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The time has come to continue my series of articles on Compact Disc technology. The first five appeared in the MAC Audio News magazine from November 1987 to May 1988. They described general principles, the disc, data format, sub-code and Laser tracking. This article will present some of the latest news on the CD revolution (pun intended), and the next few will delve into the mystic realm of Digital-to-Analog Conversion, over-sampling and filtering; which is a much bandied-about topic recently, the air needs to be cleared.
1988 was the first year that CDs outsold LPs in dollar terms in the USA, although compact cassettes are still the biggest seller by a long way. Deutche Grammophon announced that its latest recordings will be only available on CD and cassette. Nimbus, the English CD manufacturer, gave up making vinyl discs over three years ago. Over 400 million audio CDs were made during 1988, earning Philips 8 million dollars in inventors royalties. There are about 50 plants around the world producing audio CDs, over 20 of them are in Europe.
This year will see the further emergence of optical disc technology for cheap computer mass storage in both WORM (Write Once, Read Many) and erasable-recordable formats. CD-ROM (Read Only Memory) is already making a big commercial impact, being ideal for the retrieval of masses of information. A CD holds about 650 million characters, or approximately 250 thousand A4 typewritten pages. The first major influence on the public I see for CD-ROM is in libraries, where they will replace large cumbersome reference works, with fast contextual searches able to be done by small computers. Graphics and sound can be incorporated in entries with text. Soon cheap CD-ROM drives will be common place for PCs (Personal Computers), probably with erasable discs. Mastering a CD now costs about $3000 to $5000, a short run of 1000 discs costs about $1000.
For all the Video enthusiasts, Philips launched CDV (Compact Disc Video) in October last year (1988). The standard sized CDs hold about six minutes of stereo sound and vision. A further 20 minutes of sound only can also be included, which can be played on a conventional CD player. Although, the CDV system from GE and RCA can store up to an hour or sound and vision using a sophisticated compression technique, picture deterioration occurs in fast moving scenes. Polygram predicts an world wide demand for these of 350 million by 1992, which I find hard to believe from its low key exposure in the last year.
Tandy announced that it was developing an erasable audio compact disc, called Thor-CD, but it has still yet to see the light of day. A rival Japanese system from Taiyo Yuden is also being developed, but unlike Thor, it uses a standard Polycarbonate/Aluminium disc. A guiding spiral track is pre- recorded on the disc, and it can only be recorded once. An American system called Inspire from Alphatronics Inc. has a capacity of 600 million characters on a 5 1/4 inch read/write optical disc for PCs, combining portability of a floppy disc, the speed of a Winchester (a hard disc drive; not the gun) and the safety of a CD. The drives and discs are still expensive, but with increased production, prices will rapidly fally to as low as $25 for a disc. It complies with international standards (ISO, ECMA, ANSI and MITI 23); Sony and 3M are already producing discs.
The system uses tracks of minute magnetic domains about 1 micrometre across that are permanently imbedded in the disc. They may be polarised vertically up or down, and are unaffected by strong stray magnetic fields, a huge advantage over convetional magnetic media. To read a domain, a Laser beam is focussed at low power and the light polarisation of the reflected beam indicates the magnetic polarisation of the domain. To write a domain, the laser beam is focussed at higher power for a few nanoseconds, heating the domain significantly. The magnetic coercive force required to re-magnetise the domain becomes very low, and a small coil under the disc readily sets its new magnetic state.
Current developments indicate that optical disc capacities may increase 10 fold and data transmission rates 100 fold in the next few years, making an ultimate digital audio system seem more possible, with say an 8 cm disc giving 70 minutes of 22 bit stereo sampling at 100 thousand samples per second, giving a flat frequency response to over 40 KHz and more than 120 db S/N ratio. Heres hoping.
Originally published in MAC Audio News No. 178, November 1989, pp 19-21.