CD - Compact Disc
The Compact Disc popularly known as CD is the data storage
device used to store and retrieve datas encodedin digital format. This
optical disc is made of a special polycarbonate plastic known as Polymethyle
Meta Acrylic. Laser beam generating from the Laser diode of the CD drive
is used to read and write the CD. Using the 780 nanometer near infrared
Laser beam, the data can be stored in the Bits and Pits of the CD. Based
on the Reflection and Scattering of the Laser light, The Optoelectronic
tracking module in the CD drive retrieve the data.
The CD has a thickness of 1.2 mm and weighs 15-20 grams. Its reflecting
side is coated with a thin layer of Aluminium and protected with a layer
of Lacquer. Standard CD has a diameter of 120 mm and can store 700 MB(megabytes)
uncompressed data. This can run around 80 minutes. Mini CD and Digital
Video CD are also available to suit the requirements of the user.
CD may be Recordable CD and Rewritable CD.
Recordable CD
It is injection molded with a "blank" data spiral.
A photosensitive dye is then applied, after which the discs are Metalized
and lacquer-coated. The write laser of the CD Recorder changes the color
of the dye to allow the read laser of a standard CD Player to see the
data, just as it would with a standard stamped disc. The resulting discs
can be read by most CD-ROM drives and played in most audio CD players.
Re-Writable CD
Rewritable CD allows "Write and Erase" functions
so that it is easy to re-use it. Unlike the readable CD, the Rewritable
CD has a Phase change compound coated in its reflective surface. This
phase change substance is the compound of silver, antimony, tellurium
and indium. This compound changes its physical state on heating depending
on the temperature applied. When the temperature rises above its melting
point (around 600 degree) it changes to liquid form and in its crystallization
point (around 200 degree) it becomes solid. Unlike the ordinary CD, in
Rewritable CD, the bumps are represented by the phase changes in the compound.
When it is in the 'Crystalline form', it stays translucent so that laser
light can reflect back. When the compound becomes 'Amorphous' due melting,
it becomes opaque and laser light will not reflect back. These changes
during melting can 'lock' the phase change in place.
The erasing process changes the crystalline and amorphous states of the
compound through melting. The high temperature from the Laser beam will
change the states of the compound so that the data in the form of translucent
and opaque areas in the reflective layer will be erased. During writing,
the high power laser beam melts points corresponding to the bumps of the
conventional CD. They block the read laser so that it will not reflect.
Capacity
The CD can store large quantity of data as series of tiny
indentations known as "pits", encoded in a spiral track molded
into the top of the polycarbonate layer. The areas between pits are known
as "lands". Each pit is approximately 100 micrometer deep by
500 nm wide, and varies from 850 nm to 3.5 micrometer in length. CD-ROM
capacities are normally expressed with binary prefixes, subtracting the
space used for error correction data. A standard 120 mm, 700 MB CD-ROM
can actually hold about 737 M. Scanning velocity of the CD is 1.2-1.4
m/s equivalent to approximately 500 rpm at the inside of the disc, and
approximately 200 rpm at the outside edge. (A disc played from beginning
to end slows down during playback).
CD Reading
Laser technology is used to read the optically recorded
data in the form of Bits and Pits on a CD. CD's recording surface has
about 20000 or more tracks. The distance between the tracks, the pitch,
is 1.6 ¥ìm. A CD is read by focusing a 780 nm wavelength(Near
Infrared)Semiconductor Laser through the bottom of the polycarbonate layer.
The change in height between pits and lands results in a difference in
intensity in the light reflected. The data can be read from the disc by
measuring the intensity change with a Photodiode. The digital information
is stored and varied as the length of pits and distance between them.
The pits and reflective surface represents logic 0 and logic 1. The pits
and lands themselves do not directly represent the zeros and ones of Binary
Data. Instead, Non returned to zero Inverted (NRZI) encoding is used:
a change from pit to land or land to pit indicates a one, while no change
indicates a series of zeros. There must be at least two and no more than
ten zeros between each one, which is defined by the pit length..
Burn Speed
The CD burn speed is adjustable. The CD burner can write
at multiple speeds. 1x is the writing speed closest to the reading speed.
That is, it takes 60 minutes to write 60 minute duration data. At 2x rate,
the burner will take a half of that, 30 minute to write 60 minutes data.
The general writing speeds are 8x, 16x, 24x, 32x etc. When writing Music
files, it is recommended to avoid writing CD in high speed because it
might cause some data loss.The normal writing speed is 16x or 24x. Produplicator
CD Duplicator provides Burning speeds up to 52x on CD-R, which can take
about 3 to 4 minutes to burn a full CD disc.
CD Damage
CDs are easily damaged from both daily use and environmental
exposure. 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,
CDs have more scratch damage on the label side whereas scratches on the
clear side can be repaired by refilling them with similar refractive plastic,
or by careful polishing.
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