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DVD


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A disk format similar to a compact disk (see CD-ROM) but containing much more data. It was introduced in 1996. DVD disks are the same 120 mm diameter as CDs with potential capacities of up to 4.7 gigabytes for a single-sided single-layer disk.

“DVD” was originally used as an abbreviation for digital video disk and subsequently for digital versatile disk. However, the DVD specification does not state that “DVD” is an abbreviation and treats it as the full name of the format.

The technology involved in DVD storage is similar to that in compact disks, but more precise. The extra capacity is achieved in a number of ways. The tracks on a DVD are closer and the pits are smaller, allowing more pits per unit area. The key to this was the use of a shorter wavelength laser (typically 635 or 650 nm in the red region for DVDs as opposed to 780 nm in the infrared for CDs). Moreover, a DVD can have two layers on the same side of the disk. The top layer is translucent and the bottom layer opaque. Data can be read from either layer by refocusing the laser. In addition DVDs may be double-sided. DVD formats also have a more efficient error-correction system. The potential capacity of a double-sided double-layer DVD is up to 17 gigabytes.

The early publicity and interest in the DVD format was as a video equivalent of the audio CD — i.e. a medium for distributing films that was cheaper than the VHS tape cassettes used by the film industry. There were delays in the introduction of DVD caused partly by disputes about format between large developers, and later by worries in the film industry about piracy.

Since then DVDs have been increasingly used in computing as a higher-capacity version of compact disks. As with compact disks, there are various types. DVD-ROM (DVD read-only memory) is similar to CD-ROM. DVD-R (DVD-recordable) is similar to CD-R. There are also different rewritable formats: DVD-RAM, DVD+RW, and DVD-RW. Unfortunately there are several mutually incompatible standards, each with its own claimed advantages. Potential problems are mitigated in practice by multiformat DVD drives that can read two or more of these formats.

See also Blu-ray, HD DVD.

http://www.dvdforum.org The DVD Forum home page

Subjects: Computing.


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