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From Wikipedia, the free encyclopedia

CD-ROM
Media type Optical disc
Capacity 194 MiB (8 cm)
650–900 MB (12 cm)
Read mechanism 150 KiB/s (1×)
10,800 KiB/s (72×)
Write mechanism 150 KiB/s (1×)
8,400 KiB/s (56×)
Usage Data storage, video, audio, open internet
Optical discs
Optical media types
Standards
Further reading

CD-ROM (pronounced /ˌsiːˌdiːˈrɒm/, an acronym of "compact disc read-only memory") is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback, the 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data.[1]

CD-ROMs are popularly used to distribute computer software, including games and multimedia applications, though any data can be stored (up to the capacity limit of a disc). Some 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). These are called enhanced CDs.

Although many people use lowercase letters in this acronym, proper presentation is in all capital letters with a hyphen between CD and ROM. It was also suggested by some, especially soon after the technology was first released, that CD-ROM was an acronym for "Compact Disc read-only-media", or that it was a more "correct" definition. This was not the intention of the original team who developed the CD-ROM, and common acceptance of the "memory" definition is now almost universal. This is probably in no small part due to the widespread use of other "ROM" acronyms such as Flash-ROMs and EEPROMs where "memory" is usually the correct term.[citation needed]

At the time of the technology's introduction it had far more capacity than computer hard drives common at the time, although the reverse is now true though some experimental descendants of it such as Holographic versatile disc may not have more space than today's biggest hard drive.

Contents

Media

CD-ROM discs 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 disc 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 shown 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.

Standard

There are several formats used for data stored on compact discs, known collectively as the Rainbow Books. These include the original Red Book standards for CD audio, White Book and Yellow Book CD-ROM. The ECMA-130 standard, which gives a thorough description of the physics and physical layer of the CD-ROM, inclusive of cross-interleaved Reed-Solomon coding (CIRC) and eight-to-fourteen modulation (EFM), can be downloaded from ECMA.[2]

ISO 9660 defines the standard file system of a CD-ROM, although it is due to be replaced by ISO 13490 (which also supports CD-R and multi-session). UDF extends ISO 13346 (which was designed for non-sequential write-once and re-writeable discs such as CD-R and CD-RW) to support read-only and re-writeable media and was first adopted for DVD. The bootable CD specification, to make a CD emulate a hard disk or floppy, is called El Torito.

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 e.g. limits on achievable symmetry and strength in mass produced media, mean that CDROM drive speeds have not massively increased since the late 90s. 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 - 13,000rpm (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 pickup heads, 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× CDROM speeds (4× DVD) or higher. Additionally, with a 700mb CDROM 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 8.5GiB (single and dual layer), with higher speed and capacity needs instead being catered to by Blu-Ray drives.

CD-ROM format

A CD-ROM sector contains 2,352 bytes, divided into 98 24-byte frames. Unlike a music 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 has a third layer of Reed–Solomon error correction.[3] A Mode-1 CD-ROM, which has the full three layers of error correction data, contains a net 2,048 bytes of the available 2,352 per sector. In a Mode-2 CD-ROM, which is mostly used for video files, there are 2,336 user-available bytes per sector. The net byte rate of a Mode-1 CD-ROM, based on comparison to CDDA 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 1× speed CD drive reads 75 consecutive sectors per second.

CD sector contents

  • A standard 74 min. CD contains 333,000 blocks or sectors.
  • Each sector is 2,352 bytes, and contains 2,048 bytes of PC (mode 1) data, 2,336 bytes of PSX/VCD (mode 2) data, or 2,352 bytes of audio.
  • The difference between sector size and data content are the header information and the error-correcting codes, that are big for data (high precision required), small for VCD (standard for video) and none for audio.
  • If extracting the disc in raw format (standard for creating images) always extract 2,352 bytes per sector, not 2,048/2,336/2,352 bytes depending on data type (basically, extracting the whole sector). This fact has two main consequences:
    • Recording data CDs at very high speed (40×) can be done without losing information. However, as audio CDs do not contain a third layer of error-correcting codes, recording these at high speed may result in more unrecoverable errors or 'clicks' in the audio.
    • On a 74 minute CD, one can 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
    • The sync pattern for Mode 1 CDs is 0xff00ffffffffffffffff00ff[4]
  • An image size is always a multiple of 2,352 bytes (the size of a block) when extracting in raw mode.[5]
Layout type ← 2,352 byte block →
CD digital audio: 2,352
Digital audio
CD-ROM (mode 1): 12
Sync.
4
Sector id.
2,048
Data
4
Error detection
8
Zero
276
Error correction
CD-ROM (mode 2): 12
Sync.
4
Sector id.
2,336
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) and rewritable (CD-RW) discs are manufactured by a similar method, but 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".

Capacity

The CD-ROM can easily contain all the encyclopedia's words and images, plus audio & video clips

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.

Capacities of Compact Disc types (90 and 99 minute discs are not standard)
Type Sectors Data max. size Audio max. size Time
(MB) Approx. (MiB) (MB) (min)
8 cm 94,500 193.536 184.570 222.264 21
283,500 580.608 553.711 666.792 63
650 MB 333,000 681.984 650.391 783.216 74
700 MB 360,000 737.280 703.125 846.720 80
800 MB 405,000 829.440 791.016 952.560 90
900 MB 445,500 912.384 870.117 1,047.816 99
Note: megabyte (MB) and minute (min) values are exact; MiB values are approximate.

CD-ROM drives

An old 4× 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 (ATA), SCSI, S-ATA, 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.

CD-ROM drive can sometimes be a misnomer for newer drives that are capable for reading and burning DVDs, the CD's successor which is now the standard optical disc drive.

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

The rate at which CD-ROM drives can transfer data from the disc is gauged by a speed factor relative to music CDs: 1× or 1-speed which gives a data transfer rate of 150 KiB/s in the most common data format. 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 up to 4,000 rpm (compared to the 500 rpm maximum for 1× speed), giving a transfer rate of 1.2 MiB/s. Above 12× speed, vibration and heat can become a problem. CD-ROM drives above this speed tackle the problem in several ways. Constant angular velocity (CAV) drives spin the disc at a constant rate, leading to faster data transfer when reading from the outer parts of the disc, but slower towards the centre. 20x 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 (to 26,000) rpm and 7.62 MiB/s, CD is spinning faster when reading inner part and slower when moving to read outer part of the CD[citation needed]. Future speed increases based simply upon spinning the disc faster are particularly limited by the strength of polycarbonate plastic used in CD manufacturing. At 52×, the linear velocity of the outermost part of the disk is around 65 to 163 meters per second (235–588 km/h or 147-368 mph; 0.12 m × π × 10,400 rpm / 60 = 65 m/s and 0.12 m × π × 26,000 rpm / 60 = 163 m/s). This presents a danger of injury should the disk disintegrate (if the CD is damaged or defective) due to the rotationally induced centrifugal force (the centripetal force tension in the disc medium failing to keep this in equilibrium) in the disc at these speeds. Due to this the read speed in the innermost part of the CD is usually limited to slower speeds. (Indeed, severe damage to computer hardware was the consistent result as CD manufacturers tested the limits of the polycarbonate in controlled environments.) 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 CD discs at 32× speed (4.69 MiB/s).

The 1× speed rating for CD-ROM (150 KiB/s) is different than 1x speed rating for audio CD (172.3 KiB/s) and is not to be confused with the 1× speed rating for DVDs (1.32 MiB/s).

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 Drive.
Common data transfer speeds for CD-ROM drives
Transfer speed KiB/s Mbit/s RPM
150 1.2288 500
300 2.4576 1,000
600 4.9152 2,000
1,200 9.8304 4,000
10× 1,500 12.2880 5,000
12× 1,800 14.7456 6,000
20× 3,000 24.5760 10,000
32× 4,800 39.3216 16,000
36× 5,400 44.2368 18,000
40× 6,000 49.1520 20,000
48× 7,200 58.9824 24,000
50× 7,500 61.4400 25,000
52× 7,800 63.8976 26,000
56× 8,400 68.8128 28,000
72× 10,800 88.4736 36,000

Copyright issues

There has been a move by the recording industry to make audio CDs (CDDAs, Red Book CDs) 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 are as of October 2001 pushing 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.[6] This is a counterpart to the 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

References

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

  1. ^ EP patent 689208 "Method for block oriented addressing" - for block layouts see columns 1 and 2
  2. ^ Data Interchange on Read-only 120 mm Optical Data Disks (CD-ROM). ECMA. June 1996. http://www.ecma-international.org/publications/standards/Ecma-130.htm. Retrieved 2009-04-26. 
  3. ^ Note that the CIRC error correction system used in the CD audio format has two interleaved layers.
  4. ^ "ECMA-130 standard". http://www.ecma-international.org/publications/standards/Ecma-130.htm. Retrieved 2009-09-07. 
  5. ^ "Optical Media FAQs" (PDF). http://www.memorex.com/downloads/whitepapers/WhitePaper_Reference_Guide_Optical_Media_Mar2406.pdf. Retrieved 2007-01-06. 
  6. ^ Schoen, Seth. "Harry Potter and the Digital Fingerprints", Electronic Frontier Foundation, July 20, 2007. Retrieved October 24, 2007.

Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

English

Pronunciation

  • IPA: /si-di-rɒm/

Acronym

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Wikipedia

CD-ROM or CD ROM

  1. Compact Disk - Read Only Memory. A device that can read CDs (Compact Disks), but can not write to them. Additionally, this is also a class of Compact Disks that are manufactured with the data and shipped, but cannot be written to at all.

Gaming

Up to date as of February 01, 2010

From Wikia Gaming, your source for walkthroughs, games, guides, and more!

A CD-ROM is a type of optical disc media that usually allows for about 700 megabytes of storage space. These were used to replace floppy disks once games and programs became too large. The PlayStation also used CD-ROMs instead of cartridges like Nintendo used because of their increased storage space.
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