Compact Disc

A compact disc or CD is an optical disc originally used to store music but also used for data. Compact discs are made from a 1.2mm thick disc of polycarbonate plastic coated with a much thinner aluminium layer which is protected by a film of lacquer. The lacquer is usually printed with a label. CDs are available in a range of sizes but the most commonly available is 120mm in diameter. A 120mm disc can store about 74 minutes of music or about 650 megabytes of data. The format of the disk, known as the 'Red Book' standard, was laid out by the Dutch electronics company Philips, who own the rights to the licensing of the 'CD' logo that appears on the disk. In broad terms the format is a two channel (left and right, for stereo) 16-bit PCM encoding at 44.1KHz.

The information on a standard CD is encoded as a spiral track of "pits" moulded into the top of the polycarbonate layer. Each pit is approximately 125 nm deep, 500 nm wide, and varies from 850 nm to 3.5 μm long. The spacing between the tracks is 1.5 μm. A CD is read by shining light from a 780 nm wavelength semiconductor laser through the bottom of the polycarbonate layer, and monitoring the light reflected by the aluminium coating. The light from the laser forms a spot of approximately 1.7 μm diameter on the metal surface. Since the CD is read through the bottom of the disc, each "pit" appears as an elevated "bump" to the reading light beam; the areas without bumps are known as "land".

Light striking the "land" areas is reflected normally and detected by a photodiode. Light striking a bump, however, undergoes destructive interference with light reflecting from the land surrounding the bump and no light is relected. This occurs because the height of each bump is one quarter of the wavelength of the laser light (in the polycarbonate medium), leading to a half-wavelength phase difference in light reflecting from the land to that of light reflecting from the bump.

Attempts are currently (early 2002) being made by record companies to market so called 'copy-protected' compact disks. These rely on deliberate errors being introduced into the data recorded on the disk; the intent is that the error-correction in a music player will enable music to be played as normal, while computer CD-ROM drives will fail with errors (though not all current drives fail, and copying software is being adapted to cope with these damaged data tracks.) Philips have stated that such disks, which are written with deliberately degraded data on them which fails the Red Book specification, will not be permitted to have the CD logo on their packaging; it also seems likely that Philips's new models of CD recorders will be designed to be able to record from these 'protected' disks.

Compact discs cannot be easily recorded; they are manufactured by etching a glass plate and using that plate to press metal. However there are also CD-R discs, which can be recorded by a laser beam using a CD-R writer (most often on a computer, though standalone units are also available), and can be played on (most) compact disc players. The recording on a CD-R is permanent, the medium cannot record more than once, hence the process is also called "burning a CD". CD-RW on the other hand is a medium that allows multiple recordings on the same disc over and over again. Many CD-Audio players cannot read CD-RW discs, however more standalone DVD players can read CD-RW than only CD-R discs. These issues are moot, however, when referring to drives installed in computers - all but the oldest CD-ROM and DVD-ROM drives can read (if not write) CD-R and CD-RWs.

The term EP is used for both a CD and a vinyl record of intermediate play-time.