1. An optical storage medium for digital data, usually audio. A compact disc is a nonmagnetic, polished metal disc with a protective plastic coating that can hold up to 74 minutes of high-fidelity recorded sound. The disk is read by an optical scanning mechanism that uses a high-intensity light source, such as a laser, and mirrors. Also called: optical disc.
2. A technology that forms the basis of media such as CD-ROM, CD-ROM/XA, CD-I, CD-R, DVI, and PhotoCD. These media are all compact disc–based but store various types of digital information and have different read/write capabilities. Documentation for compact disc formats can be found in books designated by the color of their covers. For example, documentation for audio compact discs is found in the Red Book. See also CD-I, CD-R, CD-ROM, CD-ROM/XA, DVI, Green Book (definition 2), Orange Book (definition 2), PhotoCD, Red Book (definition 2).
3. See CD.
(or CD) Disc for storing digital information, about 12 cm/4.5 in across, mainly used for music, when it can have over an hour's playing time. The compact disc is made of aluminum with a transparent plastic coating; the metal disc underneath is etched by a laser beam with microscopic pits that carry a digital code representing the sounds. During playback, a laser beam reads the code and produces signals that are changed into near-exact replicas of the original sounds.
CD-ROM, or Compact-Disc Read-Only Memory, is used to store written text, pictures, and video clips in addition to music. The discs are ideal for large works, such as catalogs and encyclopedias.
CD-I, or Compact-Disc Interactive, is a form of CD-ROM used with a computerized reader, which responds intelligently to the user’s instructions.
Recordable CDs, called WORMs (“write once, read many times”), are used as computer discs, but are as yet too expensive for home use.
Video CDs, on sale 1994, store an hour of video. High-density video discs, first publicly demonstrated 1995, can hold full-length features.
Erasable CDs, which can be erased and recorded many times, are also used by the computer industry. These are coated with a compound of cobalt and gadolinium, which alters the polarization of light falling on it. In the reader, the light reflected from the disc is passed through polarizing filters and the changes in polarization are converted into electrical signals.