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CD-RW Media and Encoding
CD-RW media are more similar to CD-R media than they are to regular CD-ROMs. CD-R media replaces the physically molded pits in the surface of the disk with a sensitive dye layer that can be burned to simulate how a standard CD works. CD-R media technology is described here; it makes sense to understand it before continuing in this section.
CD-RW media are formed in the same basic way that CD-R media are; they start with a polycarbonate base and a molded spiral pre-groove to provide a base for recording. There are several layers applied to the surface of the disk, with one of them being the recording layer where ones and zeroes are encoded. The recording layer for CD-RW is different of course than it is for CD-R. The problem with CD-R is that the dye layer used is permanently changed during the writing process, which prevents rewriting.
CD-RW media replaces this dye with a special phase-change recording layer, comprised of a specific chemical compound that can change states when energy is applied to it, and can also change back again. Much the way water can change to steam, or ice, depending on its temperature, there are some types of chemicals that can not only change their state after having heat or other conditions applied, but even retain that state when the heat is removed. They can later be returned to their original state through another, different process.
The material used in CD-RW disks has the property that when it is heated to one temperature and then cooled, it will crystallize, while if it is heated to a higher temperature and then cools, it will form a non-crystalline structure when cooled. (Many metals are like this; in fact, different types of iron are formed by controlled heating and cooling to modify its internal structure).
When the material is crystalline, it reflects more light than when it doesn't; so in the crystalline state it is like a "land" and in the non-crystalline state, a "pit". By using two different laser power settings, it is possible to change the material from one state to another, allowing the rewriting of the disk. The change of phase at each point on the disk's spiral is what encodes ones and zeros into the disk. The spiral and other structures are the same as for CD-R; what changes is how the pits are encoded.
CD-RW media have one very important drawback: they don't emulate the pits and lands of a regular CD as well as the dye layer of a regular CD-R, and therefore, they are not backward compatible to all regular audio CD players and CD-ROM drives. Also, the fact that they are written multiple times means that they are multi-session disks by definition, and so are not compatible with non-multi-session-capable drives. These issues are discussed in more detail here.