A Compact Disc (also known as a CD) is an optical disc used to store digital data, originally developed for storing digital audio. The CD, available on the market since late 1982, remains the standard playback medium for commercial audio recordings to the present day. All CDs are pressed from a digital source, with the most common sources being low error-rate CD-Rs or files from an attached computer hard drive containing the finished product. Glass mastering is performed in a class 100 or better clean room or a self-enclosed clean environment within the mastering system. During glass mastering, glass is used as a substrate to hold the CD master, hence the name, while it is created and processed. CD moulding machines are specifically designed high temperature polycarbonate injection moulders. They have an average throughput of 550-900 discs per hour, per moulding line. For quality control, both the stamper and the moulded discs are tested before a production run. Samples of the disc (test pressings) are taken during long production runs and tested to ensure consistency of quality. The pressed discs are analyzed on a signal analysis machine. If no defects are found the CD continues into printing to have a label screen or offset printed on the top of the disc and then onward to be packaged, and passed to distribution.
Standard CDs have a diameter of 120 mm and can hold up to 80 minutes of audio. There is also the Mini CD, with diameters ranging from 60 to 80 mm; they are sometimes used for CD singles, storing up to 24 minutes of audio. The technology was later adapted and expanded to include data storage (CD-ROM), write-once audio and data storage (CD-R), rewritable media (CD-RW), Super Audio CD (SACD), Video Compact Discs (VCD), Super Video Compact Discs (SVCD), PhotoCD, PictureCD, CD-i, and Enhanced CD. CD-ROMs and CD-Rs remain widely used technologies in the computer industry. The CD and its extensions have been extremely successful: in 2004, worldwide sales of CD audio, CD-ROM, and CD-R reached about 30 billion discs. By 2007, 200 billion CDs had been sold worldwide.
The logical format of an audio CD (officially Compact Disc Digital Audio or CD-DA) is described in a document produced in 1980 by the format's joint creators, Sony and Philips. The document is known colloquially as the "Red Book" after the color of its cover. The format is a two-channel 16-bit PCM encoding at a 44.1 kHz sampling rate per channel. Four-channel sound is an allowed option within the Red Book format, but has never been implemented. Monaural audio has no existing standard on a Red Book CD; mono source material is usually presented as two identical channels on a 'stereo' track.
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