
In
optical storage
Optical storage refers to a class of data storage systems that use light to read or write data to an underlying optical media. Although a number of optical formats have been used over time, the most common examples are optical disks like th ...
, constant angular velocity (CAV) is a qualifier for the rated speed of any
disc containing information, 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
In physics, angular velocity (symbol or \vec, the lowercase Greek letter omega), also known as the angular frequency vector,(UP1) is a pseudovector representation of how the angular position or orientation of an object changes with time, i ...
, contrasted with a
constant linear velocity (CLV).
A typical
CD-ROM
A CD-ROM (, compact disc read-only memory) is a type of read-only memory consisting of a pre-pressed optical compact disc that contains computer data storage, data computers can read, but not write or erase. Some CDs, called enhanced CDs, hold b ...
drive operates in CLV mode, in contrast to a floppy or hard disk drive, or gramophone, which operates in CAV mode. In CAV mode, the spindle motor turns at a constant speed, which makes the medium pass by the read/write head faster when the head is positioned at the outside of the disk. In contrast, in CLV mode, the spindle motor speed varies so that the medium passes by the head at the same speed regardless of where on the disk the head is positioned.
If the disk is recorded at the same areal density throughout, then when read or written in CAV mode, the data rate is higher for the outer tracks than for the inner tracks (because for the same rotation on the inner track, more distance is travelled on the outer track), whereas in CLV mode, the data rate is the same everywhere.
An advantage of CAV mode over CLV is that the drive mechanism is easier to engineer (and less expensive to build). Another advantage is that a device can switch from reading one part of a disk to reading another part more quickly, because in CLV mode, when the device moves the head in or out, it must change the speed of the disc.
Dimensions

In case of a 12 cm standard diameter disc, data at the outermost edge (6 cm from center) is accessed at 2.4 times the speed of data at the inner edge of the data-containing program area (2.5 cm from center).
For a
miniature disc with a diameter of 8 cm (radius of 4 cm), the speed ratio of outer to inner data edge is 1.6.
This means that, for example, if a disc is accessed at a constant angular velocity of ×24, the equivalent
linear velocity is ×24 while accessed at the outermost edge of the data area while being ×10 at the innermost data area.
Uses
Gramophone record
A phonograph record (also known as a gramophone record, especially in British English) or a vinyl record (for later varieties only) is an analog sound storage medium in the form of a flat disc with an inscribed, modulated spiral groove. The g ...
s have always used CAV, including
CEDs which provide video signals.
Some high-speed CD and DVD drives can use CAV. It allows for shorter
access time
Access time is the time delay or latency between a request to an electronic system, and the access being initiated or the requested data returned.
In computer and software systems, it is the time interval between the point where an instructio ...
s because the rotation speed (angular velocity) does not need to be changed when the laser seeks across the disc, similarly to the
magnetic head of a
hard disk drive
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk drive platter, pla ...
. With a given angular (rotation) speed, the linear speed at the outermost track is nearly 2.4 times as fast as that of the innermost track.
CAV was used in the
LaserDisc
LaserDisc (LD) is a home video format and the first commercial optical disc storage medium. It was developed by Philips, Pioneer Corporation, Pioneer, and the movie studio MCA Inc., MCA. The format was initially marketed in the United State ...
format for interactive titles; it was also used with
special edition
The terms special edition, limited edition, and variants such as deluxe edition, collector's edition or expanded edition are used as a marketing incentive for various kinds of products, originally published products related to the arts, such as b ...
s 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 30 minutes on each side (these discs are also known as "standard play"), as opposed to 60 minutes on each side for CLV-based discs ("extended play").
CAV is used with the
Nintendo GameCube Game disc and Wii Optical disc, the
original Xbox discs, and Sega's
GD-ROM
The GD-ROM (gigabyte disc read-only memory) is a proprietary optical disc format developed as a collaboration between Sega and Yamaha for the Dreamcast and other Sega systems.
Specification
A double-density format based on the CD-ROM that cou ...
.
Partial constant angular velocity (P-CAV)
If the used media (e.g.
DVD-RAM
DVD-RAM (DVD Random Access Memory) is a DVD-based disc specification presented in 1996 by the DVD Forum, which specifies rewritable DVD-RAM media and the appropriate DVD writers. DVD-RAM media have been used in computers as well as camcorde ...
variants) has a linear (writing) speed limitation that can not be attained with physically supported angular (rotation) speeds at the inner edge of the recordable area, the disc writing can start off with a constant angular velocity, until the increasing linear velocity has reached the writing speed limitation of the disc.
At that point, the drive transitions into writing with a constant linear velocity.
[QPxTool glossary: P-CAV (partial constant angular velocity)]
/ref>
See also
* Constant linear velocity (CLV)
* Zone bit recording
In computer storage, zone bit recording (ZBR) is a method used by disk drives to optimise the tracks for increased data capacity. It does this by placing more sectors per zone on outer tracks than inner tracks. This contrasts with other approache ...
(ZCAV)
*
References
{{reflist
External links
* http://www.pcguide.com/ref/hdd/geom/tracksZBR-c.html
Rotating disc computer storage media
Audio storage
Video storage
Optical computer storage