Working of compact disk - computer architecture, Computer Engineering

Working of compact disk:

A CD is built from 1.2 mm thick, approximately all-pure polycarbonate plastic and its weighs is approximately 15-20 grams. From the core outward components are at the center (spindle) hole, the clamping area (stacking ring), the first-shift area (clamping ring), the information (data) area the second-transition area (mirror band), , and the rim.

A thin layer of aluminum or, rarely, gold is applied to the surface to make it reflective, and is protected by a film of lacquer that is usually spin coated directly on top of the reflective layer, on which the label print is applied. General printing methods for CDs are and offset printing and screen-printing

CD data are stored as a series of tiny indentations called -pits? which is encoded in a spiral track molded into the top of the polycarbonate layer. The areas among pits are called -lands?.  Each pit is about 100 nm deep by 500 nm wide, and varies from 850 nm to 3.5 µm in the length.

The distance among the tracks, the pitch is 1.6 µm. CD is read by focusing a 780 nm wavelength (near infrared) semiconductor laser throughout the bottom of the polycarbonate layer. The modification in height among pits (in fact ridges as seen by the laser) and lands results in a difference in intensity in the light reflected. By measuring the intensity change having photodiode, the data may be read from the disc.

The lands and pits both do not directly represent the 0 and 1 of binary data. Instead of it, Non-return- to-zero, inverted (NRZI) encoding is used for this purpose: a change from pit to land or land to pit denote a 1, whereas no change denote a series of 0. There have to be at least 2 and no more than 10 zeros between each one that is defined by the length of the pit. This is decoded in turn by reversing the Eight -to-Fourteen Modulation which is used in mastering the disc, and afterwards reversing the Cross-Interleaved Reed-Solomon Coding, at last revealing the raw data stored on the disc.

CDs are susceptible to damage from both every day use and environmental contact. Pits are much nearer to the label side of a disc, so that dirt and defects on the clear side can be out of focus at the playback time. Thus, CDs suffer from scratch and damage on the label side while scratches on the clear side may be repaired by refilling them with same refractive plastic, or by polishing carefully. First music CDs were known to suffer from or "laser rot", or "CD rot", in which the internal reflective layer degrades. When this take place the CD may become unplayable.

Posted Date: 10/13/2012 6:29:28 AM | Location : United States

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