CD (Compact Disc)

A CD, short for Compact Disc, is a small rotating disc on which data is stored digitally. The data is read by a laser (concentrated light beam that travels in a straight line). A standard disc has a diameter of 12 centimetres and is very thin. It is made of polycarbonate (a type of plastic) covered with a reflective metal layer that bounces light, and then covered with a protective layer.

CDs were introduced in the 1980s for music playback, and later expanded to computer data storage. They displaced part of the market for vinyl records and cassette tapes. Following the development of DVDs, online storage, and internet music services, their use has declined, though they continue to be used for storage and distribution.

How it works

The disc surface contains a spiral track that begins at the centre and extends outward. This track contains tiny indentations called pits and flat areas called lands. A laser hits the disc surface; the way light reflects back is converted into digital numbers (0 and 1). A reader device translates these numbers into sound or data.

Types

CD-Audio is designed for music. CD-ROM is for computer data, written once and not rewritable. CD-R is writable once by the user. CD-RW is rewritable multiple times. The storage capacity of a standard CD is around 700 megabytes of data, or roughly 74–80 minutes of audio.

Care and lifespan

Scratches on the surface, heat, and humidity can damage readability. The metal layer can oxidize over time, leading to data loss. For this reason, organizations storing critical data use backup copies.