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[ The PC Guide | Systems and Components Reference Guide | CD-ROM Drives | Recordable CD (CD-R) ]

CD-R Media and Encoding

The reason that regular CDs have never been able to write CD media is that the ones and zeros are encoded using physical changes to the disk: pits that are physically etched into the plastic substrate. CD-R technology was faced with the difficult task of finding a way to conveniently create these pits, without the special equipment required when mastering CDs (which can etch the pits, but costs a lot of money).

CD-R media addresses this issue by doing away with pits and lands entirely and using a different kind of media. CD-R media starts with a polycarbonate substrate, just like regular CDs do. Instead of physical etching this substrate, it is stamped with a spiral pre-groove, similar to the spiral found on a regular CD except that it is intentionally "wobbled". This groove is what the CD-R drive uses to follow the data path of the disk during recording. If the disk were totally blank then writing the spiral tracks would be a very complex process; recall that they are spaced only 1.6 microns apart.

On top of the polycarbonate, a special photosensitive dye layer is deposited; on top of that a metal reflective layer is applied (such as a gold or silver alloy) and then finally, a plastic protective layer. It is these different layers that give CD-R media their different visual appearance from regular CDs.

The key to the media is the dye layer (and the special laser used in the drives.) It is chosen so that it has the property that when light from a specific type and intensity of laser is applied to it, it heats up rapidly and changes its chemical composition. (When folks talk about creating a CD-R as "burning" a disk, they're basically correct.) As a result of this change in chemical composition, the area "burned" reflects less light than the areas that do not have the laser applied. This system is designed to mimic the way light reflects cleanly off a "land" on a regular CD, but is scattered by a "pit", so an entire disk is created from burned and non-burned areas, just like how a regular CD is created from pits and lands. The result is that the created CD media will play in regular CD players as if it were a regular CD, in most cases.

Since the media is being physically altered by a process of heat and chemistry, the change is permanent and irreversible. Once any part of the CD has been written, the data is there forever. Some drives allow you to record some information in one sitting, and then more information later on, if the disk is not yet full. This is called multi-session recording, and requires a CD player capable of recognizing multi-session disks in order to use the burned disk.

CD-R media are generally 74 minutes in length. There are some disks with less capacity, and some with more, but all 74-minute disks have the same capacity, about 650 MB. Some media seems to work better than others in some drives. The cost of the media has been on a steady decline as the technology matures, and at under $5 each the disks are now a very reasonable medium for archival and long-term storage. I personally think they are not an appropriate avenue for backup, given their write-once nature.

Next: CD-R Drives


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