Compact Disc
A Compact Disc or CD is an optical disc used to store digital data, originally developed for storing digital audio. The CD, available on the market in late 1982, remains the standard physical medium for commercial audio recordings as of 2007. An audio CD consists of one or more stereo tracks stored using 16-bit PCM coding at a sampling rate of 44.1 kHz. Standard CDs have a diameter of 120 mm and can hold approximately 80 minutes of audio. There are also 80 mm discs, sometimes used for CD singles, which hold approximately 20 minutes of audio. Compact Disc technology was later adapted for use as a data storage device, known as a CD-ROM, and to include record-once and re-writable media (CD-R and CD-RW). CD-ROMs and CD-Rs remain widely used technologies in the personal-computer industry as of 2007. The CD and its extensions have been extremely successful: in 2004, the annual worldwide sales of CD-Audio, CD-ROM, and CD-R reached about 30 billion discs.
History
In 1979 Philips and Sony set up a joint task force of engineers to design the new digital audio disc. Prominent members of the task force were Joop Sinjou, Kees Immink and Toshi tada Doi. After a year of experimentation and discussion, the taskforce produced the "Red Book", the Compact Disc standard. Philips contributed the general manufacturing process, based on video LaserDisc technology. Philips also contributed the Eight-to-Fourteen Modulation, EFM, which offers both a long playing time and a high resilience against disc handling damage such as scratches and fingerprints, while Sony contributed the error-correction method, CIRC. The Compact Disc Story[1], told by a former member of the taskforce, gives background information on the many technical decisions made, including the choice of the sampling frequency, playing time, and disc diameter. According to Philips, the Compact Disc was thus "invented collectively by a large group of people working as a team[2]."
The Compact Disc reached the market in late 1982 in Asia and early the following year in other markets; for example, it was released in the United States in March, the first CDs available being 16 Japanese-made titles from CBS/Sony. This event is often seen as the "Big Bang" of the digital audio revolution. The new audio disc was enthusiastically received, especially in the early-adopting classical music and audiophile communities and its handling quality received particular praise. As the price of players sank rapidly, the CD began to gain popularity in the larger popular and rock music markets.
The CD was originally thought of as an evolution of the gramophone record, rather than primarily as a data storage medium. Only later did the concept of an 'audio file' arise, and the generalizing of this to any data file. From its origins as a music format, Compact Disc has grown to encompass other applications. In June 1985, the CD-ROM (read-only memory) and, in 1990, CD-Recordable were introduced, also developed by Sony and Philips
Physical details
A Compact Disc is made from a 1.2 mm thick disc of almost pure polycarbonate plastic and weighs approximately 16 grams. A thin layer of Super Purity Aluminium (or rarely gold, used for its data longevity, such as in some limited-edition audiophile CDs) is applied to the surface to make it reflective, and is protected by a film of lacquer. The lacquer is normally printed directly and not with an adhesive label. Common printing methods for compact discs are screen-printing and offset printing. CD data is stored as a series of tiny indentations (pits), encoded in a tightly packed spiral track moulded into the top of the polycarbonate layer. The areas between pits are known as 'lands'. Each pit is approximately 100 nm deep by 500 nm wide, and varies from 850 nm to 3.5 μm in length. The spacing between the tracks, the pitch, is 1.6 μm. A CD is read by focusing a 780 nm wavelength semiconductor laser through the bottom of the polycarbonate layer. The difference in height between pits and lands leads to a phase difference between the light reflected from a pit and that from its surrounding land. By measuring the intensity with a photodiode, it is possible to read the data from the disc. The pits and lands themselves do not directly represent the zeros and ones of binary data. Instead, Non-return-to-zero, inverted encoding is used: a change from pit to land or land to pit indicates a one, while no change indicates a zero. This in turn is decoded by reversing the Eight-to-Fourteen Modulation used in mastering the disc, and then reversing the Cross-Interleaved Reed-Solomon Coding, finally revealing the raw data stored on the disc.
Pits are much closer to the label side of a disc so that defects and dirt on the clear side can be out of focus during playback. Discs consequently suffer more damage because of defects such as scratches on the label side, whereas clear-side scratches can be repaired by refilling them with plastic of similar index of refraction.
