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A digital video disc (DVD), also sometimes known as a digital versatile disc, is an optical data disc that typically holds multimedia information — but may be used for data as well. A DVD can hold between approximately 4.7 and 17 gigabytes (GB) of information depending on how many layers of data it has. There are several formats of DVD that allow users different ways to manipulate the data on the disc.
The most common data recorded to a digital video disc are multimedia files, such as video or audio. Other discs of this type may hold data, like large databases or the installation files for computer programs. Most users recognize DVDs to be used for films and television. These discs usually contain video files, audio tracks, and a multimedia menu. The menu often allows users to choose from several viewing options, such as playing alternate audio tracks with the film, or viewing extra footage.
The standard physical size for a digital video disc is 4.7 inches (12 cm) in diameter. The discs are typically circular, and have a center hole where the DVD reader can insert a drum that turns or spins it. Some smaller discs, called MiniDVDs, are 3.2 inches (8 cm) in diameter. For the most part, DVDs are made of several layers of plastic that, in total, measure 0.05 inches (1.2 mm) in thickness.
The layers of plastic that comprise a digital video disc are usually pressed with a pattern of very tiny indentations. These bumps, arranged in a long line spiraling out from the center of the disc, usually contain all the digital data. The DVD reader spins the disc in order to read these lines of code.
Traditional compact discs (CDs) work in a similar manner, but only hold 650 megabytes (MB) of digital information, whereas a single-layer DVD can hold 4.7 gigabytes (GB), or around seven times as much. This is mostly because the data indentations on a DVD are smaller than those on a CD. Also, DVDs usually use the Moving Picture Experts Group’s (MPEG) standard for video compression format, known as MPEG-2, to allow even more data to be saved on the disc.
Many DVDs have more than one spiral of data. Some discs have material printed on both sides. Another option is to create a multi-layer disc. These have the standard printed data spiral, but are covered with yet another layer of data, and the two layers are separated by a thin, semi-transparent layer of gold. A standard, multi-layer, single-sided DVD can hold up to 8.5 GB, and a multi-layer, double-sided disc can hold approximately 17 GB of data.
There are several formats for DVDs. Some discs are only readable. This means that the companies that publish the digital video disc do not allow private users to manipulate the data on it. These read-only DVDs are often encrypted as well, so that users cannot make unauthorized copies of the disc.
Other discs are able to be manipulated by private users. Some, typically known as DVD-Recordable (DVD-R) discs, allow individuals to save data to the disc only once. Discs labeled DVD-ReWritable (DVD-RW) often allow a user to save and erase data multiple times to the same disc.