Science: CD players with split personalities: New technology that allows video material to be stored on standard compact discs is about to invade your sitting room, says Steve Homer

JUST WHEN you thought that the technology of the sitting room could not become any more complicated, your CD player is about to become a movie machine.

At the end of last month Philips, Matsushita and Sony, the world's three largest consumer electronics companies, announced that they had agreed a common format for storing video material on standard-size CDs. The potential market that this opens up is enormous.

Though it could seriously dent sales of pre-recorded videos in a relatively short time, the technology has drawbacks, not least that discs can only store 74 minutes of video. But it is far cheaper to press a CD than to duplicate a VHS cassette and a lot easier to transport and store a CD.

The sound quality is very close to CD (much better than VHS tape). For material such as music videos, instant access to your favourite tracks will be a real boon. The technology has pleasantly surprised its inventors, who have gone from describing the image quality as 'near VHS' to 'better than VHS'.

It will first be on show this October when Philips launches an upgrade cartridge for its CD-I players.

The basic technology used for storing video on CDs has been developed over several years. Each frame of video is encoded and broken up into blocks. Then each block is compared with the previous frame and only the differences are processed. More data is removed by using 'motion prediction'; by seeing, for example, that a ball is moving across a picture so only the basic description of the ball has to be stored and its detail moved about.

The third way of reducing the amount of data is seeing the commonality in a picture. One piece of blue sky is much like the next, so only a small amount of information needs to be stored to tell the decompressing technology how the shades of sky will vary. All this reduces the amount of data that needs to be stored by about 150 times. So instead of some 30 seconds' worth of video being stored on a CD, 74 minutes can be squeezed on.

For years, hundreds of people have been working on getting this system right and they have achieved near- miracles. The final result is better than they ever expected. Now standardised under the ungainly title of MPEG 1, this international format and its soon-to-be-finalised big brother, MPEG 2, have been accepted by everyone from broadcasters to telephone companies, satellite operators, video-recorder manufacturers and cable television companies.

The problem is that what has been agreed is the way of encoding information, which can be used for many things; no set format for CDs was actually prescribed.

Each CD has a table of contents at the beginning, so a CD player can see where different tracks are. When Philips and its partners dreamt up the idea of storing data on CDs instead of just music (the CD-ROM format), they defined ways of telling the player that this was not an audio CD and that the player should switch off. This was to protect audio systems as it was thought that data pushed through an amplifier might do serious damage to expensive speakers. In the late Eighties, Philips, the main inventor of the CD, wanted to take things further and came up with the idea for CD-I, CD-Interactive. These discs would store text, sound, pictures, animation and, eventually, full-motion video (FMV).

MPEG 1 is the FMV dream come to fruition. But before FMV was fully ready to be sent out into the demanding consumer market, JVC in Japan saw an opening for a CD karaoke machine. In discussions, Philips and JVC designed a simplified format. The system was launched in Japan in October and JVC did dollars 180m worth of business in the first six months.

A British company, Nimbus, had proposed a slightly different method of encoding the information on to the disc. This would have allowed ordinary CD players to play video discs. The idea was that any CD player that had an output socket for digital signals would read data from the disc and simply output it without any processing. This data could then have been transferred into a fairly simple MPEG 1 decoder box that could have fed straight into a television set.

But Philips says that only the most expensive CD players have digital output, and that while the data stream for audio CD need not be completely 'pure' - there is enough error- checking information embedded in the signal to ensure that small errors are inaudible - this does not hold good for compressed video, which pushes CD technology right to the limit. Nimbus, however, counters that its system works fine on all the machines it has been tried on.

This autumn, Philips will launch an add-on FMV cartridge for its CD-I player. The company has already signed a deal with Paramount to release 50 films on CD-I. These discs will be in the new format, so if another company makes a CD movie player based on the new standard, the film discs will play on those as well. Samsung of Korea has said it will launch a video CD player in the United States next spring and Commodore has announced a CD games machine that will support the new standard. Other manufacturers seem likely to launch CD movie machines over the next 12 months or so.

Philips will also release games, educational and entertainment products (including the Playboy Massage Disc) using full-motion video for CD-I.

However, this is putting the way the technology is likely to develop back to front. It is more likely that CD player manufacturers will start building players that take notice of the requirements of the new format. These machines will be 'video CD- ready' and all you will have to do is plug a converter box into a digital output at the back when the price drops low enough. Initially this converter may cost pounds 200 or more, but it could drop as low as pounds 50 within a couple of years. CD machines will also start having this technology built in and some may go all the way and implement CD-I.

In the meantime, some forward- thinking bands will release CDs that will contain maybe 10 minutes of music video. Conventional CD players will ignore this information, as will the new video CD-ready machines. Only when a converter is attached will the owner have the option of accessing this video information. As it costs no more to press a disc containing this video segment than one that just contains audio, the additional cost for a major band with large disc runs will be extremely limited.

Well-placed sources indicate that eventually the system might support new types of CDs that would be able to hold twice or perhaps four times the amount of data. In the long term, the four-hour video CD looks an interesting prospect.