DVD [Digital Video Disc]


(n.) Term that loosely refers to numerous configurations that define the physical structure and the data contained on a 12 cm disc. A DVD in any of several formats contains MPEG-2 video with either AC-3 (Dolby) audio or MPEG-2 audio (common in Europe). The term Digital Versatile Disc was originally used to indicate that the medium may contain any type of data, not just audio and video files in a proprietary format. DVD-ROM is the term most often used to describe a disc formatted with a larger capacity than a CD-ROM. A DVD-ROM does not necessarily contain video data, as the words would imply.

DVD generally refers to an optical disc with multiple data layers that exceeds the capacity of a standard CD-ROM and that may contain video, audio, or any other type of digital data. In the areas of home entertainment and computer data storage, it has the capacity to replace all existing audio tape, videotape, CD-ROM, and video game formats.

The storage capacity of a DVD is greater than that of a CD-ROM because it has a laser beam with a shorter wavelength, smaller pits on the disc surface, a denser track pitch, and more efficient channel encoding. A visible red laser with a wavelength of 635 to 650 nanometers was chosen for DVD when the specification was unified from competing technologies developed by Toshiba, Sony, and Philips. The DVD specifies a laser beam of smaller diameter than the 780-nanometer infrared laser diode used by a CD-ROM. The smallest CD-ROM pits are 1.1 micrometers in diameter. The pits on a DVD are 0.4 micrometers in diameter on a single layer disc and about 10 percent larger when a second layer is used. Because a DVD has smaller pits and narrower laser beams, the tracks can be closer together. Track pitch is 0.74 micrometers on a DVD, compared with 1.6 micrometers on a CD-ROM. The modulation and encoding scheme was also modified in the DVD format to increase the density of data.

A single-layer DVD spins at a constant linear velocity (CLV) of 3.4 meters per second, approximately the same speed as an 8X CD-ROM. At this velocity, data may be extracted at a rate of 11.08 megabits per second (Mbps). The sustained audio-video media delivery rate is 9.8 Mbps, deducting overhead (formatting and private data).

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