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A CD-ROM (, compact disc read-only memory) is a pre-pressed optical
compact disc The compact disc (CD) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Electronics compris ...

compact disc
that contains
data Data (; ) are individual facts, statistics, or items of information, often numeric. In a more technical sense, data are a set of values of qualitative property, qualitative or quantity, quantitative variable (research), variables about one or ...
. Computers can read—but not write or erase—CD-ROMs, i.e. it is a type of
read-only memory Read-only memory (ROM) is a type of non-volatile memory Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs const ...
.


History

During the 1990s, CD-ROMs were popularly used to distribute
software Software is a collection of Instruction (computer science), instructions that tell a computer how to work. This is in contrast to Computer hardware, hardware, from which the system is built and actually performs the work. At the low level lang ...

software
and
data Data (; ) are individual facts, statistics, or items of information, often numeric. In a more technical sense, data are a set of values of qualitative property, qualitative or quantity, quantitative variable (research), variables about one or ...

data
for computers and fifth generation video game consoles. Some CDs, called
enhanced CD Enhanced CD is a certification mark A certification mark (or conformity mark) on a commercial product indicates the existence of an accepted product standard or regulation Regulation is the management of complex systems according to a s ...
s, hold both computer data and audio with the latter capable of being played on a
CD player A CD player is an electronic device that plays audio compact discs, which are a digital audio, digital optical disc data storage device, data storage format. CD players were first sold to consumers in 1982. CDs typically contain recordings of au ...

CD player
, while data (such as software or digital video) is only usable on a computer (such as
ISO 9660 ISO 9660 is a file system In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer h ...
format PC CD-ROMs). The earliest theoretical work on optical disc storage was done by independent researchers in the United States including David Paul Gregg (1958) and James Russel (1965–1975). In particular, Gregg's patents were used as the basis of the
LaserDisc
LaserDisc
specification that was co-developed between
MCA MCA may refer to: Astronomy * Mars-crossing asteroid, an asteroid whose orbit crosses that of Mars Aviation * Minimum crossing altitude, a minimum obstacle crossing altitude for fixes on published airways * Medium Combat Aircraft, a 5th gener ...
and
Philips Koninklijke Philips N.V. (in Dutch Dutch commonly refers to: * Something of, from, or related to the Netherlands * Dutch people () * Dutch language () *Dutch language , spoken in Belgium (also referred as ''flemish'') Dutch may also refer t ...

Philips
after MCA purchased Gregg's patents, as well as the company he founded, Gauss Electrophysics. 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 Kornelis Antonie "Kees" Schouhamer Immink (born 18 December 1946) is a Netherlands, Dutch scientist, inventor, and entrepreneur, who pioneered and advanced the era of digital audio, video, and data recording, including popular digital media such a ...
during 1979–1980, who worked on a taskforce for
Sony , commonly known as Sony and stylized as SONY, is a Japanese multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from mult ...

Sony
and Phillips. The result was the
Compact Disc Digital Audio Compact Disc Digital Audio (CDDA or CD-DA), also known as Digital Audio Compact Disc or simply as Audio CD, is the standardization, standard format for audio compact discs. The standard is defined in the ''Red Book'', one of a series of Rainbow ...
(CD-DA), defined on 1980. The CD-ROM was later designed 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. Sony and Philips created the
technical standard A technical standard is an established norm (social), norm or requirement for a repeatable technical task which is applied to a common and repeated use of rules, conditions, guidelines or characteristics for products or related processes and produc ...
that defines the format of a CD-ROM in 1983, in what came to be called the ''Yellow Book''. One of a set of color-bound books that contain the
technical specification A specification often refers to a set of documented requirements to be satisfied by a material, design, product, or service. A specification is often a type of technical standard A technical standard is an established norm or requirement for a ...
s for all CD formats, the ''Yellow Book'' specifies a format for discs with a maximum capacity of 650 MB. The CD-ROM was announced in 1984 and introduced by
Denon is a Japanese electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplif ...
and
Sony , commonly known as Sony and stylized as SONY, is a Japanese multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from mult ...

Sony
at the first Japanese
COMDEX COMDEX (an abbreviation of COMputer Dealers' EXhibition) was a held in the of , , each November from 1979 to 2003. It was one of the largest computer s in the world, usually second only to the German , and one of the largest trade shows in a ...
computer show in 1985. CD-ROM began being used in
home video game consoles A home video game console is a video game console A video game console is an electronic or computer device that outputs a video signal or visual image to display a video game A video game is an electronic game that involves interactio ...
starting with the
PC Engine CD-ROM² The TurboGrafx-16, known as the outside North America, is a History of video game consoles (fourth generation), fourth-generation home video game console designed by Hudson Soft and sold by NEC, NEC Home Electronics. It was the first console ma ...
(TurboGrafx-CD) in 1988, while CD-ROM drives had also become available for
home computers Home computers were a class of microcomputer A microcomputer is a small, relatively inexpensive computer with a microprocessor as its central processing unit (CPU). It includes a microprocessor, Computer memory, memory and minimal input/ou ...
by the end of the 1980s. By early 1990, about 300,000 CD-ROM players were sold in Japan, while 125,000 CD-ROM discs were being produced monthly in the United States. In 1990,
Data East , also abbreviated as DECO, was a Japanese video game and electronic engineering company. The company was in operation from 1976 to 2003, and released 150 video game titles. Its main headquarters were located in Suginami, Tokyo, Suginami, Tokyo. T ...
demonstrated an
arcade system board An arcade game or coin-op game is a coin-operated entertainment machine typically installed in public businesses such as restaurants, bars and amusement arcades. Most arcade games are presented as primarily games of skill and include arcade ...
that supports CD-ROM, similar to 1980s laserdisc video games but with digital data, allowing more flexibility than older laserdisc games.


CD-ROM discs


Media

CD-ROMs are identical in appearance to
audio CD Compact Disc Digital Audio (CDDA or CD-DA), also known as Digital Audio Compact Disc or simply as Audio CD, is the standardization, standard format for audio compact discs. The standard is defined in the ''Red Book'', one of a series of Rainbow ...
s, and data are stored and retrieved in a very similar manner (only differing from audio CDs in the standards used to store the data). Discs are made from a 1.2 mm thick disc of
polycarbonate Polycarbonates (PC) are a group of thermoplastic A thermoplastic, or thermosoft plastic, is a plastic polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material c ...

polycarbonate
plastic Plastics are a wide range of syntheticA synthetic is an artificial material produced by organic chemistry, organic chemical synthesis. Synthetic may also refer to: In the sense of both "combination" and "artificial" * Synthetic chemical or s ...

plastic
, with a thin layer of
aluminium Aluminium (aluminum in and ) is a with the  Al and  13. Aluminium has a density lower than those of other common , at approximately one third that of . It has a great affinity towards , and of on the surface when exposed to air ...

aluminium
to make a reflective surface. The most common size of CD-ROM is 120 mm in diameter, though the smaller
Mini CD Mini CDs, or pocket CDs, are CDs with a smaller diameter and one-third the storage capacity of a standard 120 mm disc. Formats Amongst the various formats are the * Mini CD single, a small disc. The format is mainly used for audio CD si ...
standard with an 80 mm diameter, as well as shaped compact discs in numerous non-standard sizes and molds (e.g., business card-sized media), are also available. Data is stored on the disc as a series of microscopic indentations called "pits", with the non-indented spaces between them called "lands". A
laser A laser is a device that emits light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as h ...

laser
is shone onto the reflective surface of the disc to read the pattern of pits and lands. Because the depth of the pits is approximately one-quarter to one-sixth of the wavelength of the laser light used to read the disc, the reflected
beam Beam may refer to: Streams of particles or energy *Light beam, or beam of light, a directional projection of light energy **Laser beam *Particle beam, a stream of charged or neutral particles **Charged particle beam, a spatially localized group ...
's
phase Phase or phases may refer to: Science * State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter) In the physical sciences, a phase is a region of space (a thermodynamic system A thermodynamic system is a ...
is shifted in relation to the incoming beam, causing
destructive interference In physics, interference is a phenomenon in which two waves Superposition principle, superpose to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive interference result from the interaction of waves tha ...

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


Standard

Several formats are used for data stored on compact discs, known as the
Rainbow Books The Rainbow Books are a collection of CD format specification A specification often refers to a set of documented requirements to be satisfied by a material, design, product, or service. A specification is often a type of technical standard ...
. The ''Yellow Book'', published in 1988, defines the specifications for CD-ROMs, standardized in 1989 as the
ISO The International Organization for Standardization (ISO ) is an international standard An international standard is a technical standard A technical standard is an established norm (social), norm or requirement for a repeatable technical task w ...
/
IECIEC may refer to: Businesses and organisations * International Electrotechnical Commission The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organizati ...
10149 / ECMA-130 standard. The CD-ROM standard builds on top of the original Red Book
CD-DA Compact Disc Digital Audio (CDDA or CD-DA), also known as Digital Audio Compact Disc or simply as Audio CD, is the standardization, standard format for audio compact discs. The standard is defined in the ''Red Book'', one of a series of Rainbow ...
standard for CD audio. Other standards, such as the ''White Book'' for
Video CD Video CD (abbreviated as VCD, and also known as Compact Disc Digital Video) is a home video Home video is prerecorded media sold or rented for home viewing. The term originates from the VHS VHS (Video Home System) is a standard Sta ...
s, 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.
ISO 9660 ISO 9660 is a file system In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer h ...
defines the standard
file system In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and soft ...
for a CD-ROM.
ISO 13490 ISO/IEC 13490 (also known as ECMA-168) is the successor to ISO 9660 ISO 9660 is a file system In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and exp ...
is an improvement on this standard which adds support for non-sequential write-once and re-writeable discs such as
CD-R CD-R (Compact disc-recordable) is a digital media, digital optical disc data storage device, storage format. A CD-R disc is a compact disc that can be Write once read many, written once and read arbitrarily many times. CD-R discs (CD-Rs) are r ...

CD-R
and
CD-RW CD-RW (Compact Disc-Rewritable) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Electron ...
, as well as multiple sessions. The ISO 13346 standard was designed to address most of the shortcomings of ISO 9660, and a subset of it evolved into the UDF format, which was adopted for
DVD The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electr ...

DVD
s. The bootable CD specification was issued in January 1995, to make a CD emulate a
hard disk A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device On a reel-to-reel tape recorder (Sony TC-630), the recorder is data storage equipment and the magnetic tape is a data stora ...

hard disk
or
floppy disk A floppy disk or floppy diskette (sometimes casually referred to as a floppy or diskette) is a type of disk storage Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by vario ...

floppy disk
, and is called El Torito.


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 Compact Disc Digital Audio (CDDA or CD-DA), also known as Digital Audio Compact Disc or simply as Audio CD, is the standardization, standard format for audio compact discs. The standard is defined in the ''Red Book'', one of a series of Rainbow ...
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
byte The byte is a unit of digital information that most commonly consists of eight bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It ...
s of user data, composed of 98 frames, each consisting of 33 bytes (24 bytes for the user data, 8 bytes for error correction, and 1 byte for the subcode). Unlike audio CDs, the data stored in these sectors corresponds to any type of digital data, not audio samples encoded according to the audio CD specification. To structure, address and protect this data, the CD-ROM standard further defines two sector modes, Mode 1 and Mode 2, which describe two different layouts for the data inside a sector. A
track Track or Tracks may refer to: Routes or imprints * Ancient trackway, any track or trail whose origin is lost in antiquity * Animal track, imprints left on surfaces that an animal walks across * Desire path, a line worn by people taking the shortes ...
(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
header Header may refer to: Computers and engineering * Header (computing)In information technology, header refers to supplemental data placed at the beginning of a block of data being stored or transmitted. In data transmission, the data following the h ...
information, but differ in the remaining 2,336 bytes due to the use of
error correction In information theory Information theory is the scientific study of the quantification (science), quantification, computer data storage, storage, and telecommunication, communication of Digital data, digital information. The field was fun ...
bytes. Unlike an audio CD, a CD-ROM cannot rely on error concealment by
interpolation In the mathematical Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantitie ...

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 A cyclic redundancy check (CRC) is an error-detecting code In information theory Information theory is the scientific study of the quantification, storage, and communication Communication (from Latin ''communicare'', meaning "to sha ...
(CRC) code for error detection, and a third layer of
Reed–Solomon error correction Reed–Solomon codes are a group of error-correcting code In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and ...
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. Note that both modes, like audio CDs, still benefit from the lower layers of error correction at the frame level. 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. 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: 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
channel Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of a narrow body of water. Australia * Channel Country, region of outback Austr ...
s × 2,048 / 2,352 / 8 = 150
KB/s In telecommunications, data-transfer rate is the average number of bits (bitrate), characters or symbols (baudrate), or data blocks per unit time passing through a communication link in a data-transmission system. Common data rate units are multip ...
(150 × 210) . This value, 150 KB/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. It was intended as a bridge between CD-ROM and
CD-i The Compact Disc-Interactive (CD-I, later CD-i) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronic ...
( Green Book) and was published by
Sony , commonly known as Sony and stylized as SONY, is a Japanese multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from mult ...

Sony
and
Philips Koninklijke Philips N.V. (in Dutch Dutch commonly refers to: * Something of, from, or related to the Netherlands * Dutch people () * Dutch language () *Dutch language , spoken in Belgium (also referred as ''flemish'') Dutch may also refer t ...

Philips
, and backed by
Microsoft Microsoft Corporation is an American multinational corporation, multinational technology company, technology corporation which produces Software, computer software, consumer electronics, personal computers, and related services. Its best-know ...

Microsoft
, in 1991, first announced in September 1988. "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.
Video CD Video CD (abbreviated as VCD, and also known as Compact Disc Digital Video) is a home video Home video is prerecorded media sold or rented for home viewing. The term originates from the VHS VHS (Video Home System) is a standard Sta ...
s,
Super Video CD Super Video CD (Super Video Compact Disc or SVCD) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronic ...
s,
Photo CD Photo CD is a system designed by Kodak The Eastman Kodak Company (referred to simply as Kodak ) is an American public company that produces various products related to its historic basis in analogue photography. The company is headquar ...

Photo CD
s,
Enhanced Music CD The Blue Book is a compact disc standard developed in 1995 by Philips and Sony. It defines the Enhanced Music CD format (E-CD, also known as CD-Extra, CD-Plus and CD+), which combines CD-DA, audio tracks and CD-ROM, data tracks on the same disc. T ...
s and
CD-i The Compact Disc-Interactive (CD-I, later CD-i) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronic ...
use these sector modes. The following table shows a comparison of the structure of sectors in CD-ROM XA modes:


Disc images

When a disc image of a CD-ROM is created, this can be done in either "raw" mode (extracting 2,352 bytes per sector, independent of the internal structure), or obtaining only the sector's useful data (2,048/2,336/2,352/2,324 bytes depending on the CD-ROM mode). The file size of a disc image created in raw mode is ''always'' a multiple of 2,352 bytes (the size of a block). Disc image formats that store raw CD-ROM sectors include CCD/IMG, CUE/BIN, and MDS/MDF. The size of a disc image created from the data in the sectors will depend on the type of sectors it is using. For example, if a CD-ROM mode 1 image is created by extracting only each sector's data, its size will be a multiple of 2,048; this is usually the case for 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.


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 are in turn used to manufacture multiple copies of the final disc with the pits already present. Recordable (
CD-R CD-R (Compact disc-recordable) is a digital media, digital optical disc data storage device, storage format. A CD-R disc is a compact disc that can be Write once read many, written once and read arbitrarily many times. CD-R discs (CD-Rs) are r ...

CD-R
) and rewritable (
CD-RW CD-RW (Compact Disc-Rewritable) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Electron ...
) 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 In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in ...
material in a process that is often referred to as "
burning Combustion, or burning, is a high-temperature exothermic In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physica ...
".


Capacity

CD-ROM capacities are normally expressed with
binary prefixes A binary prefix is a unit prefix A unit prefix is a specifier or mnemonic that is prepended to units of measurement A unit of measurement is a definite magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity de ...
, subtracting the space used for error correction data. A standard 120 mm, ''700 MB'' CD-ROM can actually hold about 703 MB of data with error correction (or 847 MB total). In comparison, a single-layer
DVD-ROM The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital media, digital optical disc data storage format invented and developed in 1995 and released in late 1996. Currently allowing up to 17.08 Gigabyte, GB o ...

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


CD-ROM drives

CD-ROM discs are read using CD-ROM drives. A CD-ROM drive may be connected to the computer via an IDE (
ATA ATA or Ata may refer to: Computing * AT Attachment (ATA/ATAPI), the old name of Parallel ATA Parallel ATA (PATA), originally , also known as ATA or Integrated Drive Electronics, IDE is standardization, standard Electrical connector, interface ...
),
SCSI Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interface ...
,
SATA Serial ATA (SATA, abbreviated from Serial AT Attachment) is a computer bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implemen ...

SATA
,
FireWire IEEE 1394 is an interface standard In telecommunication Telecommunication is the transmission of information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and thus defi ...
, or
USB Universal Serial Bus (USB) is an industry standard ''Industry Standard'' is a 1982 album by Dixie Dregs, The Dregs. It is their only album featuring vocals (by Alex Ligertwood of Santana (band), Santana and Patrick Simmons of The Doobie Brot ...
interface or a proprietary interface, such as the Panasonic CD interface, LMSI/Philips, Sony and Mitsumi standards. Virtually all modern CD-ROM drives can also play
audio CDs#REDIRECT Compact disc {{redirect category shell, {{R move {{R from other capitalization, printworthy {{R yes print ...
(as well as
Video CD Video CD (abbreviated as VCD, and also known as Compact Disc Digital Video) is a home video Home video is prerecorded media sold or rented for home viewing. The term originates from the VHS VHS (Video Home System) is a standard Sta ...
s and other data standards) when used with the right software.


Laser and optics

CD-ROM drives employ a near-
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior ...

infrared
780 nm
laser diode A packaged laser diode shown with a penny for scale The laser diode chip removed and placed on the eye of a needle for scale A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor A semiconductor mater ...

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 Compact Disc Digital Audio (CDDA or CD-DA), also known as Digital Audio Compact Disc or simply as Audio CD, is the standardization, standard format for audio compact discs. The standard is defined in the ''Red Book'', one of a series of Rainbow ...
, the data transfer rate is 150 KB/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 Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is the number of turns in one minute The minute is a unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the ...
at the inner edge to 200 rpm at the outer edge. The 1× speed rating for CD-ROM (150 KB/s) is different from the 1× speed rating for
DVD The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electr ...

DVD
s (1.32 MB/s).


Speed advancements

By increasing the speed at which the disc is spun, 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 KB/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
Samsung The Samsung Group (or simply Samsung) ( ko, 삼성) is a South Korean Multinational corporation, multinational manufacturing Conglomerate (company), conglomerate headquartered in Samsung Town, Seoul, South Korea. It comprises numerous affil ...

Samsung
Electronics introduced the SCR-3230, a 32x CD-ROM drive which uses a ball
bearing Bearing may refer to: * Bearing (angle), a term for direction * Bearing (mechanical), a component that separates moving parts and takes a load * Bridge bearing, a component separating a bridge pier and deck * Bearing BTS Station in Bangkok See also ...
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 52× or 10,400 rpm and 7.62 MB/s. Higher spin speeds are limited by the strength of the polycarbonate plastic of which the discs are made. At 52×, the linear velocity of the outermost part of the disc is around 65 m/s. However, improvements can still be obtained using multiple laser pickups as demonstrated by the
KenwoodKenwood may refer to: Places ; England * Hampstead Heath#Kenwood, Kenwood (or Ken Wood), a part of Hampstead Heath, London, the location of ** Kenwood House * Kenwood, in the parish of Kenton, Devon * Kenwood, St. George's Hill, John Lennon's hom ...
TrueX 72× which uses seven laser beams and a rotation speed of approximately 10×. The first 12× drive was released in late 1996. 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 In optical storage, constant linear velocity (CLV) is a qualifier for the rated speed of an optical disc drive In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes ...
, 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.


Workarounds

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,
GigaRec CD-R (COMPACT DISC – Recordable) is a digital media, digital optical disc data storage device, storage format. A CD-R disc is a compact disc that can be Write once read many, written once and read arbitrarily many times. CD-R discs (CD-Rs) a ...
and double-density compact disc ( Purple Book standard) recorders, their utility was nullified by the introduction of consumer
DVD-ROM The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital media, digital optical disc data storage format invented and developed in 1995 and released in late 1996. Currently allowing up to 17.08 Gigabyte, GB o ...

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 (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


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 Compact Disc Digital Audio (CDDA or CD-DA), also known as Digital Audio Compact Disc or simply as Audio CD, is the standardization, standard format for audio compact discs. The standard is defined in the ''Red Book'', one of a series of Rainbow ...
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. Manufacturers of CD writers (
CD-R CD-R (Compact disc-recordable) is a digital media, digital optical disc data storage device, storage format. A CD-R disc is a compact disc that can be Write once read many, written once and read arbitrarily many times. CD-R discs (CD-Rs) are r ...

CD-R
or
CD-RW CD-RW (Compact Disc-Rewritable) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Electron ...
) 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 Source Identification Code (SID), an eight character code beginning with "
IFPI The International Federation of the Phonographic Industry (IFPI) is the organisation that represents the interests of the recording industry A record, recording or records may refer to: An item or collection of data Computing * Record (comput ...
" that is usually stamped on discs produced by CD recording plants.


See also

* ATA Packet Interface (ATAPI) * Optical recording (history of) * CD/DVD authoring *
Compact Disc Digital Audio Compact Disc Digital Audio (CDDA or CD-DA), also known as Digital Audio Compact Disc or simply as Audio CD, is the standardization, standard format for audio compact discs. The standard is defined in the ''Red Book'', one of a series of Rainbow ...
*
Computer hardware Computer hardware includes the physical parts of a computer, such as the computer case, case, central processing unit (CPU), Computer monitor, monitor, Computer mouse, mouse, Computer keyboard, keyboard, computer data storage, graphics card, so ...

Computer hardware
*
DVD-Audio DVD-Audio (commonly abbreviated as DVD-A) is a digital format for delivering high-fidelity High fidelity (often shortened to Hi-Fi or HiFi) is the high-quality reproduction of sound. It is important to audiophile An audiophile is a perso ...
*
DVD-ROM The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital media, digital optical disc data storage format invented and developed in 1995 and released in late 1996. Currently allowing up to 17.08 Gigabyte, GB o ...

DVD-ROM
*
MultiLevel Recording MultiLevel Recording (ML, also known as M-ary) was a technology originally developed by Optex CorporationEarman, Allen, "Optical Data Storage With Electron Trapping Materials Using M-ary Data Channel Coding," Proceedings of the Optical Data Storage ...
*
Optical disc drive In computing, an optical disc drive (ODD) is a disk drive, disc drive that uses laser light or electromagnetic waves within or near the visible light spectrum as part of the process of reading or writing data to or from optical discs. Some ...
* Phase-change Dual * Thor-CD * DVP Media, patent holder for self-loading and self configuring CD-ROM technology * List of optical disc manufacturers


Notes


References

{{Authority control Compact disc Rotating disc computer storage media Ecma standards Optical computer storage Optical computer storage media Rainbow Books Video game distribution