compact disc

Disk 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. A laser beam etches the compact disc with microscopic pits that carry a digital code representing the sounds; the pitted surface is then coated with aluminium. 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 disks are ideal for large works, such as catalogues 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.

Increasingly the dominance of the CD is being challenged by the development of alternative formats. The DVD has the same dimensions as a CD but can store over double the capacity of data. DVD drives are replacing CD-ROM drives in personal computers and as a medium for storing computer programs. Solid-state memory cards capable of exchanging data between PCs, digital cameras, PDAs, and MP3 music players are increasingly being used.