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Media type Optical disc
Capacity 194 MiB (8 cm), 650–900 MiB (12 cm)
Read mechanism 150 KiB/s (1×), 10,800 KiB/s (72×)
Write mechanism 150 KiB/s (1×), 8,400 KiB/s (56×)
Standard ISO/IEC 10149[1]
Usage Data storage

A CD-ROM /ˌsˌdˈrɒm/ is a pre-pressed optical compact disc which contains data. The name is an acronym which stands for "Compact Disc Read-Only Memory". Computers can read CD-ROMs, but cannot write to CD-ROMs which are not writable or erasable.

From the mid-1990s until the mid-2000s, CD-ROMs were popularly used to distribute software for computers and video game consoles. Some CDs, called enhanced CDs, hold both computer data and audio with the latter capable of being played on a CD player, while data (such as software or digital video) is only usable on a computer (such as ISO 9660[2] format PC CD-ROMs).

An early CD-ROM format was developed by Sony and Denon, introduced at a Japanese computer show in 1984. It was an extension of Compact Disc Digital Audio, and adapted the format to hold any form of digital data, with a capacity of 540 MiB.[3] The Yellow Book is the technical standard that defines the format of CD-ROMs. One of a set of color-bound books that contain the technical specifications for all CD formats, the Yellow Book, standardized by Sony and Philips in 1988, has a capacity of 650 MiB.

CD-ROM discs[edit]


A CD-ROM in the tray of a partially open CD-ROM drive.






CD-ROM format[edit]




Before being stored on a disc with the techniques described above, each CD-ROM sector is scrambled to prevent some problematic patterns from showing up.[5] These scrambled sectors then follow the same encoding process described in the Red Book in order to be finally stored on a CD.


Format ← 2,352 byte sector structure →
CD digital audio: 2,352 (Digital audio)

The net byte rate of a Mode-1 CD-ROM, based on comparison to CD-DA audio standards, is 44,100 Hz × 16 bits/sample × 2 channels × 2,048 / 2,352 / 8 = 153.6 kB/s = 150 KiB/s. This value, 150 KiB/s, is defined as "1× speed". Therefore, for Mode 1 CD-ROMs, a 1× CD-ROM drive reads 150/2 = 75 consecutive sectors per second.

The playing time of a standard CD is 74 minutes, or 4,440 seconds, contained in 333,000 blocks or sectors. Therefore, the net capacity of a Mode-1 CD-ROM is 682 MB or, equivalently, 650 MiB. For 80 minute CDs, the capacity is 737 MB (703 MiB).

CD-ROM XA extension[edit]

CD-ROM XA is an extension of the Yellow Book standard for CD-ROMs that combines compressed audio, video and computer data, allowing all to be accessed simultaneously.[8] It was intended as a bridge between CD-ROM and CD-i (Green Book) and was published by Sony and Philips in 1991.[4] "XA" stands for eXtended Architecture.



Format ← 2,352 byte sector structure →

Disc images[edit]


On a 74-minute CD-R, it is possible to fit larger disc images using raw mode, up to 333,000 × 2,352 = 783,216,000 bytes (~747 MiB). This is the upper limit for raw images created on a 74 min or ≈650 MiB Red Book CD. The 14.8% increase is due to the discarding of error correction data.





CD-ROM capacities are normally expressed with binary prefixes, subtracting the space used for error correction data. A standard 120 mm, 700 MB CD-ROM can actually hold about 737 MB (703 MiB) of data with error correction (or 847 MB total). In comparison, a single-layer DVD-ROM can hold 4.7 GB of error-protected data, more than 6 CD-ROMs.

Type Sectors Data max. size Audio max. size Time
8 cm 94,500 193.536 184.570 222.264 21
283,500 580.608 553.711 666.792 63
650 MB 333,000 681.984 650.391 783.216 74
700 MB 360,000 737.280 703.125 846.720 80
800 MB 405,000 829.440 791.016 952.560 90
900 MB 445,500 912.384 870.117 1,047.816 99

CD-ROM drives[edit]

Further information: Optical disc drive
A view of a CD-ROM drive's disassembled laser system
The laser system of a CD-ROM drive


Laser and optics[edit]


Transfer rates[edit]

CD-ROM drives are rated with a speed factor relative to music CDs. If a CD-ROM is read at the same rotational speed as an audio CD, the data transfer rate is 150 KiB/s, commonly called "1×". At this data rate, the track moves along under the laser spot at about 1.2 m/s. To maintain this linear velocity as the optical head moves to different positions, the angular velocity is varied from 500 rpm at the inner edge to 200 rpm at the outer edge. The 1× speed rating for CD-ROM (150 KiB/s) is different from the 1× speed rating for DVDs (1.32 MiB/s).


Faster 12× drives were common beginning in early 1997. Above 12× speed, there are problems with vibration and heat. CAV drives give speeds up to 30× at the outer edge of the disc with the same rotational speed as a standard constant linear velocity (CLV) 12×, or 32× with a slight increase. However, due to the nature of CAV (linear speed at the inner edge is still only 12×, increasing smoothly in-between) the actual throughput increase is less than 30/12: in fact, roughly 20× average for a completely full disc, and even less for a partially filled one.


Other methods of improving read speed were trialled such as using multiple optical beams, increasing throughput up to 72× with a 10× spin speed, but along with other technologies like 90~99 minute recordable media and "double density" recorders, their utility was nullified by the introduction of consumer DVD-ROM drives capable of consistent 36× CD-ROM speeds (4× DVD) or higher. Additionally, with a 700 MB CD-ROM fully readable in under 2½ minutes at 52× CAV, increases in actual data transfer rate are decreasingly influential on overall effective drive speed when taken into consideration with other factors such as loading/unloading, media recognition, spin up/down and random seek times, making for much decreased returns on development investment. A similar stratification effect has since been seen in DVD development where maximum speed has stabilised at 16× CAV (with exceptional cases between 18× and 22×) and capacity at 4.3 and 8.5 GiB (single and dual layer), with higher speed and capacity needs instead being catered to by Blu-ray drives.

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; i.e. a 12×/10×/32× CD drive can, CPU and media permitting, write to CD-R discs at 12× speed (1.76 MiB/s), write to CD-RW discs at 10× speed (1.46 MiB/s), and read from CDs at 32× speed (4.69 MiB/s).

Common data transfer speeds for CD-ROM drives
Transfer speed KiB/s Mbit/s MiB/s [n 2] RPM
150 1.2288 0.146 200–500
300 2.4576 0.293 400-1,000
600 4.9152 0.586 800–2,000
1,200 9.8304 1.17 1,600–4,000
10× 1,500 12.288 1.46 2,000–5,000
12× 1,800 14.7456 1.76 2,400–6,000
72× 6,750–10,800 up to 88.4736 up to 10.5 2,000 (multi-beam)

Copyright issues[edit]



See also[edit]


  2. ^ To three significant figures.


  1. ^ a b ISO (1995). "ISO/IEC 10149:1995 – Information technology – Data interchange on read-only 120 mm optical data disks (CD-ROM)". Retrieved 2010-08-06. 
  2. ^ a b "CD Yellow Book Standards". Retrieved 2015-11-16. 
  3. ^ Japanese PCs (1984) (14:24), Computer Chronicles
  4. ^ a b "What is Yellow Book?". Retrieved 2013-09-23. 
  6. ^ "Birth Announcement: ISO/IEC 13346 and ISO/IEC 13490". Retrieved 2013-09-23. 
  7. ^ a b McFadden, Andy (2002-12-20). "What is XA? CDPLUS? CD-i? MODE1 vs MODE2? Red/yellow/blue book?". CD-Recordable FAQ. Retrieved 2008-05-04. 
  8. ^ What are CD-ROM Mode-1, Mode-2 and XA?, Sony Storage Support
  9. ^ "Gateway Support - What is CD-ROM/XA?". Retrieved 2013-09-23. 
  11. ^ Schoen, Seth (July 20, 2007). "Harry Potter and the Digital Fingerprints". Electronic Frontier Foundation. Retrieved October 24, 2007.