DVD is an optical disc storage media format that can be used for storing data, 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. Unlike CDs, all DVDs must contain a file system. This file system is called UDF, and is an extension of the ISO 9660 Standard used for Data-CDs.
DVDs are made from a 0.6 mm thick disc of polycarbonate plastic coated with a much thinner (reflective) aluminium layer. Two such discs are glued together to form a 1.2 mm double-sided disc. The basic DVD disc is thinner than a CD to make it possible to use a lens with a higher numerical aperture.
A single-layer DVD can store 4.7 Gbyte, which is around seven times more than a standard CD-ROM. By employing a read 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 subcode 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.
DVD Media can contain:
DVD-ROM (read only, manufactured by a press)
DVD-R/RW (R=Recordable once, RW = ReWritable)
DVD-RAM (random access rewritable; after-write checking of data integrity is always active.)
DVD+R/RW (R=Recordable once, RW = ReWritable)
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.
DVD-5: single sided, single layer, 4,704,317,440 bytes, 4.7 gigabytes (GB), or 4.38 gibibytes (GiB)
DVD-9: single sided, double layer, 8.5 GB (7.92 GiB)
DVD-10: double sided, single layer on both sides, 9.4 GB (8.75 GiB)
DVD-14: double sided, double layer on one side, single layer on other, 13.3 GB (12.3 GiB)
DVD-18: double sided, double layer on both sides, 17.1 GB (15.9 GiB)
The capacity of a DVD-ROM can be visually determined by noting the number of data sides, and looking at the data side(s) of the disc. Double-layered sides are usually gold-colored, while single-layered sides are silver-colored, like a CD. One additional way to tell if a DVD contains one or two layers is to look at the center ring on the underside of the disc. If there are two barcodes, it is a dual layer disc. If there is one barcode, there is only one layer.
Each medium can contain any of the above content and can be any layer type. Double layer DVD+R discs are already on the market.
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