CD-RW
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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 never officially released CD-MO in 1988.
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[edit] Explanation
While a prerecorded compact disc has its information permanently written onto its polycarbonate surface, 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[1]. An infra-red laser beam is used to selectively heat and melt, at 400 degrees (Celsius), 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.
CD-RW discs are usually produced in the most common CD-R disc capacities such as 650 and 700 MB, while smaller and larger capacities are rarer. CD-RW recorders typically handle the most common capacities best. In theory a CD-RW disc can be written and erased roughly 1000 times[2], although in practice this number is much lower. CD-RW recorders can also read CD-R discs. When used with traditional recording software, 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 drives or players. One important difference between R and RW media is that the re-crystallized alloy of RW media will gradually decrystallize over time. Because of this, CD-RWs are not as reliable for long-term storage; however, under recommended storage conditions, CD-RW should have a life expectancy of 25 years or more (as compared to 30+ years for CD-R).
Alternatively, special packet writing software may be used. This method uses a different format and can selectively update the CD; however, this comes at a significant cost in storage capacity and compatibility, and seems to be less popular as Nero and the disc-burning software built into recent versions of Microsoft Windows have replaced Easy CD Creator as the dominant bundled burning software.[citation needed] (While Nero does have packet writing software, it does not appear to be included in the OEM versions.)
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 have considerably lower reflectivity than CD-Rs and commercially pressed discs, requiring a more sensitive laser optics. Also, CD-RWs cannot be read in some CD-ROM drives built prior to 1997. This is why CD-ROM drives of the age must 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 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. 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.
[edit] Applications 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 unsuitable for repeated small-scale file additions and deletions, thus making the use of CD-RW as a true removable disk impractical.
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.
[edit] CD-MO
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. 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 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 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 recordable 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 relatively more economical CD-R disks essentially caused the format to be abandoned before commercialization [1][2], 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.
[edit] Speed specifications
| Spec[3] | Speed |
|---|---|
| (Original) "slow" | 1x - 4x |
| High Speed | 4x - 10x |
| Ultra Speed | 12x - 24x |
| Ultra Speed+ | 32x |
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 bound to its speed spec, but depends first and foremost on the reading drive's capabilities, as with CD-R disks.
[edit] References
- Bennett, Hugh. "CD-E: Call it Erasable, Call it Rewritable, but will it Fly?" CD-ROM Professional Sept. 1996: 28+
- Bennett, Hugh. Understanding CD-R & CD-RW. Cupertino: Optical Storage Technology Association, Jan. 2003.
- Steinmetz, Ralf and Nahrstedt Klara. "Multimedia Fundamentals Volume 1: Media Coding and Content Processing", ISBN 0-13-031399-8.
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
