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Optical Disks - The Next Generation?

DVD - Digital Video Disc or Digital Versatile Disk - has generated a lot of media interest over the last year or so, but what exactly is it?


Although it looks like a standard CD-ROM, DVD has the potential to hold much larger amounts of data - up to 17GB, compared with 680M on a CD-ROM. Like a CD the data is recorded on the disc as a series of tiny pits, and read back using a laser. However, the amount of data held has been increased in three ways:

  • a different wavelength laser is used to read the discs, 650 or 635nm compared to 780nm for a standard CD-ROM. The shorter wavelength allows the use of smaller pits and tracks that are closer together.
  • dual substrate - rather than reading a single layer, the new disc can have two bonded layers. The laser will shine through the first semi-transmissive layer, reading the data from the second, deeper layer, then will automatically switch focus to read the top layer.
  • double sided. Future DVD discs will be double sided.

The amount of data a DVD holds therefore depends on whether it is double or single sided, and double or single layer. A single layer, single sided DVD will hold 4.7GB, large enough to hold most Hollywood movies, 135 minutes of high resolution MPEG-2 video with up to eight channels of audio, providing much better audio than most current televisions can make use of. The double layer, single sided DVD will hold around 8.5GB. A double sided, double layer DVD will hold 17GB, and the first such disc was recently displayed by Imation (formerly 3M) at the DVD Forum in Brussels last September.

In addition to the increased storage DVD will also transfer data a much higher rate than the standard CD-ROM, 26.16mbit/s standard channel rate compared with 4.32 mbit/s.

The DVD Consortium

DVD came into being as a merger between two rival formats, MMCD (MultiMedia Compact Disc), backed by Sony and Philips and SD (Super Density) backed by Toshiba, Time Warner and others. The new combined format was announced in September 1995 by the DVD Consortium, which now also includes Hitachi, JVC, Matsushita, Pioneer and Thomson. The Consortium aims to prevent the problems with multiple standards which beset the video tape industry (VHS versus Betamax), by producing a standard and licencing the technology to other firms. It should be noted that DVD is not a single standard. Obviously there are different encodings for audio, video and data, and discs for use in the US will use Dolby AC-3 audio encoding, all others the MPEG-2 audio encoding. Additionally discs may also contain 'regional codes', which will prevent discs produced in one region (e.g. Europe or the USA) being played on a player from another region. This is mainly to prevent movies being released on DVD in a country before they are released at the cinemas.

The Consortium will also address compatibility issues, setting up neutral labs to test compatibility between different player brands and different discs. In Europe this testing will be organised by Phillips.

The biggest problem facing the Consortium has been finding a suitable copyright protection method. While copyright is an issue for computer software producers, it is much more of an issue for movie makers, causing some to believe that they way forward is for two different, incompatible drives, one supporting an anti-copy mechanism for movies, and another for DVD-ROM. On the 29th October 1996 the Copyright Protection Technical Working Group issued a number of copy protection principles. The movie industry had been pressing for a more comprehensive copy protection system, but bowed to pressure from the computer industry who felt such a system would place too high a burden on CPU power. The system proposed by IBM and Intel will encrypt only some of the frames of a movie. This will not scramble the picture entirely, but will reduce the quality sufficiently to make the movie unwatchable if it is illegally copied. A more sophisticated copyright system may be implemented at some point in the future. DVD-ROM players will not have to support the copyright protection scheme, though it is likely that most will in order to advertise 'full DVD compatibility', and since initially most titles will be DVD-Movies.


As with CDS, where there are CD-Audio and CD-ROM, two formats of DVD will be available, DVD-ROM for data and DVD-Video for video. The DVD-ROM drives will be able to deliver video and audio much as CD-ROM players can, but currently for full screen, full motion video additional hardware will be required. DVD-Video will be the first format released with popular movie titles likely to lead the way. Potentially, because of its digital nature allowing immediate search/rewind, and very large storage capacity, interactivity could be added, for example putting several camera angles on the disc and allowing the viewer to switch between them. Video will be stored in MPEG-2 format, and audio as Dolby AC-3 audio encoding in the US and MPEG-2 audio encoding elsewhere, providing up to 8 different sound tracks (eight for MPEG-2, six for AC-3) and 32 closed caption (subtitle) tracks. This will allow, for example, different languages to be included on a single disk. An audio only format will also be available, but has yet to finalised.

To maintain backwards compatibility and ensure rapid uptake of DVD, all DVD players will read audio CDs and DVD-ROM drives will read existing CD-ROMs, except for current recordable CD-Rs. Currently Kodak Photo-CDs are not supported, but it is likely that future DVD-ROM players would support them.

What is available?

At the time of writing DVD discs and players are not yet available in the UK. DVD-Video players have been available in Japan since November 1996, and about 65 titles are expected by the end of the year. DVD-Video players are expected to be released in the US in the first quarter of 1997, selling for around $500, and Europe in the second quarter. DVD-ROM players should follow shortly, with Toshiba being the first manufacturers to announce delivery. Phillips estimate that 25 million DVD drives will be sold in the year 2000. Costs, which may be fairly high initially, are expected to fall rapidly to near CD-ROM prices as more drives become available and the cost of MPEG-2 decoders falls. Since DVD drives will be able to read CD-ROMs, CD-ROMs are likely to be replaced over the next few years.

Long-term DVD manufacturers are looking to replace video tape, and DVD offers a number of advantages, such as instant search and rewind and better quality audio and video than standard tape. Video tape however is cheap, readily available and recordable, and certainly until recordable DVD is available video tapes market is unlikely to be threatened. This is not likely to be until after 2000. Currently the erasable format looks like it will be 2.6GB, and the write-once version 4.7GB. The write-once DVD-ROM players are likely be available sometime before erasable DVD-Video.

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