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In computing, sound reproduction, and video, an optical disc is a flat, circular, usually polycarbonate disc where data is stored in the form of pits (or bumps) within a flat surface, usually along a single spiral groove that covers the entire recorded surface of the disc. This data is generally accessed when a special material on the disc (often aluminium) is illuminated with a laser diode. The pits distort the reflected laser light. Most optical discs, with the exception of a few such as black CD-ROMs designed for the original Sony PlayStation, have a characteristic prismatic or iridescent appearance created by the grooves in the reflective layer.
David Paul Gregg developed an analog optical disc for recording video and patented it in 1961 and 1969 (U.S. Patent 3,430,966). Of special interest is U.S. Patent 4,893,297, first filed in 1968 and issued in 1990, so that it will be a source of royalty income for Pioneer’s DVA until 2007. It encompasses systems such as CD, DVD, and even Blu-ray Disc. Gregg's company, Gauss Electrophysics, was acquired, along with Gregg's patents, by MCA in the early 1960s.
In a parallel manner, and probably inspired by the developments in the U.S., a small group of physicists started their first optical videodisc experiments at Philips Research in Eindhoven, The Netherlands in 1969. In 1975, Philips and MCA decided to join forces. In 1978, much too late, the long waited laserdisc was introduced in Atlanta. MCA delivered the discs and Philips the players. It turned out to be a total technical and commercial failure, and quite soon the Philips/MCA cooperation came to an end. In Japan and the U.S., Pioneer has been successful with the videodisc until the advent of DVD.
Philips and Sony formed a consortium in 1979 to develop a digital audio disc, which resulted in the very successful introduction of the compact disc in 1983.
The promotion of standardised optical storage is undertaken by the Optical Storage Technology Association (OSTA).
While Optical Discs are significantly more durable than earlier audio/visual and data formats, they are susceptible to damage from daily usage and environmental factors. Libraries and archives should enact optical media preservation procedures to ensure continued usability.
The information on an optical disc is stored sequentially on a continuous spiral track from the innermost track to the outermost track.
A computer's peripheral device used to read or write an optical disc is an optical disc drive (ODD).
Additional recommended knowledge
First-generation optical discs
Optical discs were initially used for storing music and software. The Laser disc format stored analog video, but it fought an uphill battle against VHS (mainly due to cost and non-record ability); other first-generation disc formats are designed to store solely digital data.
Most first-generation disc devices use an infrared laser as a read head. The minimum size of a laser spot is proportional to the wavelength of the laser, making wavelength one factor limiting the information density. Infrared is just beyond the long-wavelength end of the visible light spectrum, so it supports less density than any visible (to humans) color of light. One example of capacity achieved with an infrared laser is 700 MB of net user data for a 12 cm compact disc. (However, many factors affect density besides minimum spot size--for example, a multi-layered disc using infrared would hold more data than an otherwise identical disc with a single layer, and other issues--such as whether CAV, CLV, or zoned CAV is used, how data is encoded, and how much margin is left clear at the center and edge--also affect how close a disc can come to taking advantage of the minimum spot size over 100% of the disc surface.)
Second-generation optical discs
Second-generation optical discs were created to store large amounts of data, including TV-quality digital video. Many, though not all, such discs use a visible light laser (usually red); the shorter wavelength and greater Numerical aperture allow a tighter beam, allowing the pits and lands of the disc to be smaller. In the case of the DVD format, this allows 4.7 GB of storage on a standard 12 cm, single-sided, single layer disc; alternately, smaller media such as the MiniDisc and DataPlay formats can have capacity approximately comparable to a much larger standard compact disc.
Third-generation optical discs
Major third-generation optical discs are currently in development. They are designed for distribution of high-definition video and support greater capacities. This is accomplished by the use of short-wavelength visible light lasers and even greater numerical apertures. Blu-ray Disc and HD-DVD both use blue-violet lasers, though Blu-ray has a greater numerical aperture, resulting in smaller pit size and therefore greater data capacity per layer.
In practice, effective capacity for multimedia presentations can be drastically improved by using enhanced video data compression codecs such as H.264 and VC-1 as well.
Next generation optical discs
The following formats are so advanced they can be considered to be ahead of current (third gen) discs. All of the following discs have the potential of over 1 terabyte of space.
Recordable and writable optical discs
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Optical_disc". A list of authors is available in Wikipedia.|