||This article needs additional citations for verification. (February 2013)|
|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×)|
CD-ROMs are popularly used to distribute computer software, including video games and multimedia applications, though any data can be stored (up to the capacity limit of a disc). 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 format PC CD-ROMs).
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, created by Sony and Philips, was the first extension of Compact Disc Digital Audio. It adapted the format to hold any form of data.
CD-ROMs are identical in appearance to audio 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 of polycarbonate plastic, with a thin layer of aluminium to make a reflective surface. The most common size of CD-ROM is 120 mm in diameter, though the smaller Mini CD standard with an 80 mm diameter, as well as numerous non-standard sizes and shapes (e.g., business card-sized media) are also available.
Data is stored on the disc as a series of microscopic indentations. A laser is shone onto the reflective surface of the disc to read the pattern of pits and lands ("pits", with the gaps between them referred to as "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's phase is shifted in relation to the incoming beam, causing destructive interference and reducing the reflected beam's intensity. This pattern of changing intensity of the reflected beam is converted into binary data.
Several formats are used for data stored on compact discs, known as the Rainbow Books. The Yellow Book defines the specifications for CD-ROMs, standarized as the ISO/IEC 10149 / ECMA-130 standard. The CD-ROM standard builds on top of the original 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 ISO/IEC 10149 / ECMA-130 standard gives a thorough description of the physics and physical layer of the CD-ROM, including cross-interleaved Reed-Solomon coding (CIRC) and eight-to-fourteen modulation (EFM). It can be downloaded for free from ISO or ECMA.
ISO 9660 defines the standard file system for a CD-ROM. ISO 13490 is an improvement on this standard which adds support for non-sequential write-once and re-writeable discs such as CD-R and CD-RW, 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 DVDs. The bootable CD specification, to make a CD emulate a hard disk or floppy disk, is called El Torito.
- Standard ECMA-130: Data Interchange on Read-only 120 mm Optical Data Disks (CD-ROM)
- ISO - Freely Available Standards - ISO/IEC 10149:1995
CD-ROM format 
Like audio CDs, a CD-ROM sector contains 2,352 bytes, divided into 98 24-byte frames. 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 (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 (including the original sector structure for audio CDs in the case of mixed mode CDs).
Both Mode 1 and 2 sectors use the first 16 bytes for header information, but differ in the remaining 2,336 bytes due to the use of error correction bytes. Unlike an audio CD, a CD-ROM cannot rely on error concealment by interpolation, and therefore requires a higher reliability of the retrieved data. In order to achieve improved error correction and detection, a CD-ROM adds a third layer of Reed–Solomon error correction. Mode 1, used mostly for digital data, 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, contains no error detection or correction bytes, having therefore 2,336 available data bytes per sector. Note that both modes, as well as audio CDs, still benefit from a lower layer of error correction at a sub-sector level.
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 (Green Book) and was developed by Sony and Philips. "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 CDs and Super Video CDs use these sector modes.
The following table shows a comparison of the structure of sectors in CD-DA and CD-ROMs:
|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)|
|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)|
The sync pattern contained in the first 12 bytes is
The net byte rate of a Mode-1 CD-ROM, based on comparison to CD-DA audio standards, is 44100 Hz × 16 bits/sample × 2 channels × 2,048 / 2,352 /8 = 153.6 kB/s = 150 KiB/s. The playing time is 74 minutes, or 4,440 seconds, so that the net capacity of a Mode-1 CD-ROM is 682 MB or, equivalently, 650 MiB. A standard 74 min. CD contains 333,000 blocks or sectors. A 1× speed CD drive reads 75 consecutive sectors per second.
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 type). An image size created in raw mode is always a multiple of 2,352 bytes (the size of a block). 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, it is possible to fit larger 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.
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 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|
|Note: megabyte (MB) and minute (min) values are exact; MiB values are approximate.|
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), SCSI, SATA, FireWire, or USB interface or a proprietary interface, such as the Panasonic CD interface. Virtually all modern CD-ROM drives can also play audio CDs (as well as Video CDs and other data standards) when used in conjunction 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 
CD-ROM drives are rated with a speed factor relative to music CDs (1× or 1-speed which gives a data transfer rate of 150 KiB/s). 12× drives were common beginning in early 1997. Above 12× speed, there are problems with vibration and heat. Constant angular velocity (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. Thankfully, 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 a large number of 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 and "double density" recorders, their utility was nullified by the introduction of consumer DVDROM 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.
If a CD-ROM is read at the same rotational speed as an audio CD, the data transfer rate is 150 KiB/s, commonly referred to as "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.
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 KiB/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 Electronics introduced the SCR-3230, a 32x CD-ROM drive which uses a ball bearing 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 MiB/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 disk is around 65 m/s. However, improvements can still be obtained by the use of multiple laser pickups as demonstrated by the Kenwood TrueX 72× which uses seven laser beams and a rotation speed of approximately 10×.
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).
The 1× speed rating for CD-ROM (150 KiB/s) is different than the 1× speed rating for DVDs (1.32 MiB/s).
|Transfer speed||KiB/s||Mbit/s||MiB/s ||RPM|
|20×||1,200–3,000||up to 24.576||up to 2.93||4,000 (CAV)|
|32×||1,920–4,800||up to 39.3216||up to 4.69||4,800 (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)|
|72×||6,750–10,800||up to 88.4736||up to 10.5||2,000 (multi-beam)|
Copyright issues 
There have been moves by the recording industry to make audio CDs (Compact Disc Digital Audio) unplayable on computer CD-ROM drives, to prevent the copying of music. This is done by intentionally introducing errors onto the disc that the embedded circuits on most stand-alone audio players can automatically compensate for, but which may confuse CD-ROM drives. Consumer rights advocates as of October 2001 pushed to require warning labels on compact discs that do not conform to the official Compact Disc Digital Audio standard (often called the Red Book) to inform consumers which discs do not permit full fair use of their content.
In 2005, Sony BMG Music Entertainment was criticised when a copy protection mechanism known as Extended Copy Protection (XCP) used on some of their audio CDs automatically and surreptitiously installed copy-prevention software on computers (see 2005 Sony BMG CD copy protection scandal). Such discs are not legally allowed to be called CDs or Compact Discs because they break the Red Book standard governing CDs, and Amazon.com for example describes them as "copy protected discs" rather than "compact discs" or "CDs".
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.
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. This is a counterpart to the The Source IDentification Code (SID)—the Source Identification Code, an eight character code beginning with "IFPI" that is usually stamped on discs produced by CD recording plants.
See also 
- CD/DVD authoring
- Compact Disc Digital Audio
- Computer hardware
- MultiLevel Recording
- Optical disc drive
- Phase-change Dual
- ISO (1995). "ISO/IEC 10149:1995 – Information technology – Data interchange on read-only 120 mm optical data disks (CD-ROM)". Retrieved 2010-08-06.
- Data Interchange on Read-only 120 mm Optical Data Disks (CD-ROM). ECMA. June 1996. Retrieved 2009-04-26.
- Birth Announcement: ISO/IEC 13346 and ISO/IEC 13490
- Note that the CIRC error correction system used in the CD audio format has two interleaved layers.
- 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.
- What are CD-ROM Mode-1, Mode-2 and XA?, Sony Storage Support
- "Optical Media FAQs" (PDF). Archived from the original on 2006-10-22. Retrieved 2007-01-06.
- to three significant figures
- Schoen, Seth (July 20, 2007). "Harry Potter and the Digital Fingerprints". Electronic Frontier Foundation. Retrieved October 24, 2007.