• The beginning of CD and CD standards.
  
• Formats
• What is CD-R and CD-RW?
• CD-R & DVD Glossary
• CD-RW FAQ
• Copy Protection Standards
• Audio Formats Explained
• Buffer Underrun Preventer Guide

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.