We were recently asked to differentiate CD-R and CD-RW. Both systems can be used to "write" or record onto CD-ROM. The difference is that CD-R creates a CD-ROM disk which can only be read, but CD-RW creates one which can be erased and rewritten as many as 1,000 times. The reason why the two media co-exist is that a CD-R blank disk costs approximately $5 while a CD-RW blank disk is priced at around $25.
While media prices are expected to come down, there probably will continue to be a difference between the two media. A CD-RW disk has erasable heat-sensitive dielectric layers, instead of a single dye layer. Using a variable-power laser, a CD-RW drive writes pseudopits using relatively high power and erases it with somewhat lower power. As the CD-RW laser writes onto a blank disk, it changes the chemical nature of the disk's recording substrate from crystalline to amorphous. When played back, the crystalline areas reflect the playback laser's beam; the amorphous areas scatter the playback laser's beam, like a physical pit of a mass-replicated stamped CD. The much greater complexity of the CD-RW medium means that a cost differential will continue to exist for some time.
CD-R and CD-RW both are quite stable. The information will be retrievable for at least 20 to SO years even if a disk is used regularly.
A CD-R disk is truly portable. It can be read using any CD-ROM drive. A CD-RW disk can only be read on a CD-RW drive. However, a CD-RW drive is multi-read-capable, meaning that it can read 41 formats of CD-ROM. A CD-RW drive costs two to three times as much as a conventional CD-ROM drive.
A library wishing to record information on CDs which is to be retained for only a few months should consider CD-RW technology, but only after determining that the savings realized by reusing the media is not cancelled out by the additional costs of the CD-RW drives and the CD-RW blank disks.
One of the best sources for information about CD-R and CD-RW technology is the magazine New Media, a monthly published by HyperMedia Communications.
[Contact: www.newmedia.com ]