Mode 2, which is more appropriate for image or video data where perfect reliability may be a little bit less important , contains no additional error detection or correction bytes, having therefore 2, available data bytes per sector. Note that both modes, like audio CDs, still benefit from the lower layers of error correction at the frame level.
Before being stored on a disc with the techniques described above, each CD-ROM sector is scrambled to prevent some problematic patterns from showing up. The playing time of a standard CD is 74 minutes, or 4, seconds, contained in , blocks or sectors.
For 80 minute CDs, the capacity is MB. XA Mode 2 Form 2 has 2, bytes of user data, and is similar to the standard Mode 2 but with error detection bytes added though no error correction. The file size of a disc image created in raw mode is always a multiple of 2, 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,; this is usually the case for ISO disc images. The 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.
CD-ROM drives employ a near- infrared 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. At this data rate, the track moves along under the laser spot at about 1. To maintain this linear velocity as the optical head moves to different positions, the angular velocity is varied from about rpm at the inner edge to rpm at the outer edge.
By increasing the speed at which the disc is spun, data can be transferred at greater rates. Higher spin speeds are limited by the strength of the polycarbonate plastic of which the discs are made. 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 s.
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,—13, rpm i.
High rotational speeds also produce undesirable noise from disc vibration, rushing air and the spindle motor itself. 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 numerous sequential read errors and retries are encountered.
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. 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.
From Wikipedia, the free encyclopedia. Pre-pressed compact disc containing computer data. Optical disc Optical disc drive Optical disc authoring Authoring software Recording technologies Recording modes Packet writing Burst cutting area. Optical media types. History of optical storage media High-definition optical disc format war. Main article: Compact Disc manufacturing. This section needs expansion with: Information on access time latency.
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