An Overview of CD-RW Technology
Today's CD-RW drives enable end-users to create CD's almost
as effortlessly as copying floppies. This ease of use comes
with plenty of practical value, including the ability to harness
the high capacity disks for rewriteable mass storage, copying
CD-ROMs, or compiling customized audio CD's. In addition,
the ability to randomly erase and rewrite data to the same
disc as many as 1,000 times makes CD-RW considerably more
appealing than earlier CD-Recordable (CD-R).
CD-RW drives are dual-function, offering both CD-R and CD-RW
recording, so users can choose which recordable media is going
to be the best for a particular job. Another advantage of
CD-RW drives is its ability to read nearly all of the existing
variations of CD-ROMs. CD-RW also offers an excellent value
- CD-RW media cost as little as $2 per disk for 650MB of storage
and the price of the drives themselves is hundreds of dollars
less than that of the early CD-R units. Also, improvements
in recording software have made archiving files as easy as
dragging and dropping them onto the CD-RW drive icon in Windows
Explorer.
CD-RW technology uses optical phase change technology, but
does not incorporate magnetic fields like the phase change
technology used with magneto optical technology. A CD-RW disc's
phase-change medium consists of a polycarbonate substrate,
molded with a spiral groove and other data, onto which a stack
(usually five layers) is deposited. The recording layer is
sandwiched between dielectric layers that draw excess heat
from the phase-change layer during the writing process. In
place of the CD-R discs dye-based recording layer, CD-RW commonly
uses a crystalline compound composed of silver, indium, antimony
and tellurium. This rather exotic mix has a very special property:
when heated to one temperature and cooled it becomes crystalline,
but if heated to a higher temperature, it becomes amorphous
upon cooling. The crystalline areas allow the metalisized
layer to reflect the laser better while the non-crystalline
portion absorbs the laser beam, so it is not reflected.
Normal music CDs and CD-ROMs are made from pre-pressed discs
and encased in plastic. The actual data is stored through
pits, or tiny indentations, on the silver surface of the internal
disc. To read the disc, the CD-ROM shines a laser onto the
surface, and by interpreting the way in which the laser light
is reflected from the disc it can tell whether the area under
the laser is indented or not.
Thanks to sophisticated laser focusing and error detection
routines, this process is pretty much ideal. However, there’s
no way the laser can change the indentations of the silver
disc, which in turn means there’s no way of writing new
data to the disc once its been created. Thus, the technological
developments to enable CD-ROMs to be written or rewritten
to have necessitated changes to the disc media as well as
to the read/write mechanisms in the associated CD-R and CD-RW
drives.
At the start of 1997 it appeared likely that CD-R and CD-RW
drives would be superseded by DVD technology almost before
they had got off the ground. In the event, during that year
DVD Forum members turned on each other triggering a DVD standards
war and delaying product shipment. Consequently, the writable
and rewritable CD formats were given a new lease of life.
For professional users, developers, small businesses, presenters,
multimedia designers and home recording artists the recordable
CD formats offer a range of powerful storage applications.
Their big advantage over alternative removable storage technologies
such as MO, LIMDOW and PD is that of CD media compatibility;
CD-R and CD-RW drives can read nearly all the existing flavours
of CD-ROMs and discs made by CD-R and CD-RW devices can be
read on both (MultiRead-capable) CD-ROM drives and current
and all future generations of DVD-ROM drive. A further advantage,
itself a consequence of their wide compatibility, is the low
cost of media; CD-RW media is cheap and CD-R media even cheaper.
Their principal disadvantage is that there are limitations
to their rewriteability; CD-R, of course, isn't rewritable
at all and until recently CD-RW discs had to be reformatted
to recover the space taken by 'deleted' files when a disc
becomes full, unlike the competing technologies which all
offer true drag-and-drop functionality with no such limitation.
Even now, however, CD-RW rewriteability is less than perfect,
resulting in a reduction of a CD-RW disc's storage capacity.
