A compact disc (or CD) is an optical disc used to store digital data, originally developed for storing digital audio. It is the standard playback format for commercial audio recordings today.
A standard compact disc, often known as an "audio CD" to differentiate it from later variants, stores audio data in a format compliant with the red book standard. An audio CD consists of several stereo tracks stored using 16-bit PCM coding at a sampling rate of 44.1 kHz. Standard compact discs have a diameter of 120 mm, though 80 mm versions exist in circular and "business-card" forms. The 120 mm discs can hold 74 minutes of audio, and versions holding 80, 90 or even 99 minutes have been introduced. The 80 mm discs are used as "CD-singles" or novelty "business-card CDs". They hold about 20 minutes of audio.
Compact disc technology was later adapted for use as a data storage device, known as a CD-ROM.
The design of the CD was originally conceived as an evolution of the gramophone record, rather than primarily as a data storage medium. Only later did the concept of an 'audio file' arise, and the generalizing of this to any data file. As a result, the original CD format has a number of limitations; no built-in track names or disc naming for example. Online services such as CDDB were developed to work around these shortcomings in the computer age.
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