Compact disc

Compact disc (or CD) is an optical disc used to store digital data, originally developed for storing digital audio. It is the standard playback format for commercial audio recordings today.

Physical details

Compact discs are made from a 1.2 mm thick disc of polycarbonate plastic coated with a much thinner layer of Super Purity Aluminium (or rarely, gold, used for its data longevity, such as in some limited-edition audiophile CDs) 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 two sizes. By far the most common is 120 mm in diameter, with a 74-minute audio capacity and a 650-MB data (See storage capacity; this form factor has also erroneously been called "CD5" since it is about five inches across). Such a standard disc weighs 15 grams. They are also available as 80-mm discs, a format which is mainly used for audio CD singles in some regions (e.g. Japan), much like the old vinyl single. Each such "miniCD" or "Maxi CD" can hold 21 minutes of music, or 180 MB of data (this form factor has also been called "CD3", since it is about three inches across). Other unique shapes and smaller form factors have also been sold or given away as promotional items. Examples include Business Card CDs in the shape of a rectangular card and CDs shaped like the map of a country etc.

Related Topics:
Mm - Polycarbonate - Plastic - Super Purity - Aluminium - Gold - Audiophile - Lacquer - Print - Label - Silkscreening - Offset printing - Diameter - Storage capacity - Inch - Japan - Vinyl single - MiniCD

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There is a 15-mm hole in the centre of the disc, usually used by some form of clamp or clip device within the player to hold it in place and allow it to be rotated by a motor.

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The information on a standard CD is encoded as a spiral track of pits moulded into the top of the polycarbonate layer (The areas between pits are known as lands). Each pit is approximately 125 nm deep by 500 nm wide, and varies from 850 nm to 3.5 ?m long. The spacing between the tracks is 1.6 ?m. To grasp the scale of the pits and land of a CD, if the disc is enlarged to the size of a stadium, a pit would be approximately the size of a grain of sand. The spiral begins at the center of the disc and proceeds outwards to the edge, which allows the different size formats available.

Related Topics:
Moulded - Nm - ?m

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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 is a sixth of the wavelength of the laser light, leading to a third-wavelength phase difference between the light reflected from a pit and from its surrounding land. The sixth, 125 nm, (and not a quarter) of the wavelength was chosen to have a good trade-off between the push-pull radial tracking signal and the full-aperture read-out signal. The interference reduces the intensity of the reflected light compared to when the laser is reflected by just a land. By measuring this intensity with a photodiode, one is able to read the data from the disc. The pits and lands themselves do not represent the zeroes and ones of binary data. Instead 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, finally revealing the raw data stored on the disc.

Related Topics:
Wavelength - Semiconductor laser - Phase - Interference - Photodiode - Binary data - Eight-to-Fourteen Modulation

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A 74-minute (650MB) disc has roughly 333,000 sectors while the 80 minute (700MB) one has approximately 360,000 sectors allowing greater storage. 90 and 99 minute versions follow the same logical extension cramming more sectors at the cost of reliability across drives. 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. Consequently, discs are much easier to ruin by scratching their label side, whereas clear-side scratches can be repaired by refilling them with plastic of similar index of refraction.

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