Constant angular velocity


Constant angular velocity
Comparison of several forms of disk storage showing tracks (not-to-scale); green denotes start and red denotes end.
* Some CD-R(W) and DVD-R(W)/DVD+R(W) recorders operate in ZCLV, CAA or CAV modes.

In optical storage, 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. A drive or disc operating in CAV mode maintains a constant angular velocity, contrasted with a constant linear velocity (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 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.

See also


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