Constant angular velocity

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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:[citation needed] 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. In other words, The data-rate on an LP decreases as the stylus moves towards the center. This is due to less information passing per revolution from the longer run length as opposed to the middle where it moves over less vinyl. The amount of information stored in the vinyl is directly proportional to the linear length of a track. More track length passes underneath the stylus per unit time on the outside of the disc, than near the centre of the disc. Thus, data-rate decreases.

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 shorter than those on the outside of the disk, this constant angular 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, though some would think that the "constant angular velocity" principle applied on all disk-shaped media.

Uses[edit]

Constant Angular Velocity (CAV) LaserDisc, showing the NTSC field setup and individual scanlines.

Gramophone records have always used CAV, including CEDs which provide video signals.

Some high speed CD and DVD drives use CAV.[specify]

CAV was used in the LaserDisc format for interactive titles; it was also used with special editions of certain films.[1] 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, the original Xbox discs, as well as the Sega GD-rom.[citation needed]

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References[edit]

  1. ^ "The Criterion Collection: Ghostbusters". voyagerco.com. Voyager Media Store. Archived from the original on 1997-02-13. Retrieved 2012-04-30.