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  Compact Disc

Introduction to Compact Disc (Cd):

A compact disc or Cd is an optical disc employed to store digital data. Cd was initially developed to store sound recordings completely, but later it also permitted the preservation of another kinds of data. Audio CDs (compact disks) have been commercially presented since October in 1982. In the year 2010, they stay the standard physical storage medium for audio.

Standard CDs (compact disks) have a diameter of 120mm and can hold up to 80 minutes of uncompressed audio (700MB of data). The Mini CD (compact disks) has several diameters ranging from 60 to 80mm; they are occasionally employed for CD (compact disks) singles or device drivers, storing equal to 24 minutes of audio.

The technology was ultimately adapted and extended to include data storage CD-ROM, write - one time audio and data storage CD-R, rewritable media CD-RW, VCD (video compact discs), SVCD (super video compact discs), photo CD, picture CD, CD-I, and improved CD.

A CD is prepared from 1.2mm thick, approximately pure polycarbonate plastic and weighs 15-20grams.

From the center outward, components are: the first transition area (clamping ring), the centre (spindle) hole, the clamping area (stacking ring), the information (data) area, the second transition area (mirror band),  and the rim.

An aluminium's thin layer or, more rarely, gold is applied to the surface make it reflective. The metal is guarded through a film of lacquer spin coated directly on the reflective layer. On the lacquer layer the label is printed. Common printing techniques for CDS (compact disks) are offset printing and screen printing.

CD data are stored like a series of tiny indentations termed as "pits", covered in a spiral track model into the top of polycarbonate layer. The areas among pits are termed as "lands". Every pit is about 100nm deep through 500nm wide, and changes from 850nm to 3.5ìm in length.

A. A Polycarbonate layer contains the data encoded through using bumps.

B. The laser is reflected by a shiny layer.

C. A layer of lacquer assists keep the shiny layer shiny.

D. Artwork is screen printed on the disc's top.

E. The CD is read by the laser beam and is reflected back to a sensor, that converts it into electronic data.

The distance among the tracks, the pinch is 1.6ìm. A CD is read through focusing a 780nm wavelength (close to infrared) semiconductor laser by the bottom of the polycarbonate layer. The alteration in height among pits (in fact ridges as seen through the laser) and lands results in a variation in intensity in the light reflected. Through measuring the intensity change with a photodiode, the data from the disc can be read.

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