Constant angular velocity
Encyclopedia
In optical storage
Optical storage
Optical storage is a term from engineering referring to the storage of data on an optically readable medium. Data is recorded by making marks in a pattern that can be read back with the aid of light, usually a beam of laser light precisely focused on a spinning disc. An older example, that does...

, constant angular velocity (CAV) is a qualifier for the rated speed of an optical disc drive, and may also be applied to the writing speed of recordable discs
Optical disc
In computing and optical disc recording technologies, an optical disc is a flat, usually circular disc which encodes binary data in the form of pits and lands on a special material on one of its flat surfaces...

. A drive or disc operating in CAV mode maintains a constant angular velocity
Angular velocity
In physics, the angular velocity is a vector quantity which specifies the angular speed of an object and the axis about which the object is rotating. The SI unit of angular velocity is radians per second, although it may be measured in other units such as degrees per second, revolutions per...

, contrasted with a constant linear velocity
Constant linear velocity
In optical storage, constant linear velocity is a qualifier for the rated speed of an optical disc drive, and may also be applied to the writing speed of recordable discs. CLV implies that the angular velocity varies during an operation, as contrasted with CAV modes...

 (CLV).

Like all spinning-disk media, the CD-ROM drive includes a spindle motor that turns the media containing the data to be read. The spindle motor of a standard CD-ROM is very different from that of a hard disk or floppy drive in one very important way: it does not spin at a constant speed. Rather, the speed of the drive varies depending on what part of the disk (inside vs. outside) is being read.

Standard hard disks and floppy disks spin the disk at a constant speed. Regardless of where the heads are, the same speed is used to turn the media. This is called constant angular velocity (CAV) because it takes the same amount of time for a turn of the 360 degrees of the disk at all times. Since the tracks on the inside of the disk are much smaller than those on the outside of the disk, this constant speed means that when the heads are on the outside of the disk they will traverse a much longer linear path than they do when on the inside. Hence, the linear velocity is not constant.

Some high speed CD and DVD drives use CAV.

CAV was used in the LaserDisc
Laserdisc
LaserDisc was a home video format and the first commercial optical disc storage medium. Initially licensed, sold, and marketed as MCA DiscoVision in North America in 1978, the technology was previously referred to interally as Optical Videodisc System, Reflective Optical Videodisc, Laser Optical...

 format for interactive titles, as well as special editions of certain films. CAV allowed for perfect still frames, as well as random access to any given frame on a disc. Playing time, however, was cut in half from 60 minutes to 30 minutes.

CAV is used with Nintendo GameCube Game Disc and Wii Optical Disc, as well as the Sega GD-rom.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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