CD-RW

CD-RW

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A CD-RW is a rewritable optical disc
Optical disc
In computing and optical disc recording technologies, an optical disc is a flat, usually circular disc which encodes binary data in the form of pits and lands on a special material on one of its flat surfaces...

. 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 disc require a more sensitive laser optics
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

. 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
PMA (CD)
- PCA, PMA, PA, SUA :PCA: The Power Calibration Area is used to determine the correct power level for the laser.PMA: The Program Memory Area of a CDRW is a record of the data recorded on an unfinished or unfinalized disc....

, TOC and pregap
Pregap
The pregap on a Red Book audio CD is the portion of the audio track that precedes "index 01" for a given track in the table of contents . The pregap is typically two seconds long and usually, but not always, contains silence...

, 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
Confidentiality
Confidentiality is an ethical principle associated with several professions . In ethics, and in law and alternative forms of legal resolution such as mediation, some types of communication between a person and one of these professionals are "privileged" and may not be discussed or divulged to...

. 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
Wear leveling
Wear leveling is a technique for prolonging the service life of some kinds of erasable computer storage media, such as Flash memory used in solid-state drives and USB Flash drives...

 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.

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 1990 and set in the Orange Book, part 1
Rainbow Books
The Rainbow Books are a collection of standards defining the formats of Compact Discs.Red BookYellow BookThe Rainbow Books are a collection of standards defining the formats of Compact Discs.Red Book...

, 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. Dy
Dysprosium
Dysprosium is a chemical element with the symbol Dy and atomic number 66. It is a rare earth element with a metallic silver luster. Dysprosium is never found in nature as a free element, though it is found in various minerals, such as xenotime...

Fe
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

Co
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

 or less often Tb
Terbium
Terbium is a chemical element with the symbol Tb and atomic number 65. It is a silvery-white rare earth metal that is malleable, ductile and soft enough to be cut with a knife...

Fe
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

Co
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

 or Gd
Gadolinium
Gadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with...

Fe
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

Co
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

) up to its Curie point
Curie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...

 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
MiniDisc
The disc is permanently housed in a cartridge with a sliding door, similar to the casing of a 3.5" floppy disk. This shutter is opened automatically by a mechanism upon insertion. The audio discs can either be recordable or premastered. Recordable MiniDiscs use a magneto-optical system to record...

 and other magneto-optical formats. Reading of the discs relied on the Kerr effect
Magneto-optic Kerr effect
Magneto-optic Kerr effect is one of the magneto-optic effects. It describes the changes of light reflected from magnetized media.-Definition:The light that is reflected from a magnetized surface can change in both polarization and reflected intensity...

. 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
CD-R
A CD-R is a variation of the Compact Disc invented by Philips and Sony. CD-R is a Write Once Read Many optical medium, though the whole disk does not have to be entirely written in the same session....

 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 http://www.ip.philips.com/download_attachment/162/sl00272productlistCD.pdfhttp://scp.s-scptuj.mb.edus.si/~murkos/Teorija%20in%20vaje/RSM/techref%20-%20%20HW%20za%20PCje%20-%20film%20Modherboard,%20IDE,Modem.BIOS,opti%E8ni%20diski%20-%20CD%20ob%20knjigi%20OPRAVKA%20RA%C8%20MRE%8EA/Chapter13.pdf, 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.

How


Rewritable media can with suitable optical drive according to some manufacturers be re-written up to . The CD-RW technology is based on the phase change
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....

 technology, so the degree of reflection reached is only , compared to the reflection from CD-R discs. The properties of the medium and the write and erase procedure is defined in the Orange Book Part III
Rainbow Books
The Rainbow Books are a collection of standards defining the formats of Compact Discs.Red BookYellow BookThe Rainbow Books are a collection of standards defining the formats of Compact Discs.Red Book...

.

To keep rotational speed precise any track have a slight superimposed sinusoidal
Sine wave
The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...

 excursion of at a frequency of . In addition an frequency modulation
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...

 is applied to provide the recorder with an absolute time reference. The grooves have a width of and pitch of .

The media for CD-RW has basically the same layers as CD-R media. The reflective layer is, however, a silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

-indium
Indium
Indium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...

-antimony
Antimony
Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...

-tellurium (AgInSbTe) alloy, which has in its original state, a polycrystalline structure and reflective
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

 properties. When writing the laser beam uses its maximum power and heat the material to . This causes liquefaction
Melting
Melting, or fusion, is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid...

 of the material. In this state, the alloy loses its polycrystalline structure, assumes an amorphous
Amorphous solid
In condensed matter physics, an amorphous or non-crystalline solid is a solid that lacks the long-range order characteristic of a crystal....

 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, just like recordable CDs. Unlike amplification and extinction of the laser light by superimposing the reflected light with the emitted as with pressed CDs. To erase the disk, the write beam heats the amorphous regions with low power to about . The alloy is not melted, but returns to the polycrystalline state and is thus again reflective.

Speed specifications

Spec http://www.cdrfaq.org/faq04.html Speed
(Original, "slow") 1x - 4x
High Speed 4x - 10x
Ultra Speed 12x - 24x
Ultra Speed+ 32x


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
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 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 spec 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
Forward compatibility
Forward compatibility or upward compatibility is a compatibility concept for systems design, as e.g. backward compatibility. Forward compatibility aims at the ability of a design to gracefully accept input intended for later versions of itself...

), 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 spec, but depends first and foremost on the reading drive's capabilities, as with CD-R discs.

External links