Compact disc

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The read side of a compact disc

A compact disc (CD) is a type of optical disc. It is used to store digital data. A laser burns microscopic (and invisible to the naked eye) pits in the disc's material. When played a laser detects their presence. CDs were first used to store music and other sounds: they are called audio CDs. The sound on a CD is played using a compact disc player, developed by Philips and Sony.

Later, compact discs were made that could be used to store computer files in the same way as audio compact discs. These are called CD-ROMs (compact disc read-only memory). The computer reads the disc using a CD-ROM drive. Another use is to store MPEG videos cheaply; these CDs are called VCDs (video CDs). They are especially popular in Asia. In Indonesia they are used instead of the more expensive DVD. The quality of a VCD is about the same as a VHS tape.

The diameter of a normal CD is 120 mm. The middle hole in a CD is about the size of a five cent coin (about 1.5 cm). The person who decided the size was Dutch and used the size of an old Dutch coin (old because the Dutch now have switched to the Euro), called a "dubbeltje", or dime. A CD usually holds a maximum of 74-80 minutes of sound or 650-700 megabytes of data. CDs can be manufactured to hold up to 99 minutes of audio, or slightly under a gigabyte of data, by making the tracks smaller. Discs with more than 74 minutes or 650 MB are non-standard, and some CD players are not able to play them.

Some CDs are smaller; they are 80mm in diameter. They can store about 30% of a normal disc, and were intended to be used for music "singles". Mass-manufacturing made it more convenient to use a full-size CD and leave most of it empty. This greatly limited the appeal and sale of 80mm CDs.

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