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Compact Disc ReWritable (CD-RW) is a rewritable optical disc format. Known as CD-Erasable (CD-E) during its development, CD-RW was introduced in 1997, and was preceded by the failed CD-MO in the 1988.

How does CD Rewriting work ?
While a prerecorded compact disc has its information permanently stamped into its polycarbonate plastic substrate, a CD-RW disc contains a phase-change alloy recording layer composed of a phase change material, most often AgInSbTe, an alloy of silver, indium, antimony and tellurium. An infra-red laser beam is employed to selectively heat and melt the crystallized recording layer into an amorphous state or to anneal it at a lower temperature back to its crystalline state. The different reflectance of the resulting areas make them appear like the pits and lands of a prerecorded CD.

A CD-RW recorder can rewrite 700 MB of data to a CD-RW and a CD-RW disc can be written roughly 1000 times. CD-RW recorders can also write CD-R discs. 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 writing sessions must be closed before they can be read in CD-ROM drive or players. The UDF 1.5 file system allows CD-RWs to be randomly rewritten, but limits disc storage capacity to roughly 530MB.

Written CD-RW discs do not meet Red Book or Orange Book Part II standards for prerecorded or recordable CDs (e.g. reduced signal levels). CD-RWs are also considerably more opaque than CD-Rs and commercially pressed discs, requiring more sensitive laser optics. Consequently, CD-RWs cannot be read in CD-ROM drives built prior to 1997. CD-R is considered a better technology for archival purposes as disc contents cannot be modified and manufacturers claim greater longevity.

CD-RW discs need to be blanked before reuse. Different blanking methods can be used, including 'full' blanking in which the entire surface of the disc is cleared, and 'fast' blanking in which only meta-data areas are cleared: PMA, TOC and pregap, comprising a few percent of the disc. Fast blanking will obviously be much quicker, and is usually sufficient to allow rewriting the disc. Full blanking removes traces of the former data, often for confidentiality. Data from 'fast' blanked CD-RW discs can be recovered by some software like the Linux version of PhotoRec.

Uses and limitations
CD-RW discs never gained the widespread popularity of CD-R, partly due to their higher per-unit price, lower recording and reading speeds, and compatibility issues with CD reading units, as well as between CD-RW formats of different speeds specifications.

Also, compared to other forms of rewritable media such as Zip drives, Jaz drives, Magneto-optical and flash memory based media, the CD-RW format uses the standard CD-ROM and CD-R file systems and storage strategies, which are inherently not suitable for repeated small-scale file additions and deletions, but rather for medium to large scale, single operation, cumulative archiving, thus making the use of CD-RW as a true removable disk impractical.

CD-RW also have a shorter rewriting cycles life (ca. 1000) compared to virtually all of the previously exposed types storage of media (typically well above 10000 or even 100000), something which however is less of a drawback considering that CD-RWs are usually written and erased in their totality, and not with repeated small scale changes, so normally wear leveling is not an issue.

Their ideal usage field is in the creation of test disks, temporary short or mid-term backups, and in general, where an intermediate solution between online and offline storage schemes is required.

Prior to the introduction of the CD-RW technology, a standard for magneto-optical recordable and erasable CDs called CD-MO was introduced in 1988 and set in the Orange Book, part 1, and was basically a CD with a magneto-optical recording layer. The CD-MO standard also allowed for an optional non-erasable zone on the disk, which could be read by normal CD-ROM reader units.

Data recording (and erasing) was achieved by heating the magneto-optical layer's material (eg. TbFeCo, DyFeCo to a very high degree, or GdFeCo) up to its Curie point thus erasing all previous data and then using a magnetic field to write the new data, in a manner very similar to Sony's MiniDisc. Reading of the discs relied on the Kerr effect. This was also the first major flaw of this format: it could only be read in special drives and was physically incompatible with non magneto-optical enabled drives, in a much more radical way than the later CD-RWs.

The format never caught on commercially, as it was mostly marketed as a replacement for tape backup devices, intended for use in bulky, usually external proprietary drives (each company made its own device) which very frequently used proprietary data backup software and proprietary, non-standard recording formats (even at a physical level) and file systems.

Recording speed was, also, low, as it was typical for a magneto-optical device of this era, meeting CD 1x or 2x speed at best, if not less.

These combined factors rendered the disks unreadable on standard CD drives or on other similar devices, or even on the same device without the specific backup software. A similar situation was also present for early CD-WORM media, which suffered from massive standarization problems.

Also, since the CD-MO was otherwise physically identical to 'normal' CDs, it still adopted a spiral-groove recording scheme, which rendered it hard to use as a normal floppy or as a medium for repeated, small scale deletions and recordings. There were (and are) however some magneto-optical drives and media with the same form factor that don't have this limitation.

This early introduction along with the lack of standards for software, file systems and disks, low recording speeds, physical incompatibility, along with the cost of the recording devices and the disks themselves back in the early 1990s, as well as the introduction of the relatively more economical CD-R disks and (especially) faster and more compact drives restricted the CD-MO to niche markets, and the format was almost forgotten, being essentially replaced by phase change CD-RW and other, better specified magneto-optical media. As of 2006, it is very hard to find actual CD-MO record-capable drives or even CD-MO disks.

The speed of CD-RW writing
Like CD-R, CD-RW have hardcoded speed specs which limit the allowable recording speeds to certain fairly restrictive ranges, but unlike the former they also have a minimum writing speed under which the disks cannot be reliably recorded, something dictated by the phase change material's heating and cooling time constants, and the required laser energy levels.

Since the CD-RW disks need to be blanked either entirely or 'on the fly' before recording actual data, writing too slowly or with too low energy on a high speed unblanked disk will cause the phase change layer to cool off before blanking has been achieved, preventing the actual data from being reliably written.

Similarly, using inappropriately high amounts of laser energy will cause the material to get overheated and become 'insensitive' to the actual data, a situation which is typical of slower disks used in a higher powered faster spec drive.

For these reasons, in general older CD-RW drives lacking appropriate firmware and hardware cannot handle newer, high speed CD-RW disks (poor forward compatibility), while newer drives can generally record to older CD-RW disks, provided their firmware can set the correct speed, delay and power settings for the task.

The actual reading speed of CD-RW disks however is not directly correlated or binded to its speed spec, but depends first and foremost on the reading drive's capabilities, like what happens with CD-R disks.

Wikipedia information about CD-RW
. This article is licensed under the GNU Free Documentation License . It uses material from the Wikipedia article 'CD-RW'

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