CD-ROM

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CD-ROM (an abbreviation for "Compact Disc Read-only memory") in its simplest form, is a compact disc that contains data only accessible by a computer. All modern CD-ROM drives can also read audio CDs. It is possible to produce composite CDs containing both data and audio with the latter capable of being played on a CD player, whilst data or perhaps video can be viewed on a computer. These are called Enhanced CDs.

1 Manufacture
2 Capacity
3 CD-ROM drives
4 Copyright issues
5 Data formats
6 See also
7 References
8 External links

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Manufacture

CD-ROMs are always mass-produced by a process called "stamping", whereas CD-Rs and CD-RWs are recorded, or "burned". Some devices have the ability to record multiple discs at once from a single source. The contents of a CD-R may be in logical CD-ROM format (Yellow Book) but the disc itself is physically a CD-R (Orange Book).<ref>Parker, Dana J. The CD-Recordable Handbook. Cyberage Books, 1996. ISBN 0910965188</ref>

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Capacity

The standard CD-ROM holds 650 or 300,000 MiB of data. The CD-ROM is popular for distribution of software, especially multimedia applications, and large databases. A CD weighs less than 30 grams.

To put the CD-ROM's storage capacity into context, the average novel contains 60,000 words. Assume that average word length is 10 letters—in fact it's considerably less than 10—and that each letter occupies one byte. A novel therefore might occupy 600,000 bytes (600 kB, without layout information). One CD can therefore contain over 1,000 novels. If each novel occupies at least one centimetre of bookshelf space, then one CD can contain the equivalent of over ten metres of bookshelf. However textual data can be compressed by more than a factor of ten, using compression algorithms, so a CD-ROM can accommodate at least 100 metres of bookshelf space.

In comparison a DVD typically may contain at least 4.4 GiB of data, nearly 7 times the amount of a CD-ROM.

Capacities of Compact Disc types
Type	Sectors	Data max size		Audio max size		Time
Type	Sectors	(MB)	(MiB)	(MB)	(MiB)	(min)
8 cm	94,500	193.536	≈ 184.6	222.264	≈ 212.0	21
	283,500	580.608	≈ 553.7	666.792	≈ 635.9	63
650 MB	333,000	681.984	≈ 650.3	783.216	≈ 746.9	74
700 MB	360,000	737.280	≈ 703.1	846.720	≈ 807.4	80
	405,000	829.440	≈ 791.0	952.560	≈ 908.4	90
	445,500	912.384	≈ 870.1	1,047.816	≈ 999.3	99

Note: Megabyte (MB) and minute (min) values are exact.

CD capacities are always given in binary units, although decimal SI prefixes are usually used: A "700 MB" CD has a nominal capacity of about 700 MiB. DVD capacities on the other hand are given in decimal units: A "4.7 GB" DVD has a nominal capacity of about 4.38 GiB.

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CD-ROM drives

Image:CD-ROM drive.jpg

CD-ROMs are read using CD-ROM drives and written with CD recorders (often referred to as "burners"). CD-ROM drives—now almost-universal on personal computers—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 and Video CDs with the right software.

CD-ROMs are written and read using hills and valleys on the disc that are microscopic. The light/dark and dark/light (pit edge) transitions are translated into 1 bits, the areas with no transitions are translated into 0 bitsTemplate:Fact. The CD-ROM reads the hills and valleys by using laser reflecting off of the CD-ROM into a detector that can sense the difference in reflection.

CD-ROM drives are rated with a speed factor relative to music CDs: 1x or 1-speed which gives a data transfer rate of 150 kilobytes per second in the most common data format. For example, an 8x CD-ROM data transfer rate would be 1.2 megabytes per second. Above 12x speed, there are problems with vibration and heat. Constant angular velocity (CAV) drives give speeds up to 20x but due to the nature of CAV the actual throughput increase over 12x is less than 20/12. 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 52x or 7.62 megabytes per second, though this is only when reading information from the outer parts of a disc. Future speed increases based simply upon spinning the disc faster are particularly limited by the strength of polycarbonate plastic used in CD manufacturing, though improvements can still be obtained by the use of multiple laser pickups as demonstrated by the Kenwood TrueX 72x which uses seven laser beams and a rotation speed of approximately 10x.

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; ie a 12x/10x/32x CD drive can, CPU and media-permitting, write to CD-R disks at 12x speed (1.76 megabytes/s), write to CD-RW discs at 10x speed (1.46 megabytes/s), and read from CD discs at 32x speed (4.69 megabytes/s).

The 1x speed rating for CDs (150 kilobytes/s) is not to be confused with the 1x speed rating for DVDs (1.32 megabytes/s).

Some of the initial versions of CD Drives had a mechanism different from the tray or slot loaders of modern day drives. They could read CDs only when they were inserted in special cartridges. The "CD Caddy" resembled the floppy disk because of its protective casing. It never became popular, however, possibly because it would be cheaper for manufacturers to produce CDs without cartridges (CD caddies, although resembling ordinary jewel cases, were probably more expensive to produce because of their additional metal components and, unlike jewel cases, they could not hold a printed booklet or inlay cards). Furthermore, distributors using the cartridge format would still have to offer loose CDs for owners of non-caddy drives in order not to lose part of the market, whereas caddy owners could still play loose CDs by loading them into an openable cartridge which was then inserted into the drive (this of course defeated their entire purpose and was more fiddly than loading a disc into an ordinary drive).

Some might argue that caddies would be a good idea given the somewhat fragile nature of some cheaply produced modern CDs - whereas some early CDs had the recording medium sandwiched between two layers of plastic, offering good protection from scratches, some modern discs have a single layer of plastic with the medium stuck to one side, offering comparatively little protection from scratches to the top side of the disc, other than via the printed label paint - some particularly cheap recordable discs have even been known to have their recording layer peel away from the plastic substrate entirely, and of course become useless.

To use a CD-ROM disk in the drive, press the button (should be the only one visible) on the front of the drive to eject the tray, insert the disk so it lines up with the indent in the tray, and either push the tray back in or press the button again. Each of these closing methods are effective and, contrary to a common misconception, giving the tray a push to begin the retracting process does no damage to the drive. Over time retracting does affect the performance of a disc drive, but only on certain models that are not dependable.

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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 copying of the music. This is done by intentionally introducing errors onto the disc that the analogue circuits on most stand-alone audio players can automatically compensate for, but 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 of which discs do not permit full fair use of their content.

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 SID - the Source Identification Code, an eight character code beginning with "IFPI" that is usually stamped on discs produced by CD recording plants.

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Data formats

There are several formats used for CD-ROM data: the Rainbow Books, which include the Green Book, White Book and Yellow Book CD-ROM. ISO 9660 defines the standard file system of a CD-ROM, although it is due to be replaced by ISO 13490. UDF format is used on user-writable CD-R and CD-RW discs that are intended to be extended or overwritten. The bootable CD specification, to make a CD emulate a hard disk or floppy, is called El Torito (apparently named after the restaurant chain).

Informative CD-ROMs may contain links to webpages with additional information. To keep them up to date these are sometimes indirect: they link to webpages maintained by the producer of the CD-ROM which contain the links to external webpages.

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