CD-ROM - Media

An acronym for Compact Disk Read Only Memory, a computer storage medium based on the use of the standard 5 in (25 mm) compact disk, licensed by Sony and Philips, and usually used for digital audio. One CD-ROM disk can store more than 600 megabytes of computer information, which is considerably more than a comparable size of hard magnetic disk. Most CDs are read-only, and data is installed during manufacture; unlike the hard disk, the data on such a CD-ROM cannot be altered. The main applications have been in providing access to large volumes of information such as encyclopedias and databases. A CD-writer is a device, approximately the size of an external disk drive, which can be used to write data to writable CD-ROMS, thus allowing the CD-ROM to be used in a way similar to a floppy disk.

Optical disc authoring
Optical disc Optical disc image Recorder hardware Authoring software Recording technologies Recording modes Packet writing
Optical media types
Laserdisc Compact Disc/CD-ROM: CD-R, CD-RW DVD: DVD-R, DVD-R DL, DVD+R, DVD+R DL, DVD-RW, DVD+RW, DVD+RW DL, DVD-RAM Blu-ray Disc: BD-R, BD-RE HD DVD: HD DVD-R
Standards
Rainbow Books File systems ISO 9660 Joliet Rock Ridge Amiga extensions to Rock Ridge El Torito Apple ISO9660 Extensions Universal Disk Format Mount Rainier

CD-ROM (an abbreviation "Compact Disc Read-Only Memory") is a compact disc that contains data accessible by a computer.

Media

CD-ROM discs are identical in appearance to audio CDs, and data is stored and retrieved in a very similar manner (only differing from audio CDs in the standards used to store the data).

Layout Type	← 2,352 bytes →
CD Audio block:	2,352 bytes of Audio
CD-ROM (MODE1) sector:	12	4	2,048 bytes of user data	4	8	276
CD-ROM (MODE2) sector:	12	4	2,236 bytes of user data

Legend
12	sync
4	sector ID
	user data
4	error detection
8	null
276	error correction

Manufacture

Pre-pressed CD-ROMs are mass-produced by a process of stamping, where a glass master disc is created and used to make "stampers", which in turn are used to manufacture multiple copies of the final disc with the pits already present.

Capacities of Compact Disc types

Type	Sectors	Data max size		Audio max size		Time
		(MB)	(MiB)	(MB)	(MiB)	(min)
8 cm	94,500	193.536	≈ 184.6	222.264	≈ 212.0	21
	283,500	580.608	≈ 553.7	666.792	≈ 635.9	63
650 MB	333,000	681.984	≈ 650.3	783.216	≈ 746.9	74
700 MB	360,000	737.280	≈ 703.1	846.720	≈ 807.4	80
	405,000	829.440	≈ 791.0	952.560	≈ 908.4	90
	445,500	912.384	≈ 870.1	1,047.816	≈ 999.3	99

Note: Megabyte (MB) and minute (min) values are exact.

Transfer Rates

The rate at which CD-ROM drives can transfer data from the disc is gauged by 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. 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).

Common transfer speeds:

Data Transfer Speeds

Transfer Speed	Megabytes/s	Megabits/s	Mebibits/s
1x	0.15	1.2	1.1444
2x	0.3	2.4	2.2888
4x	0.6	4.8	4.5776
8x	1.2	9.6	9.1553
10x	1.5	12.0	11.4441
12x	1.8	14.4	13.7329
20x	3.0	24.0	22.8882
32x	4.8	38.4	36.6211
36x	5.4	43.2	41.1987
40x	6.0	48.0	45.7764
48x	7.2	57.6	54.9316
50x	7.5	60.0	57.2205
52x	7.8	62.4	59.5093

Loading Mechanisms

Current CD-ROM drives use either a tray-loading mechanism, where the disc is loaded onto a motorised or manually-operated tray, or a slot-loading mechanism, where the disc is slid into a slot and drawn in by motorised rollers. Although the idea behind this – a tougher plastic shell to protect the disc from damage – was sound, it did not gain wide acceptance among disc manufacturers due to the increased cost of production and the concern that the discs would not be compatible with drives that did not use caddies. 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. Such discs are not legally allowed to be called CDs or Compact Discs because they break the Red Book standard governing CDs, and Amazon for example describes them as "copy protected discs" rather than "compact discs" or "CDs". The CD-ROM itself may contain "weak" sectors to make copying the disc more difficult, and additional data that may be difficult or impossible to copy to a CD-R or disc image, but which the software checks for each time it is run to ensure an original disc and not an unauthorised copy is present in the computer's CD-ROM drive.