Compact disc

A compact disc.

A compact disc or CD is an optical disc originally used to store music, in the form of digital audio, and now also capable of being used as a data storage device, whence it is called a CD-ROM. CD-ROM reading devices are frequently included as a component in personal computers. In general, audio CDs are distinct from CD-ROMs, and CD players intended for listening to audio cannot make sense of the data on a CD-ROM, though personal computers can generally play audio CDs. It is possible to produce composite CDs containing both data and audio with the latter capable of being played on a CD player, whilst data or perhaps a video track can be viewed on a computer. Lately, with the advent of MP3 technology, audio player devices have been developed that can interpret MP3-formatted tracks on a CD-ROM and play them like a traditional audio CD. The advantage of MP3 is that it increases CD storage capacity by up to ten times without significant degradation in sound quality.

History

The compact disc was developed in 1980 in cooperation of Sony and Philips. 1982 began the mass production in Langenhagen near Hanover, Germany. The advent of the CD-ROM was pioneered by Bill Gates.

Technical details

Compact discs are made from a 1.2 mm thick disc of polycarbonate plastic coated with a much thinner aluminium layer which is protected by a film of lacquer. The lacquer can be printed with a label. CDs are available in a range of sizes but the most commonly available is 120 mm in diameter. A 120 mm disc can store about 74 minutes of music or about 650 megabytes of data. Discs that can store about 700 megabytes (80 minutes of music) have become more common. There are also less common 90, 99 and 100 minute discs, but they are not compatible with all CD writers or readers. The format of the disc, known as the 'Red Book' standard, was laid out by the Dutch electronics company Philips, who own the rights to the licensing of the 'CDDA' 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 44.1 kHz. Reed-Solomon error correction allows the CD to be scratched (to a certain degree) without degradation of the contents.

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 by 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 reflected. 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.

Copy protection

Starting in early 2002, attempts were made by record companies to market so-called 'copy-protected' compact discs. These rely on deliberate errors being introduced into the data recorded on the disc. 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 discs, which are written with deliberately degraded data on them that fails the Red Book specification, will not be permitted to have the CD logo on their packaging; and it seems likely that Philips' new models of CD recorders will be designed to be able to record from these 'protected' discs.

Recordability

Compact discs cannot be easily recorded, as they are manufactured by etching a glass plate and using that plate to press metal. However there are also CD-recordable 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. CD-R recordings are permanent and cannot be recorded more than once, so the process is also called "burning" a CD. CD-RW is a medium that allows multiple recordings on the same disc over and over again. Many CD audio players cannot read CD-RW discs, but more standalone DVD players can read CD-RW than only CD-R discs. For drives installed in computers, all current CD-ROM and DVD-ROM drives can read and write CD-R and CD-RW discs.

Naming conventions

The correctness of the spellings "disk" and "disc is not trivial: see " class="external">http://www.bartleby.com/61/16/C0521600.html.

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