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How does a CD-R write?
CD-R drives focus a laser beam through the
polycarbonate base onto the organic dye layer. When the dye is heated by the laser during
the writing phase, a physical change occurs in the dye. The laser burns the dye and
creates tiny mounds. Although these are physically different from the pits in manufactured
CDs, they are still called "pits" by those in the industry so as not to confuse
everyone.
The "pits" alter the reflectiveness
of the gold layer. To a CD player or CD-ROM drive, they appear like the stamped pits in a
standard CD-ROM. This is a reason why CD-R media can be read by nearly any CD player or
CD-ROM drive.
How does a CD-R read?
Actually, CD-R drives read data in exactly the same
way as CD players and drives do. The laser beam is focused onto the reflective surface of
the disc. Changes in the reflectiveness of the surface, caused by the pits and lands, are
read as binary "ones" and "zeros." These pits are recorded along a
track that spirals from the inside edge of the disc outward.
How do they squeeze so much
information into a CD?
Whereas all drives -- whether they are magnetic or optical
-- contain data in tracks, how they spin the disk makes a big difference between
performance and capacity. Magnetic disks spin at a Constant Angular Velocity (CAV). This
means that the disk moves faster on the outer tracks than the inner tracks. Therefore, the
density of the data is less in the outer tracks, resulting in smaller capacities. A
phonograph record is an example of a CAV format with a spiral track.
Compact discs spin at a Constant Linear Velocity
(CLV). This means that the disc spins faster when the laser beam is focused on the inner
tracks than when it is on the outer tracks. The advantage of this format is that more
information can be stored in the outer tracks of the disc.
The weakness of this scheme is an increase in the time the drive
needs to locate your data. This is because when the drive moves the laser beam from the
inner to the outer tracks, the system has to wait until the disc slows down to maintain a
constant speed in the direction of the track.
The laser beam of a CD-R drive is extremely narrow -- less than
one micron in diameter -- so it burns a pit only 0.6 microns wide. These pits are densely
packed onto a track in a spiral form like those on a phonograph record. The distance
between two tracks is 1.6 microns. To get an idea how small this is, the width of a human
hair would cover about 50 tracks.
Because of this format, bits of data are much more densely spaced
on a compact disc than those on magnetic disks. For this reason, compact discs have 7 to
10 times more tracks than their magnetic hard disk cousins. In fact, CDs have capacities
of up to 650 MB on a 120mm (5.25 inches) disc.
How about CD-RW?
CD-RW disks have a much lower reflectivity than CD-R or
pressed CD-ROM disks and thus cannot be read in the older CD players (though new multiread
CD-ROM players, with lasers that can adjust to read CD-RW disks are able to do so) Also,
CD-RW media are more expensive than the CD-R media. Theoreticaly, you can have about 1,000
rewrites on a CD-RW disc.
Special Thanks to Mr
Michael Tan of Convergent Systems
for the provision of the Yamaha CD-RW |
