Технология перезаписываемых оптических дисков CD-RW и CD-MO.

Optical disk technology

CD RW (Compact Disc Rewritable) is a digital optical disc storage format introduced in 1997. A CD RW compact 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 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. 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. for CD authoring), 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.