CD-ROM
| Optical discs |
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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 1.2 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 ("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-ROM[1]. 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 this because its design originated in an El Torito restaurant in Irvine, California.
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.[2] 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 extracting the disc in RAW format (standard for creating images) always extract 2352 bytes per sector, not 2048/2336/2352 bytes depending on data type (basically, extracting the whole sector). This fact has two main consequences:
- Recording data CDs at very high speed (40x) can be done without losing information. However, if done the same with PlayStation or Audio CD it will result in an unreadable PlayStation disc or an audio CD with lots of clicks because there are no error correction codes and the errors are more likely to occur at high speed recording.
- On a 74 minute CD can fit very large RAW images, up to 333,000 × 2352 = 783,216,000 bytes (747 MiB). This should be the upper limit for a RAW image created from a 74 min CD. If the stored standard data (backup files), it can burn only 333,000 × 2048 = 681,984,000 bytes (650 MiB).
- Please note that an image size is always a multiple of 2352 bytes (extracting SECTORS), if extracted in RAW mode.
| Layout Type | ← 2,352 bytes → | |||||
| CD Audio block: | 2,352 bytes of Audio | |||||
|---|---|---|---|---|---|---|
| CD-ROM (MODE1) sector: | 12 | 4 | 2,048 bytes of user data | 4 | 8 | 276 |
| CD-ROM (MODE2) sector: | 12 | 4 | 2,236 bytes of user data | |||
| Legend | |
|---|---|
| 12 | sync |
| 4 | sector ID |
| user data | |
| 4 | error detection |
| 8 | null |
| 276 | 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 120 mm 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 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.
| Type | Sectors | Data max size | Audio max size | Time | ||
|---|---|---|---|---|---|---|
| (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.
Durability
[1] Stability Study [2] Care, Handling and Storage of Removeable Media.
CD-ROM drives
CD-ROM discs 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:
| 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 80 mm mini CDs or any non-standard sizes, however, the Nintendo Wii seems to have defeated this problem, for it will be able to load standard size DVDs and smaller size GameCube discs in the same slot-loading drive. A small number of CD-ROM drive models, mostly compact portable units, have a top-loading mechanism where the drive lid is opened upwards and the disc is placed directly onto the spindle (similar to most portable CD players).. (Example - [3])
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. One exception is the PSP (Play Station Portable), which uses a 80 mm DVD inside a unopenable caddie. This was partly as a copy protection system as well as damage prevention. Unspecialised 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 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 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.
See also
References
This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.