compact disc (CD), a small plastic disc used for the storage of digital data. As originally developed for audio systems, the sound signal is sampled at a rate of 44,100 times a second, then each sample is measured and digitally encoded on the 43/4 in (12 cm) disc as a series of microscopic pits on an otherwise polished surface. The disc is covered with a transparent coating so that it can be read by a laser beam. Since nothing touches the encoded portion, the CD is not worn out by the playing process. Introduced in 1982, the CD offered other advantages over the phonograph record and recording tape—smaller size, greater dynamic range, extremely low distortion—and met with rapid consumer acceptance; the CD became the music carrier of choice by 1991, when sales exceeded those of audiocassettes.

Other CD formats include CD-ROM [Compact Disc-Read Only Memory], a form of CD that is read (but not written to) by computer using a CD-ROM drive and that can contain computer programs and digitized text, sound, photographs, and video; CD-R [Compact Disc-Recordable] and CD-RW [Compact Disc-ReWritable], which can be written to one time and multiple times, respectively. Interactive CDs (CD-I, CDTV, and other formats) can store video, audio, and data. Photo CD is a format that holds digitized photographs and sound. There are also CD-ROMs that require special players with built-in microcomputers.

Other optical disk formats include digital versatile (or video) discs and videodiscs. A digital versatile disk (DVD) holds far more information than a CD. DVD players are backward compatible to existing technologies, so they can also play a CD (or CD-ROM), but a CD player cannot be used with a DVD (or DVD-ROM). The videodisc, or laser disk system, uses 12-in. (30-cm) disks for video recording. Its technology, unlike that of the CD, is an analog system that uses a laser to read a variable-width track, much like a conventional phonograph record.

Molded plastic disc containing digital data that is scanned by a laser beam for the reproduction of recorded sound or other information. Since its commercial introduction in 1982, the audio CD has become the dominant format for high-fidelity recorded music. Digital audio data can be converted to analog form to reproduce the original audio signal (see digital-to-analog conversion). Coinvented by Philips Electronics and Sony Corp. in 1980, the compact disc has expanded beyond audio recordings into other storage-and-distribution uses, notably for computers (CD-ROM) and entertainment systems (videodisc and DVD). An audio CD can store just over an hour of music. A CD-ROM can contain up to 680 megabytes of computer data. A DVD, the same size as traditional CDs, is able to store up to 17 gigabytes of data, such as high-definition digital video files.

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