compact disc (CD)

  

Definition

Optically readable/writable 1.22 MM thick and 120 millimeter (4.75 inch) diameter digital medium consisting of polycarbonate substrate, a reflective metalized layer, and a protective lacquer coating. Unlike DVD and magnetic media, all CD media is only single-sided. And, unlike magnetic disks (which store data in concentric rings divided into segments called 'sectors') a CD stores data on one long (1.6 to 2.2 micron wide) spiral track, just like in the old phonograph records. The data is stored as microscopic (0.6 micron wide) grooves, cut and read by a laser beam. A CD can hold about 700 megabytes (MB) of computer data (equivalent to some 500 diskettes or about 250,000 pages of printed text) and has a data transfer-rate of 1MB per second. Alternatively, it can hold almost 80 minutes of audio (enough for Beethoven's Ninth Symphony, its design goal) and, with a 16-bit sample-size and 44.1 KHz sampling-rate, it can reproduce high quality stereo music consisting of about 65,500 sounds. CD technology was invented by the Dutch electronics firm Philips in late 1970s, and developed for commercial applications in association with Sony. Its specifications are governed by the ISO 9660 standard as described in its manual (called the Red Book). The term 'compact disc' is outdated, and the designation CD serves as an umbrella term for all CD media such as CD-Audio, CD-I, CD-R, CD-ROM, CD-RW. CD drives cannot read DVD media.

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