The CD-ROM (an abbreviation for Compact disc Read-only memory ) is a non-volatile optical data computer 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, metallized plastic disc with digital information encoded on it in a spiral from the center to the outside edge. The CD-ROM Yellow Book (CD-ROM_standards) 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 read 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. These are called Enhanced CDs.

=Manufacture=

CD-ROMs are always pressed (mass-produced), whereas CD-Rs are recorded one at a time. The contents of a CD-R may be in logical CD-ROM format (Yellow Book (CD-ROM standards)) but the disc itself is physically a CD-R (Orange Book (CD standard)).

Source: Dana J. Parker, author of The CD-Recordable Handbook. [http://www.amazon.com/exec/obidos/tg/detail/-/0910965188/qid=1111091690/sr=1-2/ref=sr_1_2/103-7169860-7230214v=glance&s=books]

=Capacity=

The standard CD-ROM can hold 650-700 megabytes of data. The CD-ROM is popular for distribution of Software, especially Multimedia applications, and large Databases. 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 (~16.5 metre) of bookshelf. However textual data can be compressed by more than a factor of ten, using computer data compression (often known as zipping ), so a CD-ROM can accommodate at least 100 yards of bookshelf space. In comparison a DVD typically contains 4.7 GB of data or more, depending upon its type. Dual layer DVD+R discs, for example, contain 8.5GB of data for a normal sized (12 cm) disc.

=CD-ROM drives=

CD-ROMs are read using CD-ROM drives and written with CD recorders (often referred to as burners ). CD-ROM drives—now almost-universal on personal computers—may be connected to the computer via an Integrated Drive Electronics (Advanced Technology Attachment) interface, a SCSI interface or a proprietary interface, such as the Panasonic CD interface. Most CD-ROM drives can also play Red Book (audio CD standard) and Video CDs with the right software.

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 (mechanical) system to balance the spinning disc in the drive to reduce vibration and noise. As of 2004, the fastest transfer rate commonly available is about 52x or 7.62 megabytes per second, though this is only when reading information from the outer parts of a disc. Future speed increases based simply upon spinning the disc faster are particularly limited by the strength of polycarbonate plastic used in CD manufacturing. Speed improvements can however still be obtained by the use of multiple laser pickups as demonstrated by the Kenwood Electronics TrueX 72x which uses seven laser beams and a rotation speed of approximately 10x.

CD-Recordable 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 (1.76 megabytes/s), write to CD-RW discs at 10x speed (1.46 megabytes/s), and read from CD discs at 32x speed (4.69 megabytes/s).

The 1x speed rating for CDs (150 kilobytes/s) is not to be confused with the 1x speed rating for DVDs (1.32 megabytes/s).

Some of the initial versions of CD Drives had a mechanism different from the tray or slot loaders of modern day drives. They could read CDs only when they were inserted in special cartridges. The CD Caddy resembled the Floppy_drive#The_3.C2.BD-inch_micro_floppy_diskette because of its protective casing. It, however, never caught on.

=Copyright Issues=

There has been a move by the Record industry to make audio CDs (CDDAs, Red Book (audio CD standard) CDs) unplayable on computer CD-ROM drives, to prevent copying of the music. This is done by intentionally introducing errors onto the disc that audio players can automatically compensate for, but confuse CD-ROM drives. 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 ( 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, which include the Green Book (CD-i standards), White Book (Video CD standards) and Yellow Book (CD-ROM standards). ISO 9660 defines the standard file system of a CD-ROM, although it is due to be replaced by ISO 13490. Universal Disk Format 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 (CD-ROM standard) (apparently named after the El Torito).

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=

= References =

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