 | |
 A traditional CD-ROM | |
| Media type | Optical disc |
|---|---|
| Capacity | 650–870 MiB |
| Read mechanism | 780 nmlaser diode, 150–10,800 KB/s (1–72×) |
| Write mechanism | Pressed mold |
| Standard | Yellow Book (ISO/IEC 10149)[1] |
| Developed by | Sony · Philips |
| Usage | Computer data storage |
| Extended to | CD-ROM XA · DVD-ROM |
| Released | 1985; 40 years ago (1985) |
ACD-ROM (/ˌsiːdiːˈrɒm/,compact disc read-only memory) is a type ofread-only memory consisting of a pre-pressed opticalcompact disc that containsdata computers can read, but not write or erase. Some CDs, calledenhanced CDs, hold both computer data and audio with the latter capable of being played on aCD player, while data (such as software or digital video) is only usable on a computer (such asISO 9660[2] format PC CD-ROMs).
During the 1990s and early 2000s, CD-ROMs were popularly used to distributesoftware anddata for computers andfifth generation video game consoles.DVDs as well asdownloading started to replace CD-ROMs in these roles starting in the early 2000s, and the use of CD-ROMs for commercial software is now rare.
The earliest theoretical work on optical disc storage was done by independent researchers in the United States includingDavid Paul Gregg (1958) andJames Russel (1965–1975). In particular, Gregg's patents were used as the basis of theLaserDisc specification that was co-developed betweenMCA andPhilips 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 byToshi Doi andKees Schouhamer Immink during 1979–1980, who worked on a taskforce forSony andPhilips.[4] The result was theCompact Disc Digital Audio (CD-DA), defined in 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 553MB.[5] Sony and Philips created thetechnical standard that defines the format of a CD-ROM in 1983,[6] in what came to be called theYellow Book. The CD-ROM was announced in 1984[7] and introduced byDenon andSony at the first JapaneseCOMDEX computer show in 1985.[8] The first CD-ROM drive released to the public, theCM 100, was introduced in July 1985 by Philips.[9][10][11] In November 1985, several computer industry participants, includingMicrosoft,Philips,Sony,Apple andDigital Equipment Corporation, met to create a specification to define afile system format for CD-ROMs.[12] The resulting specification, called the High Sierra format, was published in May 1986.[12] It was eventually standardized, with a few changes, as theISO 9660 standard in 1988. One of the first products to be made available to the public on CD-ROM was theGrolier Academic Encyclopedia, presented at theMicrosoft CD-ROM Conference in March 1986.[12]
CD-ROMs began being used inhome video game consoles starting with thePC Engine CD-ROM2 (TurboGrafx-CD) in 1988, while CD-ROM drives had also become available forhome computers by the end of the 1980s. In 1990,Data East demonstrated anarcade system board that supported CD-ROMs, similar to 1980sLaserDisc video games but with digital data, allowing more flexibility than older LaserDisc games.[13] 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.[14] 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. The firstlaptop to have an integrated CD-ROM drive as an option was 1993'sCF-V21P byPanasonic; however, the drive only supportedmini CDs up to 3.5 inches in diameter.[15]: 111 The first notebook to support standard 4.7-inch-diameter discs wasIBM'sThinkPad 755CD in 1994.[16]
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, thesubcode 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.[17][18]
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CD-ROMs are identical in appearance toaudio CDs, 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 ofpolycarbonateplastic, with a thin layer ofaluminium to make a reflective surface. The most common size of CD-ROM is 120 mm in diameter, though the smallerMini CD standard with an 80 mm diameter, as well asshaped 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". Alaser 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 thewavelength of the laser light used to read the disc, the reflectedbeam'sphase is shifted in relation to the incoming beam, causingdestructive interference and reducing the reflected beam's intensity. This is converted intobinary data.
Several formats are used for data stored on compact discs, known as theRainbow Books. TheYellow Book, created in 1983,[6][19] defines the specifications for CD-ROMs, standardized in 1988 as theISO/IEC 10149[1] standard and in 1989 as theECMA-130[20] standard. The CD-ROM standard builds on top of the originalRed BookCD-DA standard for CD audio. Other standards, such as theWhite Book forVideo CDs, further define formats based on the CD-ROM specifications. TheYellow 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 defines the standardfile system for a CD-ROM.ISO 13490 is an improvement on this standard which adds support for non-sequentialwrite-once and re-writeable discs such asCD-R andCD-RW, as well asmultiple sessions. The ISO 13346 standard was designed to address most of the shortcomings of ISO 9660,[21] and a subset of it evolved into theUDF format, which was adopted forDVDs. A bootable CD specification, calledEl Torito, was issued in January 1995, to make a CD emulate ahard disk orfloppy disk.
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 orphase transition material in a process that is often referred to as "burning".
Data stored on CD-ROMs follows the standardCD data encoding techniques described in theRed Book specification (originally defined foraudio CD only). This includescross-interleaved Reed–Solomon coding (CIRC),[22]eight-to-fourteen modulation (EFM), and the use ofpits 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 theRed Book. Like audio CDs (CD-DA), a CD-ROMsector contains 2,352bytes 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 sub code). 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.[2] Atrack (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 ofmixed mode CDs.
Both Mode 1 and 2 sectors use the first16 bytes forheader information, but differ in the remaining 2,336 bytes due to the use oferror correction bytes. Unlike an audio CD, a CD-ROM cannot rely on error concealment byinterpolation; 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-bitcyclic redundancy check (CRC) code for error detection, and a third layer ofReed–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 thelower layers of error correction at the frame level.[23]
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.[20] These scrambled sectors then follow the same encoding process described in theRed 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:[20]
| 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 × 2channels × 2,048 / 2,352 / 8 = 150KB/s (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 orsectors. Therefore, the net capacity of a Mode-1 CD-ROM is 650MB (650 × 220). For 80 minute CDs, the capacity is 703 MB.
CD-ROM XA is an extension of theYellow Book standard for CD-ROMs that combines compressed audio, video and computer data, allowing all to be accessed simultaneously.[24] It was intended as a bridge between CD-ROM andCD-i (Green Book / CD-interactive) and was published bySony andPhilips, and backed byMicrosoft, in 1991,[25] first announced in September 1988.[26] "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.[23]Video CDs,Super Video CDs,Photo CDs,Enhanced Music CDs andCD-i use these sector modes.[27]
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) | ||
When adisc 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 isalways a multiple of 2,352 bytes (the size of a block).[28] Disc image formats that store raw CD-ROM sectors includeCCD/IMG,CUE/BIN, andMDS/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 forISO 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 MBRed Book CD. The 14.8% increase is due to the discarding of error correction data.
CD-ROM capacities are normally expressed withbinary prefixes, subtracting the space used for error correction data.[29] The capacity of a CD-ROM depends on how close the outward data track is extended to the disc's outer rim.[30] 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-layerDVD-ROM can hold 4.7 GB (4.7 × 109 bytes) of error-protected data, more than 6 CD-ROMs.
| Type | Sectors | Data (mode 1) max. size | Audio max. size | Time | |
|---|---|---|---|---|---|
| (MiB) | Approx. (1 = 220) | (MiB) | (min) | ||
| 8 cm | 94,500 | 184.570 | 193.536 | 211.967 | 21 |
| 553 MB | 283,500 | 553.711 | 580.608 | 635.902 | 63 |
| 650 MB | 333,000 | 650.391 | 681.984 | 746.933 | 74 |
| 700 MB | 360,000 | 703.125 | 737.280 | 807.495 | 80 |
| 800 MB | 405,000 | 791.016 | 829.440 | 908.432 | 90 |
| 900 MB | 445,500 | 870.117 | 912.384 | 999.275 | 99 |
| Mebibyte (MiB) and minute (min) values are exact; (1 = 220) values are approximate. | |||||
![[icon]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aWiki_letter_w_cropped.svg) | This sectionneeds expansion with: Information on access time latency. You can help byadding to it.(June 2014) |
CD-ROM discs are read using CD-ROM drives. A CD-ROM drive may be connected to the computer via an IDE (ATA),SCSI,SATA,FireWire, orUSB interface or a proprietary interface, such as thePanasonic CD interface, LMSI/Philips, Sony and Mitsumi standards. Virtually all modern CD-ROM drives can also playaudio CDs (as well asVideo CDs and other data standards) when used with the right software.
CD-ROM drives employ a near-infrared 780nmlaser 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.
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 anaudio 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 500rpm 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 forDVDs (1.32 MB/s).
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 atConstant 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 untilSamsung Electronics introduced the SCR-3230, a 32× CD-ROM drive which uses a ballbearing 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 theKenwood TrueX 72× which uses seven laser beams and a rotation speed of approximately 10×.
The first 12× drive was released in late 1996.[31] 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.
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,GigaRec anddouble-density compact disc (Purple Book standard) recorders, their utility was nullified by the introduction of consumerDVD-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 2.5 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 8.5GB (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 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), if the CPU andmedia player software permit speeds that high.
| Transfer speed | KB/s | Mbit/s | MB/s[n 2] | RPM (outer to inner edge) |
|---|---|---|---|---|
| 1× | 150 | 1.2288 | 0.146 | 200–530[32][33] |
| 2× | 300 | 2.4576 | 0.293 | 400–1,060 |
| 4× | 600 | 4.9152 | 0.586 | 800–2,120 |
| 8× | 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 (CAV) |
| 24× | 1,440–3,600 | up to 29.491 | up to 3.51 | 4,800 (CAV) |
| 32× | 1,920–4,800 | up to 39.3216 | up to 4.69 | 6,400 (CAV) |
| 36× | 2,160–5,400 | up to 44.2368 | up to 5.27 | 7,200 (CAV) |
| 40× | 2,400–6,000 | up to 49.152 | up to 5.86 | 8,000 (CAV) |
| 48× | 2,880–7,200 | up to 58.9824 | up to 7.03 | 9,600 (CAV) |
| 52× | 3,120–7,800 | up to 63.8976 | up to 7.62 | 10,400 (CAV) |
| 56× | 3,360–8,400 | up to 68.8128 | up to 8.20 | 11,200 (CAV)[34] |
| 72× | 6,750–10,800 | up to 88.4736 | up to 10.5 | 2,700 (multi-beam)[35][36] |
A 2003MythBusters episode erroneously claims that a CD spinning at "52× speed" spins at 30,000 rotations per minute (rpm).[37] This miscalculation is caused by assuming alinear velocity of 52× at the inner edge of the data area of the disc, which would indeed reach dangerous speeds of over 25,000 rpm, therefore optical drives never spin discs at such speeds. In actuality, a speed of "52×" are approximately 10,000 rpm, given that drives are advertised with theirangular velocities. The angular velocity is the measured as thelinear velocity at the outermost edge of the disc, where the linear velocity (and accordingly the data transfer rate) is roughly 2.5 times higher than at the innermost edge of the data area.[32]
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 fromaudio 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 orCD-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.[38] 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.
Here comes Diamond with the first 12X CD-ROM.
Standards ofEcma International | ||
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| Application interfaces |  | |
| File systems (tape) | ||
| File systems (disk) | ||
| Graphics | ||
| Programming languages | ||
| Radio link interfaces | ||
| Other | ||
List of Ecma standards (1961 – present) | ||
| Discs | |
|---|---|
| Technology | |
| Hardware/software/content | |
| Court cases | |
| Lists | |
| Variants based on CD | |
| See also | |
| International | |
|---|---|
| National | |
| Other | |
