By Michael Sanford / Published on June 10th, 2008 / Entertainment
DVD is an optical disc storage media format that can be used for data storage, including movies with high video and sound quality. DVDs resemble compact discs: their physical dimensions are the same—12cm or the mini 8cm—but they are encoded in a different format and at a much higher density. DVDs contain a file system, called UDF, which is an extension of the ISO 9660 Standard used for CD-ROMs.
DVDs are made from a 0.6 mm thick disc of polycarbonate plastic coated with a much thinner (reflective) aluminum layer. Two such discs are glued together to form a 1.2 mm double-sided disc. 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.
A single-layer DVD can store 4.7 Gbyte, which is around seven times as much a standard CD-ROM. By employing a red laser at 650 nm (was 780 nm) wavelength and a numerical aperture of 0.6 (was 0.45), 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 has the same characteristics as classic EFM. The CD sub code is removed. As a result, the DVD format is 47 percent more efficient with respect to CD-ROM, which uses a 'third' error correction layer.
A DVD can contain:
DVD-Video (containing movies (video and sound))
DVD-Audio (containing high-definition sound)
DVD-Data (containing data)
The disc medium can be:
DVD-ROM (read only, manufactured by a press)
DVD-R/RW (R=Recordable once, RW = Rewritable)
DVD-RAM (random access rewritable)
DVD+R/RW (R=Recordable once, RW = Rewritable)
DVD-R DL (double layer)
DVD+R DL (double layer)
Two DVDs with different bottom sides.
The disc may have one or two sides, and one or two layers of data per side; the number of sides and layers determines the disc capacity.
There are several competing DVD Formats:
DVD-ROM: They are pressed similarly to CDs. The reflective surface is silver or gold colored. They can be single-sided/single-layered, single-sided/double-layered, double-sided/single-layered, or double-sided/double-layered. As of 2004, new double-sided discs have become increasingly rare.
DVD-R for Authoring: Special-purpose DVD-Rs used to record DVD masters, which can then be duplicated to pressed DVDs by a duplication plant. They require a special DVD-R recorder, and are not often used nowadays since many duplicators can now accept ordinary DVD-R masters.
DVD-R discs (strictly DVD-R for General) can record up to 4.7 GB in a similar fashion to a CD-R disc. Once recorded and finalized it can be played by most
DVD-ROM players. This format is supported by the DVD Forum.
DVD-RW discs can record up to 4.7 GB in a similar fashion to a CD-RW drive. Supported by the DVD Forum.
DVD-R DL: Derivate of DVD-R that uses double-layer recordable discs to store up to 8.5 GB of data.
DVD-RAM: (current specification is version 2.1) require a special unit to play 4.7GB or 9.4GB recorded discs (DVD-RAM disc are typically housed in a cartridge). 2.6GB discs can be removed from their caddy and used in DVD-ROM drives. Top capacity is 9.4GB (4.7GB/side). Supported by the DVD Forum.
DVD+R: Can record up to 4.7 GB single-layered/single-sided DVD+R disc, at up to 16x speed. Like DVD-R you can record only once. Supported by the DVD+RW Alliance.
DVD+RW: Can record up to 4.7 GB at up to 16x speed. Since it is rewritable it can be overwritten several times. It does not need special "pre-pits" or finalization to be played in a DVD player. Supported by the DVD+RW Alliance.
DVD+R DL: Derivate of DVD+R that uses double-layer recordable discs to store up to 8.5 GB of data. Supported by the DVD+RW Alliance.
DVD-D is a new self-destructing disposable DVD format. Like the EZ-D, it is sold in an airtight package, and begins to destroy itself by oxidation after several hours.
DVD Plus combines both DVD and CD technologies by providing the CD layer and a DVD layer.
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For more information on DVD players please visit the DVD players resource center at http://www.dvd-players-resources.info