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A CD-RW (Compact Disc-ReWritable) is a rewritable optical disc. It was introduced in 1997, and was known as "CD-Writable" during development. It was preceded by the CD-MO, which was never commercially released.
CD-RW discs require more sensitive laser optics. Also, CD-RWs cannot be read in some CD-ROM drives built prior to 1997. CD-ROM drives will bear a "MultiRead" certification to show compatibility. 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 is much quicker, and is usually sufficient to allow rewriting the disc. Full blanking removes traces of the former data, often for confidentiality. It may be possible to recover data from full-blanked CD-RWs with specialty data recovery equipment; however, this is generally not used except by government agencies due to cost.
CD-RW also have a shorter rewriting cycles life (ca. 1,000) compared to virtually all of the previously exposed types storage of media (typically well above 10,000 or even 100,000), 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 1990 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 (e.g. DyFeCo or less often TbFeCo 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 essentially identical to Sony's MiniDisc and other magneto-optical formats. Reading of the discs relied on the Kerr effect. This was also the first major flaw of this format: it could be read in only 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 was never released commercially, mostly because of its inherent incompatibility with standard CD reading units. A similar situation was also present for early CD-R media, which suffered from either physical or logical incompatibilities.
Since the CD-MO was otherwise physically identical to "normal" CDs, it still adopted their spiral-groove recording scheme, which would have rendered it hard to use as a removable medium for repeated, small scale deletions and recordings (not unlike CD-RW). There were (and are) however some magneto-optical drives and media with the same form factor that don't have this limitation. Unlike modern CD-RWs, CD-MO allowed for hybrid disks containing both an unmodifiable, pressed section, readable in standard drives and a writable MO section.
This early introduction along with the lack of standards for disk recording software, file systems and formats, physical incompatibility as well as the introduction of the more economical CD-R disks essentially caused the format to be abandoned before commercialization, and the whole idea of a rewritable CD medium to be almost forgotten until modern phase change CD-RWs appeared. Other kinds of magneto-optical media, unbound by the limitations of the typical CD-ROM filesystems, took the place intended for CD-MO.
Rewritable media can, with suitable optical drive according to some manufacturers, be re-written up to 100 000 times. The CD-RW technology is based on the phase change technology, so the degree of reflection reached is only 15–25%, compared to the 40–70% reflection from CD-R discs. The properties of the medium and the write and erase procedure is defined in the Orange Book Part III.
To keep rotational speed precise any track have a slight superimposed sinusoidal excursion of 0.3 µm at a frequency of 22.05 kHz. In addition an 1 kHz frequency modulation is applied to provide the recorder with an absolute time reference. The grooves have a width of 0.6 µm and pitch of 1.6 µm.
The media for CD-RW has basically the same layers as CD-R media. The reflective layer is, however, a silver-indium-antimony-tellurium (AgInSbTe) alloy, which has in its original state, a polycrystalline structure and reflective properties. When writing the laser beam uses its maximum power (8 - 14 mW) to heat the material to 500–700 °C. This causes liquefaction of the material. In this state, the alloy loses its polycrystalline structure, assumes an amorphous state and lose its reflectivity. The lost reflectivity serves the same function as bumps on a manufactured CDs and the opaque spots on a CD-R which will be read as a "0". The polycrystalline state of the disk forms the trenches, which is read as "1". The scanning signal when reading is created by strong or weak reflection of the laser beam (not unlike the destructive interference of light caused by and used to read "pits" in pressed CD-ROMs). To erase the disc, the write beam heats the amorphous regions with low power to about 200 °C. The alloy is not melted, but returns to the polycrystalline state and is thus again reflective.
Like CD-R, CD-RW have hardcoded speed specifications 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 discs 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 disc 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 discs used in a higher powered faster specification drive.
For these reasons, in general older CD-RW drives lacking appropriate firmware and hardware cannot handle newer, high speed CD-RW discs (poor forward compatibility), while newer drives can generally record to older CD-RW discs, 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 bound to its speed specification, but depends first and foremost on the reading drive's capabilities, as with CD-R discs.