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CD-RW (Compact Disc-Rewritable) is adigitaloptical discstorage format introduced by Ricoh in 1997.[1] A CD-RWcompact disc (CD-RWs) can be written, read, erased, and re-written.
CD-RWs, as opposed to CDs, require specialized readers that have sensitive laser optics. Consequently, CD-RWs cannot be read in many CD readers built prior to the introduction of CD-RW. CD-ROM drives with a "MultiRead" certification are compatible.
CD-RWs must be erased or blanked before reuse. Erasure methods include full blanking where the entire surface of the disc is erased and fast blanking where only metadata areas, such asPMA,TOC andpregap, are cleared. Fast blanking is quicker and usually sufficient to allow rewriting the disc. Full blanking removes all traces of the previous data,[2] and is often used for confidentiality purposes.
CD-RWs can sustain fewer re-writes compared to other storage media (ca. 1,000 compared up to 100,000). They are ideally used for test discs (e.g. forCD authoring), temporary backups, and as a middle-ground between online and offline storage schemes.
Before CD-RW technology, in 1990 a standard formagneto-optical recordable and erasable CDs called CD-MO was introduced and set in theOrange Book, part 1 as a CD with a magneto-optical recording layer. The CD-MO standard allowed for an optional non-erasable zone on the disc that could be read by CD-ROM units.
Data recording (and erasing) was achieved by heating the magneto-optical layer's material (e.g.DyFeCo or less oftenTbFeCo orGdFeCo) to itsCurie point and then using a magnetic field to write the new data, in a manner essentially identical to Sony'sMiniDisc and othermagneto-optical formats. Reading the discs relied on theKerr effect - a major format flaw. The rewrite could only be read in special drives and was incompatible with non-magneto-optical enabled drives. The format was never released commercially,[3] mostly because of incompatibility with standard CD reading units. EarlyCD-R media contained a similar compatibility flaw.
Since the CD-MO was otherwise identical to CDs, the format still adopted a spiral-groove recording scheme, rendering the disc poorly suited as a removable medium for repeated, small-scale deletions and recordings. Somemagneto-optical drives and media with the same form factor don't have this limitation. Unlike modern CD-RWs, CD-MO allowed for hybrid discs containing both an unmodifiable, pressed section, readable in standard drives, and a writable MO section.
The early introduction and no standards for disc recording software, file systems, and formats, physical incompatibility, coupled with more economical CD-R discs, led to abandoning the format.[4][5] Other magneto-optical media, unbound by limitations of the typical CD-ROM filesystems, replaced the CD-MO.
Rewritable media can, with suitable hardware, be re-written up to100,000 times. The CD-RW is based onphase change technology, with a degree of reflection at15–25%,[6] compared to40–70% for CD-R discs.[6] The properties of the medium and the write and erase procedure is defined in theOrange Book Part III.
To maintain a precise rotation speed, tracks have a slight superimposedsinusoidal excursion of0.3µm at a frequency of22.05 kHz.[6] In addition a1 kHzfrequency modulation is applied to provide the recorder with an absolute time reference.[6] Groove width is0.6 μm and pitch of1.6 μm.[6]
The media for CD-RW has the same layers as CD-R media. The reflective layer is, however, asilver-indium-antimony-tellurium (AgInSbTe) alloy with apolycrystalline structure andreflective properties in its original state. When writing the laser beam uses its maximum power (8-14 mW)[6] to heat the material to500–700 °C causing materialliquefaction. In this state, the alloy loses its polycrystalline structure and reflectivity and assumes anamorphous state. The lost reflectivity serves the same function as bumps on manufactured CDs.[7] The polycrystalline state of the disc forms the trenches.[7] The scanning signal when reading is created by strong or weak reflection of the laser beam. To erase the disc, the write beam heats the amorphous regions with low power to about200 °C. The alloy is not melted, but returns to the polycrystalline state and is again reflective.
During and after a disc authoring the distribution of data on the CD-RW varies. The following areas are present:
Each session on a multi-session disc has a corresponding lead-in, PMA, PA and lead-out. When the session is closed TOC information in the PMA is written into a lead-in area and the PCA and PMA are logically eliminated. The lead-out is created to mark the end of the data in the session.
| Specification[9] | Logo | Speed |
|---|---|---|
| (Original, "slow") | | 1×, 2×, 4× |
| High Speed |  | 8×, 10×, 12× |
| Ultra Speed |  | 16×, 20×, 24× |
| Ultra Speed+ |  | 32× |
Like a CD-R, a CD-RW has hardcoded speed specifications which limitrecording speeds to fairly restrictive ranges. Unlike a CD-R, a CD-RW has aminimum writing speed under which the discs cannot be recorded, based on the phase change material's heating and cooling time constants and the requiredlaser energy levels. Despite this, some professional audio CD recorders, such as those made by Tascam, use special techniques to bypass these limitations and can record high speed (but not ultra speed) discs in realtime.[citation needed]
Since the CD-RW discs need to be blanked before recording data, writing too slowly or with too low energy on a high speed unblanked disc will cause the phase change layer to cool before blanking is achieved, preventing the data from being properly written.
Similarly, using inappropriately high amounts of laser energy will cause the material to overheat and beinsensitive to the data, a situation typical of slower discs used in a high powered and fast specification drive.[citation needed]
For these reasons, older CD-RW drives that lack appropriate firmware and hardware are not compatible with newer, high-speed CD-RW discs, while newer drives can record to older CD-RW discs, provided their firmware correct speed, delay, and power settings can be appropriately set.
The actual reading speed of CD-RW discs, however, is not directly correlated or bound to speed specification, but depends primarily on the reading drive's capabilities.
Manyhalf-height CD and DVD writers released between 2004 and 2010, including theTSSTcorp SH-M522combo drive (2004),PioneerDVR-110D (2005),[10]Hitachi-LGGSA-4167 (2005)[11], TSSTcorpSH-S182/S183 (2006) andSH-S203/TS-H653B (2007) have officially adapted support for CD-RWUltraSpeed Plus (32×Z-CLV), while more recent DVD writers such as theSH-224DB (2013) and Blu-Ray writers such as theLG BE16NU50 (2016) have downgraded thebackwards compatibility to CD-RWUltraSpeed (24× Z-CLV).[12][13]
Slim type optical drives are subject to physical limitations, thus are not able to attain rotation speeds of half-height (desktop) optical drives. They usually support CD-RW writing speeds of 16×[14][15][16] or 24× Z-CLV in zones of 10× CLV, 16× CLV, 20× CLV and 24× CLV towards the outer edge, of which the highest speed zone depends on availability.[12][13][17][18]
CD Recordable and CD ReWriteable discs have the same basic structure but with significant detailed differences. The CD-R has a dye based recording layer, with a reflectivity of 40-70 %, while the CD-RW has a phase change recording layer with a reflectivity of 15-25 %. Both have an additional reflecting layer (gold) for the CD-R and (silver) for the CD-RW.
{{cite web}}: CS1 maint: bot: original URL status unknown (link)Rewrite Speed: 16x (CD)
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