CD-RW, or Compact Disc-ReWritable, is a CD format that provides flexible recording options. This format permits optical data to be written, re-written, read and erased multiple times.
At its conception, it was known as CD-E (CD-Erasable) but was introduced as an extension of the Orange Book standard under the moniker CD-RW in 1997. CD-RWs are designed for computer storage and backup since they can be re-written, but their re-writable format results in a disc with a smaller storage capacity than a write-once CD-R disc.
Its predecessor-the CD-MO launched in 1988-shared similarities in form and function, however did not take off due to a lack of standards and quality along with the introduction of the more cost-effective CD-R.
A CD-RW's basic composition parallels a standard CD, but for its re-write purposes, includes a six-layer design. There's a polycarbonate layer designed with pregrooves for the laser to track. It is then topped by a layer of zinc sulfide and silicone dioxide. Subsequently, a recording layer is placed on top and made with alloyed silver, indium, tellurium and antimony. The next layer is another coat of zinc sulfide and silicone dioxide. These two chemical compounds serve as dielectrics-or electric insulators to remove excess heat from the recording layer. A thin aluminum layer serves as the reflective surface for the burner's laser beam. Each CD-RW disc is finished off with a shiny acrylic top coat.
The mixture of compounds in the CD-RW's recording layer when heated by a laser beam can change into three different states depending on the laser's temperature. Heated to the laser's highest temperature will write data into the recording layer. The laser's second power is high enough to melt any recorded data which erases it, while maintaining the integrity of the layer for future re-writes. At this erase temperature, the layer converts into a crystalline state responsible for reflection. Compatible disc drives also contain a blanking option as a necessary step to re-recording. The lowest laser beam temperature will not alter the layer, but simply read the disc's data.
CD-RW has extremely specific requirements for writing speeds. They operate within a confined speed range because if they're too slow or too fast the laser's high or low energy levels will not properly record data. The reading speed of CD-RW is not related to this issue and depends on what the drive offers.
CD-RW drives are versatile in that they can record data for CD-R and CD-RW. After 1997, CD-ROM drives needed MultiRead support to be compatible with CD-RW.
Be sure to check out our pages on these other CD formats: