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 as PMA, TOC and pregap, are cleared. Fast blanking is quicker and usually sufficient to allow rewriting the disc. Full blanking removes all traces of the previous data, and is often used for confidentiality purposes, though it may be possible to recover data with specialty recovery equipment.
CD-RWs can sustain fewer re-writes compared to other storage media (ca. 1,000 compared up to 100,000). Ideal use is for test discs, temporary backups, and as a middle-ground between online and offline storage schemes.
Before CD-RW technology, in 1990 a standard for magneto-optical recordable and erasable CDs called CD-MO was introduced and set in the Orange 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 often TbFeCo or GdFeCo) to its Curie point 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 the discs relied on the Kerr 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, mostly because of incompatibility with standard CD reading units. Early CD-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. Some magneto-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. Other magneto-optical media, unbound by limitations of the typical CD-ROM filesystems, replaced the CD-MO.
Mechanism of action
Rewritable media can, with suitable hardware, be re-written up to 100 000 times. The CD-RW is based on phase change technology, with a degree of reflection at 15–25%, compared to 40–70% for CD-R discs. The properties of the medium and the write and erase procedure is defined in the Orange Book Part III.
To maintain a precise rotation speed, tracks have a slight superimposed sinusoidal excursion of 0.3 µm at a frequency of 22.05 kHz. In addition a 1 kHz frequency modulation is applied to provide the recorder with an absolute time reference. Groove width is 0.6 µm and pitch of 1.6 µm.
The media for CD-RW has the same layers as CD-R media. The reflective layer is, however, a silver-indium-antimony-tellurium (AgInSbTe) alloy with a polycrystalline structure and reflective properties in its original state. When writing the laser beam uses its maximum power (8 - 14 mW) to heat the material to 500–700 °C causing material liquefaction. In this state, the alloy loses its polycrystalline structure and reflectivity and assumes an amorphous state. The lost reflectivity serves the same function as bumps on manufactured CDs and the opaque spots on a CD-R are read as a "0". The polycrystalline state of the disc forms the trenches, which are read as "1". 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 about 200 °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:
- 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. It is used as a transition TOC while the session is still open. PMA records may contain information on up to 99 audio tracks and their start and stop times (CD-DA), or sector addresses for the start of data files for each session on a data CD.
- PA: The Program Area contains the audio tracks or data files.
- SUA: The System User Area The PCA and the PMA grouped together are sometimes denoted as the System User Area.
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.
|(Original, "slow")||1×, 2×, 4×|
|High Speed||8×, 10×, 12×|
|Ultra Speed||16×, 20×, 24×|
Like a CD-R, a CD-RW has hardcoded speed specifications which limit recording speeds to fairly restrictive ranges. Unlike a CD-R, a CD-RW has a minimum writing speed under which the discs cannot be recorded, based on 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 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 written.
Similarly, using inappropriately high amounts of laser energy will cause the material to overheat and be "insensitive" to the data, a situation typical of slower discs used in a high powered and fast specification drive.
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.
- 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.
- ECMA-395: Recordable Compact Disc Systems CD-RW Ultra-Speed (standardized Orange Book, Part III, Volume 3)
- The CD-R FAQ
- Understanding CD-R & CD-RW
- PC Technology Guide
- Upgrading and repairing PCs By Scott Mueller, page 739: "The Orange Book comes in three parts: Part I describes a format called CD-MO (magneto-optical), which was to be a rewritable format but was withdrawn before any products really came to market"
- "Wayback Machine" (PDF). 6 February 2012. Archived from the original (PDF) on 6 April 2008. Retrieved 10 April 2018.
- "Archived copy". Archived from the original on 2012-07-09. Retrieved 2011-05-15.CS1 maint: archived copy as title (link) CS1 maint: BOT: original-url status unknown (link)
- "How CD Burners Work". howstuffworks.com. 1 August 2001. Retrieved 10 April 2018.