CD-ROM

CD-ROM (an abbreviation of "Compact Disc Read-Only Memory") is a compact disc that contains data accessible by a computer. While the compact disc format was originally designed for music storage and playback, the format was later adapted to hold any form of binary data. 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, whilst data (such as software or digital video) is only usable on a computer. 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.

Media

CD-ROM discs are identical in appearance to audio CDs, and data is 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 1.2mm 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 120mm in diameter, though the smaller Mini CD standard with an 80mm 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 ("pits", with the gaps between them referred to as "lands"). A 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'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.

Standards

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-ROMParker, Dana J. The CD-Recordable Handbook. Cyberage Books, 1996. ISBN 0910965188. 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).

CD-ROM format

A CD-ROM sector contains 2352 bytes, divided into 98 24-byte frames. The CD-ROM is, in essence, a data disk, which cannot rely on error concealment, 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.Note that the CIRC error correction system used in the CD audio format has two interleaved layers. A Mode-1 CD-ROM, which has the full three layers of error correction data, contains a net 2048 bytes of the available 2352 per sector. In a Mode-2 CD-ROM, which is mostly used for video files, there are 2336 user-available bytes per sector. The net byte rate of a Mode-1 CD-ROM is 44.1k×2048/(6×98) = 153.6 kB/s. The playing time is 74 minutes, or 4440 seconds, so that the net capacity of a Mode-1 CD-ROM is 682 MB.

A 1x speed CD drive reads 75 consecutive sectors per second.

CD Sector Contents

A standard 74 min CD contains 333,000 sectors.
Each sector is 2352 bytes, and contains 2048 bytes of PC (MODE1) Data, 2336 bytes of PSX/VCD (MODE2) Data or 2352 bytes of AUDIO.
The difference between sector size and data content are the Headers info and the Error Correction Codes, that are big for Data (high precision required), small for VCD (standard for video) and none for audio.
If you extract data in RAW format (standard for creating images) you always extract 2352 bytes per sector, not 2048/2336/2352 bytes depending on data type (basically, you extract the whole sector). This fact has two main consequences:
- You can record data at very high speed (40x) without losing information, but if you try to do the same with PSX or Audio you get unreadable CD (for PSX) or audio CD with lots of clicks because there are no error correction codes (and error are more likely to occur if you record at high speed.)
- On a 74 min CD you can fit very large RAW images,up to 333,000 x 2352 = 783,216,000 bytes (747 Mb). This should be the upper limit for a RAW image created from a 74 min CD. Remember that if you store standard data (backup files), you can burn only 333,000 x 2048 = 681,984,000 bytes (the well known 650 MB limit).
Please note that an image size is ALWAYS a multiple of 2352 bytes (you extract SECTORS), if extracted in RAW mode.

A Mode-1 CD-ROM sector contains 2352 bytes:

12 bytes: sync
4 bytes: sector ID
2048 bytes: user data
4 bytes: error detection
8 bytes: null
276 bytes: error correction

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 in turn are 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 is recorded on them by a laser changing the properties of a dye or phase change material in a process that is often referred to as "burning".

Capacity

A standard 120mm CD-ROM holds 650 or 700 MiB of data. To put this 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.

CD-ROM drives

CD-ROMs are read using CD-ROM drives, which are now almost universal on personal computers. 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.

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: 1x or 1-speed which gives a data transfer rate of 150 kilobytes per second 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 8x speed spins the disc at up to 4000 rpm (compared to the 500 rpm maximum for 1x speed), giving a transfer rate of 1.2 megabytes per second. Above 12x 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 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).

Common transfer speeds:

Data Transfer Speeds
Transfer Speed	Megabytes/s	Megabits/s	Mebibits/s
1x	0.15	1.2	1.1444
2x	0.3	2.4	2.2888
4x	0.6	4.8	4.5776
8x	1.2	9.6	9.1553
10x	1.5	12.0	11.4441
12x	1.8	14.4	13.7329
20x	3.0	24.0	22.8882
32x	4.8	38.4	36.6211
36x	5.4	43.2	41.1987
40x	6.0	48.0	45.7764
48x	7.2	57.6	54.9316
50x	7.5	60.0	57.2205
52x	7.8	62.4	59.5093

Loading Mechanisms

Current CD-ROM drives use either a tray-loading mechanism, where the disc is loaded onto a motorised or manually-operated tray, or a slot-loading mechanism, where the disc is slid into a slot and drawn in by motorised rollers. Slot-loading drives have the disadvantage that they cannot usually accept the smaller 80mm mini CDs or any non-standard sizes. A very few CD-ROM drives, mostly compact portable units, have a top-loading mechanism where the drive lid is opened and the disc is placed directly onto the spindle (similar to most portable CD players). (Example - ^*)

Some early CD-ROM drives used a mechanism where CDs had to be inserted into special cartridges or caddies, somewhat similar in appearance to a 3.5" floppy diskette. Although the idea behind this – a tougher plastic shell to protect the disc from damage – was sound, it did not gain wide acceptance among disc manufacturers due to the increased cost of production and the concern that the discs would not be compatible with drives that did not use caddies. Drives that used the caddy format required "bare" discs to be placed into an openable cartridge before use, negating most of the protection offered by having discs in a permanent, integrated protective cartridge, as well as making the drives less convenient to use.

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 analogue 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 of which discs do not permit full fair use of their content.

In 2005, Sony BMG Music Entertainment were 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 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 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 unauthorised 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 SID - the Source Identification Code, an eight character code beginning with "IFPI" that is usually stamped on discs produced by CD recording plants.

References

External links

120 mm discs | Computer storage media | Audio storage

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