Science Fair Project Encyclopedia
In computing and data storage, Compact Disc Rewritable, or CD-RW, is a rewritable version of CD-ROM. Whereas standard prerecorded compact discs have their information permanently stamped into an aluminium reflecting layer, CD-RW discs have a phase-change recording layer and an additional aluminium reflecting layer. A laser beam can melt crystals in the recording layer into a non-crystalline amorphous phase, or anneal them slowly at a lower temperature back to the crystalline state. The different reflectance of the resulting areas make them appear like the 'pits' and 'lands' of a standard CD.
A CD-RW drive can write about 700MiB of data to CD-RW media around 1000 times. The number of times the CD-RW can be re-written varies depending on the quality and production techniques employed. Most CD-RW drives can also write once to CD-R media. Except for the ability to completely erase a disc, CD-RWs act very much like CD-Rs and are subject to the same restrictions; i.e., they can be extended, but not selectively overwritten, and must be closed before they can be read in a normal CD-ROM drive. A variation of UDF formatting allows CD-RWs to be randomly read and written, but limits the capacity to about 500MB.
Note that unlike CD-Rs, CD-RW discs are non-standard, in that they do not meet the Orange Book standards for CDs. Hence CD-RW media cannot be read by CD-ROM drives built prior to 1997 due to the reduced reflectivity (15% compared to 70%) of CD-RW media. CD-RW is also more expensive than CD-R, and so CD-R is sometimes considered a better technology for archival purposes. The write-once nature of CD-Rs also ensures that data cannot be accidentally modified or tampered with, and encourages better archival practices. However, due to the crystalline layer of CD-RWs (as opposed to the organic material used in CD-Rs), disc manufactures claim longer durability and better data safety of CD-RWs.
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