We know that the optical storage media is divided into CD-rom,CD-r and CD-RW which work differently .
Optical storage media
Optical storage devices, such as CD and DVD, all use optical (i.e. light) read/write
methods, unlike tapes and floppy/hard drive disks which are magnetic media. A laser
beam is used to write to and read from the optical media.
The CDs and DVDs are manufactured either from a single polycarbonate disk or
from two polycarbonate disks bonded together. A very thin layer of metal or organic
dye is used as the recording media.
While in CD-ROMS and DVD roms the data is stored as a series of pits
(equivalent to a binary value of 1) and lands (equivalent to the binary value of 0) in
the metallic optical layer. The pits are formed by a laser beam etching the surface at
the manufacturing stage. Only a single track exists which spirals out from the centre
of the disk.
The pits and lands are read by a low-powered laser beam which follows the data
stream and reads from the centre outwards in a spiral. The light reflects differently
off a pit than it does off a land and this is interpreted as 1s and 0s (i.e. data) – hence
the term digital media.
In CD-R or DVD-R a thin layer of an organic dye as the recording
media; DVDs also use an additional silver alloy or gold reflector. A laser beam
produces heated spots and unheated spots. On reading the disk, a laser beam is
capable of distinguishing between the two types of spots and effectively reads the
data stream from the centre outwards in a spiral action. This data is then interpreted
as 1s and 0s.
in CD-RW or DVD-RW the recording
layer uses a special phase-changing metal alloy. The alloy can switch between
crystalline and amorphous (non-crystalline) phases, thus changing its reflectivity to
light, depending on the laser beam power. Spots are produced which can be read by
a laser and then interpreted as 1s and 0s. The system allows data to be written,
erased and re-written many times.
Source : IGCSE Information and Communication technology book
