History

In the early 1990s two high density optical storage standards were being developed: one was the MultiMedia Compact Disc (MMCD), backed by Philips (http://www.philips.com/) and Sony (http://www.sony.com/), and the other was the Super Density Disc (SD). IBM (http://www.ibm.com/)'s president, Lou Gerstner, acting as a matchmaker, led an effort to unite the two camps behind a single standard, anticipating a repeat of the costly format war between VHS and Betamax in the 1980s.

Philips and Sony abandoned their MMCD format and agreed upon Toshiba's SD format with two modifications that are both related to the servo tracking technology. The first one was the adoption of a pit geometry that allows "push-pull" tracking, a proprietary Philips/Sony technology. The second modification was the adoption of Philips' EFMPlus. EFMPlus, created by Kees Immink, who also designed EFM, is 6% less efficient than Toshiba's SD code, which resulted in a capacity of 4.7GB as opposed to SD's original 5GB. The great advantage of EFMPlus is its great resilience against disc damage such as scratches and fingerprints. The result was the DVD specification Version 1.5, announced in 1995 and finalized in September 1996. In May 1997, the DVD Consortium was replaced by the DVD Forum , which is open to all companies.

Technical information

DVD-R writing/reading side

DVDs are made from a 0.6 mm thick disc of polycarbonate plastic coated with a much thinner reflective layer of aluminium or gold. Two such discs are glued together to form a 1.2 mm disc that can be designed to be read from one side (single sided) or both sides (double sided). The substrates are half as thick as a CD to make it possible to use a lens with a higher numerical aperture and therefore use smaller pits and narrower tracks. Discs commonly come in 8 cm or 12 cm diameters.

A single-layer DVD can store 4.7 GB, or 4,707,000,000 Bytes, which is around seven times as much as a standard CD-ROM. By employing a red laser at 650 nm wavelength (compared to 780 nm for CD) and a numerical aperture of 0.6 (compared to 0.45 for CD), the read-out resolution is increased by a factor 1.65. This holds for two dimensions, so that the actual physical data density increases by a factor of 3.5. DVD uses a more efficient coding method in the physical layer. CD's error correction, CIRC, is replaced by a powerful Reed-Solomon product code, RS-PC; Eight-to-Fourteen Modulation (EFM) is replaced by a more efficient version, EFMPlus, which uses eight-to-sixteen modulation. There is no subcode as in CD. As a result, the DVD format is 47% more efficient with respect to CD-ROM, which uses a "third" error correction layer.

Application types

There are a variety of application types for DVD:

• DVD-Audio (containing high-definition sound)
• SACD (containing high-definition sound)
• DVD-Video (containing movies (video and sound))
• DVD-VR (containing recorded video and sound, usually from TV or camcorder)
• DVD+VR (a variation of DVD-Video used for recording on +R and +RW discs)
• DVD-ROM (computer data: operating systems, applications, encyclopedias, games (8 cm format).

External links

• DVD Forum (http://www.dvdforum.org)
• DVD+RW Alliance (http://www.dvdrw.com/)
• DVD Copy Control Association and the Content Scramble System (CSS) (http://www.dvdcca.org/)
• Understanding Recordable & Rewritable DVD (http://www.osta.org/technology/dvdqa/) by Hugh Bennett
• DVD recorder formats explained (http://www.dvdrecorderworld.com/formatlist.php)