CD-ROM The CD-ROM (an abbreviation for "Compact Disc Read-Only Memory" (ROM) ) is a non-volatile optical data storage medium using the same physical format as audio compact discs, readable by a computer with a CD-ROM drive. A CD-ROM is a flat, plastic disc with digital information encoded on it in a spiral from the center to the limit, the outside edge. The CD-ROM Yellow Book standard was established in 1985 by Sony and Philips. CD-R How it works Digital information is encoded at near-microscopic size, allowing a large amount of information to be stored. CDs record binary data as tiny pits (or non-pits) pressed into the lower surface of the plastic disc; a semiconductor laser beam in the player reads these. Most CDs can not be written with a laser, but CD-R discs have colored dyes that can be "burned" (written to) once, and CD-RW (rewritable) discs contain phase-change material that can be written and overwritten several times. Most CD-ROM drives can read CD-R discs; modern drives carrying the MultiRead mark can read CD-RW discs. Manufacture A CD-ROM can be burned or pressed. Small production runs are burned; larger runs are pressed. The break-even point seems to be somewhere between 100 and 500 copies http://www.lboro.ac.uk/service/ltd/services/lt/yp/CDROM.htm, http://www.niche-publications.co.uk/cdprod/process.htm. Capacity The standard CD-ROM can hold approximately 650-700 megabytes of data, although data compression technology allows larger capacities. CD-ROM is popular for distribution of large databases, software and especially multimedia applications. A CD weighs under an ounce. To put the CD-ROM's storage capacity into context, the average novel contains 60,000 words. Assume that average word length is 10 letters - in fact it is less than 10 - and that each letter occupies one byte. A novel therefore might occupy 600,000 bytes. One CD can therefore contain over 1,000 novels. If each novel occupies half an inch of bookshelf space, then one CD can contain the equivalent of about 15 yards of bookshelf. Textual data can be compressed by more than a factor of ten, using computer compression algorithms (often known as zipping), so a CD-ROM can accommodate at least 100 yards of bookshelf space. A DVD can contain ten times as much data as a CD-ROM. Writing to and reading from CD-ROM CD-ROMs are read using CD-ROM drives, a now-common computer peripheral, and, in the case of burning, are burned with CD-Recorders, commonly referred to as CD Burners. CD-ROM drives are rated with a speed factor relative to music CDs (1x or 1-speed which gives a data transfer rate of 150 kilobytes per second in the most common data format). Above 12x speed, there are problems with vibration and heat. Constant angular velocity (CAV) drives give speeds up to 20x but due to the nature of CAV the actual throughput increase over 12x is less than 20/12. 20x was thought to be the maximum speed due to mechanical constraints until February 1998, when Samsung Electronics introduced the SCR-3230, a 32x CD-ROM drive which uses a ball bearing system to balance the spinning disc in the drive to reduce noise. CD-ROM drives may connect to an IDE (ATA) interface, a SCSI interface or a proprietary interface, such as the Panasonic CD interface. Most CD-ROM drives can also play audio CDs. Copyright Issues There is a move by the recording industry to make audio CDs (CDDAs, Red Book CDs) unplayable on computer CD-ROM drives, to prevent copying the music. This is done by intentionally introducing errors onto the disc that audio players can automatically compensate for. Consumer rights advocates are as of October 2001 pushing to require warning labels on compact discs that do not conform to the official Compact Disc Digital Audio standard (often called the Red Book) to inform consumers of which discs do not permit full fair use of their content. Manufacturers of CD writers (CD-R or CD-RW) are "encouraged" by the music industry to ensure that every drive they produce has a unique identifier, which will be encoded by the drive on every disc that it records: the RID or Recorder Identification Code. This is a counterpart to the SID - the Source Identification Code, an eight character code beginning with "IFPI" that is usually stamped on discs produced by CD recording plants. Data Formats There are several formats used for CD-ROM data, the Rainbow Books including Green Book, White Book and Yellow Book CD-ROM. ISO 9660 defines the standard file system of a CD-ROM, although it is due to be replaced by ISO 13490. UDF format is used on user-writable CD-R and CD-RW discs that are intended to be extended or overwritten. The bootable CD specification, to make a CD emulate a hard disk or floppy, is called El Torito (apparently named after the restaurant chain). Informative CD-ROMs may contain links to webpages with additional information. To keep them up to date these are sometimes indirect: they link to webpages maintained by the producer of the CD-ROM which contain the links to external webpages. See also computer hardware References External links How CDs Work from HowStuffWorks.com FAVORITE ARTICLES: History of the World    History of the United States    History of Europe    Ancient History    History    Military History    World War I    Bombing of Dresden    California    US Constitution    Frank Sinatra    Boston    Marbury v. Madison This article is licensed under the GNU Free Documentation License at http://www.gnu.org/copyleft/fdl.html You may copy and modify it as long as the entire work (including additions) remains under this license. You must provide a link to http://www.gnu.org/copyleft/fdl.html To view or edit this article at Wikipedia go to http://www.wikipedia.org/wiki/CD-ROM