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CD-R

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A Simple jQuery Image Rollover What is a CD-R:  
Compact Disc Recordable (CD-R)  or  Write Once Read Mandy (WORM )) is an optical storage medium. 
Primary use of CR-R  is  for music and  digital  data.

What is a CD-R  made of:
 

Physical make of a CD-R  is combination of  four or five layers of material.
1.  The base plate of a CD is made from 1.2 mm thick polycarbonate plastic.
2.   Photosensitive Dye to make it writable.
.dyes used (Cyanine, PhthaloCyanine,  Metallized Azo, Formazon)
3.   A thin layer of aluminium, silver, or gold is applied to the surface to make it reflective. 
4.   Thin film of lacquer is applied for protection. 
5.   Scratch resistant protective layer ( Not all the CDs have this layer). 

                                                How it is made



Tracks

The surface of the base plate has a grooved spiral starting from the center to the outer
 edge of the plate. The spiral grooves called tracks.   These tracks guide the laser beam
on the CD drive when reading from a CD,  or writing to a CD.  The width of the track is
about 0.5 micron and the spacing  between the tracks are about 1.6 microns an there are
over 22000 tracks on a CD (a micron is 1 millionth of a meter).




Sectors

The tracks are divided into smaller uniform size, called sectors, which are listed in the disc's
 Table Of Contents (TOC). In Audio CD every sector contains 3234 bytes of data, 882 bytes of which are
 reserved for error detection and correction code and control bytes, leaving 2352 bytes for data
. Total length of spiral track is about 5680 meters (3.55 miles).
 Computer CDs use only 2048 (2K) to store user data, and remainig 304 bytes used for control data.
the minimum recordable unit on a compact disc is the logical block. In theory a logical block could be
 512, 1024 or 2048 bytes in size (that is , you could fit 1, 2, or 4 logical blocks into a sector).




Grooves

This is the cross section of a CD-R viewing from the edge showing the magnified tracks which
 are grooved on the base plate.





Magnified image of grooves of an actual CD




Pits & Lands

In a pressed or Mass-Replicated CDs the data are recorded in the form of pits and lands
. The lands represent “1” and the pits represent “0” in binary computing. But it is not simply
 so that a land is a "1" data bit, and a pit is a "0" data bit. On an optical disc there are no
 data-bits but channel-bits. A channel bit is the smallest time unit used on a disc, for a CD it
 equals 1/4,321,800 sec. a "1" channel bit is a time with change from land to pit, or from pit to land.
 a "0" channel bit is a time when there is no change.



Melted dye

On CD-R discs do not have true pits and lands, but the un-melted clear areas and melted dark areas.
 To write data onto a disc, the optical drive uses laser beam to make a series of tiny marks in the dye.
 The resulting sequence of light and dark spots (pits and lands) represent the digital ones and zeros.
 The stored data are read by the detection of the reflected light. The optical drive synchronizes read
 frequency and rotation speed.