to store digital music enough to be able to use linear

tracks rather than helical scans.

Though Philips has not said much about the compression technique it will use in the digital compact cassette system, it is clearly similar to one the company has been promoting for use as a worldwide standard for digital radio broadcasts.

The system takes advantage of the fact that certain sounds cannot be heard because they are too close in frequency to other, louder sounds -- a phenomenon known as psycho-acoustic masking.

By eliminating data corresponding to such inaudible sounds, Philips can reduce the number of digital bits needed to represent the music by a factor of four.

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