In computing and optical disc recording technologies, an optical disc (OD) is a flat, usually circular disc which encodes binary data (bits) in the form of pits (binary value of 0 or off, due to lack of reflection when read) and lands (binary value of 1 or on, due to a reflection when read) on a special material (often aluminium) on one of its flat surfaces. The encoding material sits atop a thicker substrate (usually polycarbonate) which makes up the bulk of the disc and forms a dust defocusing layer. The encoding pattern follows a continuous, spiral path covering the entire disc surface and extending from the innermost track to the outermost track. The data is stored on the disc with a laser or stamping machine, and can be accessed when the data path is illuminated with a laser diode in an optical disc drive which spins the disc at speeds of about 200 to 4,000 RPM or more, depending on the drive type, disc format, and the distance of the read head from the center of the disc (inner tracks are read at a faster disc speed). The pits or bumps distort the reflected laser light, hence most optical discs (except the black discs of the original PlayStation video game console) characteristically have an iridescent appearance created by the grooves of the reflective layer. The reverse side of an optical disc usually has a printed label, sometimes made of paper but often printed or stamped onto the disc itself. This side of the disc contains the actual data and is typically coated with a transparent material, usually lacquer. Unlike the 3½-inch floppy disk, most optical discs do not have an integrated protective casing and are therefore susceptible to data transfer problems due to scratches, fingerprints, and other environmental problems.
Optical discs are usually between 7.6 and 30 cm (3 to 12 in) in diameter, with 12 cm (4.75 in) being the most common size. A typical disc is about 1.2 mm (0.05 in) thick, while the track pitch (distance from the center of one track to the center of the next) is typically 1.6 µm.
An optical disc is designed to support one of three recording types: read-only (e.g.: CD and CD-ROM), recordable (write-once, e.g. CD-R), or re-recordable (rewritable, e.g. CD-RW). Write-once optical discs commonly have an organic dye recording layer between the substrate and the reflective layer. Rewritable discs typically contain an alloy recording layer composed of a phase change material, most often AgInSbTe, an alloy of silver, indium, antimony, and tellurium.
Optical discs are most commonly used for storing music (e.g. for use in a CD player), video (e.g. for use in a Blu-ray player), or data and programs for personal computers (PC). The Optical Storage Technology Association (OSTA) promotes standardized optical storage formats. Although optical discs are more durable than earlier audio-visual and data storage formats, they are susceptible to environmental and daily-use damage. Libraries and archives enact optical media preservation procedures to ensure continued usability in the computer's optical disc drive or corresponding disc player.
For computer data backup and physical data transfer, optical discs such as CDs and DVDs are gradually being replaced with faster, smaller, and more reliable solid-state devices, especially the USB flash drive. This trend is expected to continue as USB flash drives continue to increase in capacity and drop in price. Similarly, personal portable CD players have been supplanted by portable solid-state digital audio player (MP3 players), and MP3 music purchased or shared over the Internet has significantly reduced the number of audio CDs sold annually.
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