To create a CD is a very complicated process and is called mastering. Information from a digital tape master is used to modulate a laser beam, which traces a spiral track on a spinning glass disc coated with a photosensitive material. An electromagnetic current is used to vary the intensity of the laser beam and the photosensitive coating dissolves where the laser hits, creating pits in the glass. The glass disc is called a glass master and is in turn used to create metal masters (occasionally called fathers), formed by coating the glass master with a thin layer of nickel. For mass production, however, discs called "mothers" are created from the metal masters; the mothers serve as the masters for a number of metal stampers (occasionally called "sons"). The stamps are the forms used to mould the CD's polycarbonate layer, which is then coated with aluminum and the final acrylic layer.
The data encoded by the laser in a long continuous spiral track, from the inside to the outside, about 1.6 to 2.2 microns wide (1 micron = 1/1000 of a millimeter), with pits about 0.6 microns wide engraved into the track. The thin metallic layer conforms to the contours of the substrate. The outer acrylic layer over it is impressed with the CD label.
Considerable amounts of data on a CD are devoted to error correction. This significantly reduces the total true data storage capacity of a CD. However, this is an integral and necessary part of every CD. Imagine a person accidentally touches the data surface of a CD. Without error correction information, the computer would be at a loss to determine what important information was present in that section of the CD. A fingerprint is largely washable, but what about more enduring forms of abuse? If a CD were to get permanently scratched, that whole section would be deemed useless. The error correction helps to figure out the false or missing parts of a file and therefore helps to minimize if not eliminate all forms of distortion. For example, the worst thing a scratch or a fingerprint can do to a music CD is to skip a few seconds of music. This does not mean that the CD can be subject to such forms of abuse as the error correction ability of a CD is limited. The more damage a CD sustains, the greater the chance of destroyed information.
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