Source: Wikipedia, the free encyclopedia.
KB/s (1×; 150 × 210), 10,800 KB/s (72×)
Write mechanismPressed mold
StandardISO/IEC 10149[1]
UsageData storage
Released1985; 38 years ago (1985)

A CD-ROM (/ˌsdˈrɒm/, compact disc read-only memory) is a type of read-only memory consisting of a pre-pressed optical compact disc that contains data. Computers can read—but not write or erase—CD-ROMs. 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).

During the 1990s and early 2000s, CD-ROMs were popularly used to distribute

fifth generation video game consoles
. DVD started to replace it in these roles starting in the early 2000s.


The earliest theoretical work on optical disc storage was done by independent researchers in the United States including

MCA and Philips after MCA purchased Gregg's patents, as well as the company he founded, Gauss Electrophysics.[3]
The LaserDisc was the immediate precursor to the CD, with the primary difference being that the LaserDisc encoded information through an analog process whereas the CD used digital encoding.

Key work to digitize the optical disc was performed by Toshi Doi and Kees Schouhamer Immink during 1979–1980, who worked on a taskforce for Sony and Philips.[4] The result was the Compact Disc Digital Audio (CD-DA), defined on 1980. The CD-ROM was later designed as an extension of the CD-DA, and adapted this format to hold any form of digital data, with an initial storage capacity of 553 MB.[5] Sony and Philips created the technical standard that defines the format of a CD-ROM in 1983,[6] in what came to be called the Yellow Book. The CD-ROM was announced in 1984[7] and introduced by Denon and Sony at the first Japanese COMDEX computer show in 1985.[8] In November, 1985, several computer industry participants including Microsoft, Philips, Sony, Apple and Digital Equipment Corporation met to create a specification to define a file system format for CD-ROMs.[9] The resulting specification, called the High Sierra format, was published in May 1986.[9] It was eventually standardized, with a few changes, as the ISO 9660 standard in 1988. One of the first products to be made available to the public on CD-ROM was the Grolier Academic Encyclopedia, presented at the Microsoft CD-ROM Conference in March 1986.[9]

CD-ROMs began being used in

laserdisc video games but with digital data, allowing more flexibility than older laserdisc games.[10] By early 1990, about 300,000 CD-ROM drives were sold in Japan, while 125,000 CD-ROM discs were being produced monthly in the United States.[11] Some computers that were marketed in the 1990s were called "multimedia
" computers because they incorporated a CD-ROM drive, which allowed for the delivery of several hundred megabytes of video, picture, and audio data.

On early audio CD players that were released prior to the advent of the CD-ROM, the raw binary data of CD-ROM was played back as noise. To address this problem, the subcode channel Q has a "data" flag in areas of the disc that contain computer data rather than playable audio. The data flag instructs CD players to mute the audio.[12][13]

CD-ROM discs


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

CD-ROMs are identical in appearance to

shaped compact discs in numerous non-standard sizes and molds (e.g., business card-sized media
), also exist.

Data is stored on the disc as a series of microscopic indentations called "pits", with the non-indented spaces between them called "lands". A

destructive interference
and reducing the reflected beam's intensity. This is converted into binary data.


Several formats are used for data stored on compact discs, known as the

Red Book CD-DA standard for CD audio. Other standards, such as the White Book for Video CDs
, further define formats based on the CD-ROM specifications. The Yellow Book itself is not freely available, but the standards with the corresponding content can be downloaded for free from ISO or ECMA.

There are several standards that define how to structure data files on a CD-ROM.



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 are in turn used to manufacture multiple copies of the final disc with the pits already present. Recordable (CD-R) and rewritable (CD-RW) discs are manufactured by a different method, whereby the data are recorded on them by a laser changing the properties of a dye or phase transition material in a process that is often referred to as "burning".

CD-ROM format

Data stored on CD-ROMs follows the standard CD data encoding techniques described in the Red Book specification (originally defined for audio CD only). This includes cross-interleaved Reed–Solomon coding (CIRC), eight-to-fourteen modulation (EFM), and the use of pits and lands for coding the bits into the physical surface of the CD.

The structures used to group data on a CD-ROM are also derived from the Red Book. Like audio CDs (CD-DA), a CD-ROM sector contains 2,352

track (a group of sectors) inside a CD-ROM only contains sectors in the same mode, but if multiple tracks are present in a CD-ROM, each track can have its sectors in a different mode from the rest of the tracks. They can also coexist with audio CD tracks, which is the case of mixed mode CDs

Sector structure

Both Mode 1 and 2 sectors use the first 16 bytes for

error correction bytes. Unlike an audio CD, a CD-ROM cannot rely on error concealment by interpolation; a higher reliability of the retrieved data is required. To achieve improved error correction and detection, Mode 1, used mostly for digital data, adds a 32-bit cyclic redundancy check (CRC) code for error detection, and a third layer of Reed–Solomon error correction[n 1] using a Reed-Solomon Product-like Code (RSPC). Mode 1 therefore contains 288 bytes per sector for error detection and correction, leaving 2,048 bytes per sector available for data. Mode 2, which is more appropriate for image or video data (where perfect reliability may be a little bit less important), contains no additional error detection or correction bytes, having therefore 2,336 available data bytes per sector. Both modes, like audio CDs, still benefit from the lower layers of error correction at the frame level.[17]

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.[15] These scrambled sectors then follow the same encoding process described in the Red Book in order to be finally stored on a CD.

The following table shows a comparison of the structure of sectors in CD-DA and CD-ROMs:[15]

Format ← 2,352 byte sector structure →
CD digital audio: 2,352 (Digital audio)
CD-ROM Mode 1: 12 (Sync pattern) 3 (Address) 1 (Mode, 0x01) 2,048 (Data) 4 (Error detection) 8 (Reserved, zero) 276 (Error correction)
CD-ROM Mode 2: 12 (Sync pattern) 3 (Address) 1 (Mode, 0x02) 2,336 (Data)

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

(150 × 210) . This value, 150 Kbyte/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 650 MB (650 × 220). For 80 minute CDs, the capacity is 703 MB.

CD-ROM XA extension

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.

Green Book) and was published by Sony and Philips, and backed by Microsoft, in 1991,[19] first announced in September 1988.[20]
"XA" stands for eXtended Architecture.

CD-ROM XA defines two new sector layouts, called Mode 2 Form 1 and Mode 2 Form 2 (which are different from the original Mode 2). XA Mode 2 Form 1 is similar to the Mode 1 structure described above, and can interleave with XA Mode 2 Form 2 sectors; it is used for data. XA Mode 2 Form 2 has 2,324 bytes of user data, and is similar to the standard Mode 2 but with error detection bytes added (though no error correction). It can interleave with XA Mode 2 Form 1 sectors, and it is used for audio/video data.

Enhanced Music CDs and CD-i use these sector modes.[21]

The following table shows a comparison of the structure of sectors in CD-ROM XA modes:

Format ← 2,352 byte sector structure →
CD-ROM XA Mode 2, Form 1: 12 (Sync pattern) 3 (Address) 1 (Mode) 8 (Subheader) 2,048 (Data) 4 (Error detection) 276 (Error correction)
CD-ROM XA Mode 2, Form 2: 12 (Sync pattern) 3 (Address) 1 (Mode) 8 (Subheader) 2,324 (Data) 4 (Error detection)

Disc images

When a

ISO disc images

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 MB). This is the upper limit for raw images created on a 74 min or ≈650 MB Red Book CD. The 14.8% increase is due to the discarding of error correction data.


A CD-ROM can easily store the entirety of a paper encyclopedia's words and images, plus audio & video clips

CD-ROM capacities are normally expressed with

can hold 4.7 GB (4.7 × 109 bytes) of error-protected data, more than 6 CD-ROMs.

Capacities of Compact Disc types (90 and 99 minute discs are not standard)
Type Sectors Data (mode 1) max. size Audio max. size Time
(MB) Approx. (1 = 220) (MB) (min)
8 cm 94,500 193.536 184.570 222.264 21
553 MB 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
Note: megabyte (MB) and minute (min) values are exact; (1 = 220) values are approximate.

CD-ROM drives

A view of a CD-ROM drive's disassembled laser system
The movement of the laser enables reading at any position of the CD
The laser system of a CD-ROM drive

CD-ROM discs are read using CD-ROM drives. A CD-ROM drive may be connected to the computer via an IDE (

audio CDs (as well as Video CDs
and other data standards) when used with the right software.

Laser and optics

CD-ROM drives employ a near-infrared 780 nm laser diode. The laser beam is directed onto the disc via an opto-electronic tracking module, which then detects whether the beam has been reflected or scattered.

Transfer rates

Original speed

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 Kbyte/s, commonly called "1×" (with constant linear velocity, short "CLV"). 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 about 500 rpm at the inner edge to 200 rpm at the outer edge. The 1× speed rating for CD-ROM (150 Kbyte/s) is different from the 1× speed rating for DVDs (1.32 MB/s).

Speed advancements

When the speed at which the disc is spun is increased, data can be transferred at greater rates. For example, a CD-ROM drive that can read at 8× speed spins the disc at 1600 to 4000 rpm, giving a linear velocity of 9.6 m/s and a transfer rate of 1200 Kbyte/s. Above 12× speed most drives read at Constant angular velocity (CAV, constant rpm) so that the motor is not made to change from one speed to another as the head seeks from place to place on the disc. In CAV mode the "×" number denotes the transfer rate at the outer edge of the disc, where it is a maximum. 20× was thought to be the maximum speed due to mechanical constraints until

TrueX 72× which uses seven laser beams and a rotation speed of approximately 10×.

The first 12× drive was released in late 1996.[24] 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.

Physical limitations

Problems with vibration, owing to limits on achievable symmetry and strength in mass-produced media, mean that CD-ROM drive speeds have not massively increased since the late 1990s. Over 10 years later, commonly available drives vary between 24× (slimline and portable units, 10× spin speed) and 52× (typically CD- and read-only units, 21× spin speed), all using CAV to achieve their claimed "max" speeds, with 32× through 48× most common. Even so, these speeds can cause poor reading (drive error correction having become very sophisticated in response) and even shattering of poorly made or physically damaged media, with small cracks rapidly growing into catastrophic breakages when centripetally stressed at 10,000–13,000 rpm (i.e. 40–52× CAV). High rotational speeds also produce undesirable noise from disc vibration, rushing air and the spindle motor itself. Most 21st-century drives allow forced low speed modes (by use of small utility programs) for the sake of safety, accurate reading or silence, and will automatically fall back if numerous sequential read errors and retries are encountered.


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,

DVD-ROM drives capable of consistent 36× equivalent 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 GB
(single and dual layer), with higher speed and capacity needs instead being catered to by Blu-ray drives.

Speed ratings

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 MB/s), write to CD-RW discs at 10× speed (1.46 MB/s), and read from CDs at 32× speed (4.69 MB/s).

Speed table

Common data transfer speeds for CD-ROM drives
Transfer speed
MB/s [n 2]
RPM (outer to inner edge)
150 1.2288 0.146 200–530[25][26]
300 2.4576 0.293 400–1,060
600 4.9152 0.586 800–2,120
1,200 9.8304 1.17 1,600–4,240
10× 1,500 12.288 1.46 2,000–5,300
12× 1,800 14.7456 1.76 2,400–6,360
20× 1,200–3,000 up to 24.576 up to 2.93 4,000 (
24× 1,440–3,600 up to 29.491 up to 3.51 4,800 (
32× 1,920–4,800 up to 39.3216 up to 4.69 6,400 (
36× 2,160–5,400 up to 44.2368 up to 5.27 7,200 (
40× 2,400–6,000 up to 49.152 up to 5.86 8,000 (
48× 2,880–7,200 up to 58.9824 up to 7.03 9,600 (
52× 3,120–7,800 up to 63.8976 up to 7.62 10,400 (
56× 3,360–8,400 up to 68.8128 up to 8.20 11,200 (
72× 6,750–10,800 up to 88.4736 up to 10.5 2,700 (multi-beam)[28][29]

Copyright issues

Software distributors, and in particular distributors of computer games, often make use of various copy protection schemes to prevent software running from any media besides the original CD-ROMs. This differs somewhat from audio CD protection in that it is usually implemented in both the media and the software itself. 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 unauthorized copy is present in the computer's CD-ROM drive.[citation needed]

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.[30] This is a counterpart to the Source Identification Code (SID), an eight character code beginning with "IFPI" that is usually stamped on discs produced by CD recording plants.

See also


  1. CIRC
    error correction system used in the CD audio format has two interleaved layers.
  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)". Archived from the original on 2019-01-15. Retrieved 2010-08-06.
  2. ^ a b "CD Yellow Book Standards". Archived from the original on 2019-05-21. Retrieved 2015-11-16.
  3. ^ "Optical Disc invented by David Paul Gregg in year 1958". Retrieved 2020-07-06.
  4. ^ "Shannon, Beethoven, and the Compact Disc". Archived from the original on 2015-03-18.
  5. ^ "Videodisc Update, Volumes 1-3". 1982. p. 13.
  6. ^ a b "InfoWorld Vol. 16, No. 23". InfoWorld. June 6, 1994. p. 88. Retrieved March 25, 2020.
  7. ^ "1983 | Timeline of Computer History | Computer History Museum". Retrieved 2020-07-06.
  8. ^ Japanese PCs (1985) (14:20), Computer Chronicles
  9. ^ a b c Maher, Jimmy (September 30, 2016). "A Slow-Motion Revolution".
  10. ^ "Hot Off The Press! More Revolution". RePlay. Vol. 16, no. 6. March 1991. p. 3.
  11. ^ "Special Report: A Fact of Life". RePlay. Vol. 15, no. 5. February 1990. pp. 48–9.
  12. ^ Standard ECMA-130 2nd Edition - June 1996 - Data interchange on read-only 120 mm optical data disks (CD-ROM) - 22.3 Setting of the q-channel
  13. ^ IEC 60908-1999 Audio Recording - Compact Disc Digital Audio System - 17.5 Channel Q
  14. ^ "Proceedings of the 5th Annual Federal Depository Library Conference". U.S. Government Printing Office. April 15–18, 1996. p. 11. Retrieved February 10, 2022.
  15. ^ a b c "Data Interchange on Read-only 120 mm Optical Data Disks (CD-ROM)". ECMA. June 1996. Retrieved 2009-04-26. {{cite journal}}: Cite journal requires |journal= (help)
  16. ^ "Birth Announcement: ISO/IEC 13346 and ISO/IEC 13490". 1996-02-09. Archived from the original on 2018-09-14. Retrieved 2013-09-23.
  17. ^ a b McFadden, Andy (2002-12-20). "What is XA? CDPLUS? CD-i? MODE1 vs MODE2? Red/yellow/blue book?". CD-Recordable FAQ. Archived from the original on 2019-10-02. Retrieved 2008-05-04.
  18. ^ What are CD-ROM Mode-1, Mode-2 and XA? Archived 2013-01-26 at the Wayback Machine, Sony Storage Support
  19. ^ "What is Yellow Book?". Archived from the original on 2018-10-17. Retrieved 2013-09-23.
  20. ^ "Industry Players Adopt Varied Approaches to Multimedia Technology". InfoWorld. 1989-01-16.
  21. ^ "Gateway Support - What is CD-ROM/XA?". Archived from the original on 2003-01-24. Retrieved 2013-09-23.
  22. ^ "Optical Media FAQs" (PDF). Archived from the original (PDF) on 2006-10-22. Retrieved 2007-01-06.
  23. ^ Halfhill, Tom R. (October 1996). "CDs for the Gigabyte Era". Retrieved 2022-02-11.
  24. Imagine Media
    . January 1997. p. 30. Here comes Diamond with the first 12X CD-ROM.
  25. ^ "1x CD-ROM".
  26. .
  27. ^ "CD-Recordable FAQ - section 5".
  28. ^ "Kenwood's 72X CD can't keep pace with 24X CD-RW". GCN. August 7, 2000.
  29. ^ "Kenwood Technologies Releases Kenwood 72x TrueX CD-ROM Drive".
  30. ^ Schoen, Seth (2007-06-20). "Harry Potter and the Digital Fingerprints". Electronic Frontier Foundation. Archived from the original on 2017-11-07. Retrieved October 24, 2007.
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