A compact disc (or CD) is an optical disc used for storing digital data. It was originally invented for digital audio and is also used as a data storage device, 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.
Compact discs are made from a 1.2 mm thick disc of polycarbonateplastic coated with a much thinner aluminium (originally gold) 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 120mm (about 5 inches) in diameter. A 120mm 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 however. There are also less common 90, 99 and 100 minute discs, but they are not compatible with all CD writers or readers. The mini-CD (not to be confused with the similar MiniDisc) is 80mm (about 3 inches) in diameter, holds about 140MB of data or 21 minutes of audio, and has the exact same data format as the larger one. Yet another version of the CD has a mini-CD trimmed down to fit in with business cards.
The data format of the disc, known as the 'Red Book' standard, was laid out by the Dutchelectronics company Philips, who own the rights to the licensing of the 'CDDA' logo that appears on the disc. In broad terms the format is a two-channel (left and right, for stereo) 16-bit PCM encoding at a 44.1kHzsampling rate. 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 125nm deep by 500nm wide, and varies from 850nm to 3.5μm; long. The spacing between the tracks is 1.5μm. A CD is read by shining light from a 780nm wavelengthsemiconductor 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 1.7μm 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.
To grasp the scale of the pits and land of a CD, if the disc is enlarged to the size of a regular stadium, a pit would have approximately the size of a grain of sand.
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.
The compact disc specification does not include any copy protection mechanism and discs can be easily duplicated or the contents "ripped" to a computer. 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. This approach is the subject of an evolutionary arms race or cat-and-mouse game — not all current drives fail, and copying software is being adapted to cope with these damaged data tracks. The recording industry then works on further approaches.
Philips have stated that such discs, which do not meet the Red Book specification, are not permitted to bear the trademarked Compact Disc Digital Audio logo. It also seems likely that Philips' new models of CD recorders will be designed to be able to record from these 'protected' discs.
CD-RW is a medium that allows multiple recordings on the same disc over and over again. A CD-RW does not have as great a difference in the reflectivity of lands and bumps as a pressed CD or a CD-R, so many CD audio players cannot read CD-RW discs, although the majority of standalone DVD players can.