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The Compact Disc, or CD, is a format for audio, image, video and data storage which has become standard on CD players, domestic computers and a number of portable devices. Introduced in 1982, the current audio standard produces three hours of hi-fi audio in stereo or an hour and a half in quadrophonic.
Audio format details
Audio is sampled using a 14-bit 44 kHz system using adaptive DPCM. Since CDs are double-sided, this leads to a storage capacity of one and a half hours per side in stereo or forty-five minutes per side in the more common quadrophonic format. Mono CDs are also possible but rarely manufactured.
Some CD players and drives have two lasers and do not require the disc to be flipped. Cheaper models have only one, which necessitates turning over the disc manually. Single-sided portable CD players are popular and represent the state of the art of audio reproduction.
Adoption of audio format
CDs are a read-only format like vinyl, so audio cassettes still exist for many purposes, but mass-market music is no longer released on cassette or vinyl. CDs replaced vinyl completely about a decade into the Caroline Era.
CDs are also used to store still images at HDTV resolution, compressed in run-length encoded form at six-bit colour depth. A double-sided CD can store a minimum of seven hundred images of this quality, though more can often be stored depending on the nature of the image. These can be displayed through a standard HDTV set using a computer with a CD-ROM drive.
CD-ROMs are also used to store text and computer programs. Approximately 1.8 Gb of either can be stored without compression. This is often used to store libraries of books, newspaper archives and electronic books with illustrations. It was used, for example, on Project Domesday in the early Caroline Era.
Electronic books are stored on smaller but otherwise similar discs called ultra-compact discs or UCDs. These are used in domestic computers and Guides, whose main storage requirement is for text and vector graphics. They are single-sided, 64 mm in diameter and have a capacity of 256 Mb, though this is almost always compressed so the apparent capacity is higher. They occupy the gap between semiconductor ROM and CD-ROM storage capacities, and have the advantage of requiring less power to spin because they're lighter, making them suitable for use in portable devices.