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. Microsoft and Apple Computer were early enthusiasts and promoters of CD-ROMs. John Sculley, CEO of Apple at the time, said as early as 1987 that the CD-ROM would revolutionize the use of personal computers.
CD-ROM reading devices are a standard component of most modern personal computers. In general, audio CDs are distinct from CD-ROMs, and CD players intended for listening to audio cannot make sense of the data on a CD-ROM, though personal computers can generally play audio CDs. It is possible to produce composite CDs containing both data and audio with the latter capable of being played on a CD player, whilst data or perhaps video can be viewed on a computer.
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.
A CD-ROM can be burned or pressed. Small production runs are burned; larger runs are pressed. The change over 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).
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 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.
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). For example, an 8x CD-ROM data transfer rate would be 1.2 megabytes per second. 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. As of 2004, the fastest transfer rate commonly available is about 52x or about 7.8 megabytes per second, though this is only available when reading information from the outer parts of a disc.
CD-ROM drives are often sold with three different speed ratings, one speed for write-once operations, one for re-write operations, and one for read-only operations. The speeds are typically listed in that order; ie a 12x/10x/32x CD drive can, CPU and media-permitting, write to CD-R disks at 12x speed (1.8 megabytes/sec), write to CD-RW discs at 10x speed (1.5 megabytes/sec), and read from CD discs at 32x speed (4.8 megabytes/sec).
The 1x speed rating for CDs (150 kilobytes/sec) is not to be confused with the 1x speed rating for DVDs (1.32 megabytes/sec).
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.
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.
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
- How CDs Work from HowStuffWorks.com (http://computer.howstuffworks.com/cd.htm)