| | | | Compact Disk - Read Only Memory. The original compact disk was designed for music storage but was soon adapted for storage of any digital data. Computer CD-drives can play back both music CDs and CD-ROMs but music players cannot read data disks. The original standard for CD-ROM format was set in 1983 by Philips and Sony. Physical attributes are as follows: 120 mm diameter, 1.2 mm thick plastic (polycarbonate) disk and a very thin layer of reflective metal, usually aluminium, coverd by a lacquer coating. Smaller 80mm diameter disks are also available (holding 180MB) and are often used to carry presentations, "business card" information and device drivers. When manufactured, a series of indentations or "pits" are made in the metal layer. These can be read by a laser being bounced off them by the disk player and reflected back. The way the relected laser light interferes with the incoming laser beam results in changes in intensity which can be interpreted by the reading device as binary data. A CD-ROM holds 650MB or 750MB of data arranged in a spiral pattern starting at the innermost edge and finishing at the outer. The first CD-ROMs transferred data at 150KB per second - over time that read speed gradually increased, first to 2x speed (300KB per second) and onwards until current drives read disks capable of 52x data transfer. Other CD-based formats are CD-R and CD-RW. CD-R disks can be written to (recorded) once, while CD-RW blanks can be written and re-written many times. CD-R disks can be read with a CD-ROM drive, whereas CD-RW must be read using the more recent MultiRead drives. Initial claims presented to the public about the durability of CD-ROMs (and audio CDs ) at launch proved to be slightly over-enthusiastic and the medium remains susceptible to surface scratches and degradation by oxidation, humidity and temperature, particularly in the case of fluctuations of the latter two. Original estimates were that compact disks would last for 100 years and 25-30 year guarantees are regulary given, but it is advisable to back up disks on a regular basis nevertheless. | | | |
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