CD re-directs here; see Cd for other meanings of CD.
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 a standard component of most modern 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 video can be viewed on a computer. Lately, with the advent of MP3 technology, audio player devices have been developed that can load and play MP3-formatted files from CD-ROM discs. The advantage of MP3 is that it reduces the amount of space required to store audio by around ten times, thereby increasing maximum playback time per disc from around 74 minutes to more than 700 without significant degradation in sound quality.
The compact disc was developed in 1979 by Philips and Sony. Philips invented the general manufacturing process, based on their earlier Laserdisc technology, while Sony contributed the error-correction method. 1982 saw its mass production in Langenhagen near Hanover, Germany. Microsoft and Apple Computer were early enthusiasts and promoters of CD-ROMs. John Sculley, CEO of Apple at the time, said as early as 1987 that the CD-ROM would revolutionize the use of personal computers. Originally the CD was supposed to be 60 minutes, but Sony insisted on it being 74 minutes so it could hold even the slowest versions of Beethoven's 9th Symphony. Later on, the discs would become larger, though.
Compact discs are made from a 1.2 mm thick disc of polycarbonate plastic coated with a much thinner aluminium (originally gold, although you can still buy gold cd's for their data longevity) layer which is protected by a film of lacquer. The lacquer can be printed with a label. Common printing methods for compact discs are silkscreening and offset printing. CDs are available in a range of sizes but the most commonly available is 120 mm (about 5 inches) 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 however. Less common 90, 99, and 100 minute discs also exist but are not compatible with all CD writers or readers. The mini-CD (not to be confused with the similar MiniDisc) is 80 mm (about 3 1/8 inches) in diameter, holds about 184MB 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 Dutch electronics 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 44.1kHz sampling 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 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. The spiral begins at the center of the disc and proceeds outwards to the edge. This allows the different size formats available. 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. ==Copy protection== 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. However, there has been great public outcry over copy-protected discs because many see it as a threat to fair use. See also Serial Copy Management System ==Recordability== Injection molding is used to manufacture compact discs. A 'stamper' is made from the original media (audio tape, data disc, etc.) by writing to a photosensitive dye with a laser. This dye is then etched, leaving the data track. It is then plated to make a positive version of the CD. Polycarbonate is liquified and injected into the mold cavity where the stamper transfers the pattern of pits and lands to the polycarbonate disc. The disc is then metallized with aluminum and lacquer coated. 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. 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. Recordable compact discs are injection molded with a "blank" data spiral. A photosensitive dye is then applied, and then the discs are metallized and lacquer coated. The write laser of the CD burner changes the characteristics of the dye to allow the read laser of a standard CD player to see the data as it would an injection molded compact disc.
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