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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.
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
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
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.
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
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
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
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
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
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
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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