Constant linear velocity
From Wikipedia, the free encyclopedia
Constant Linear Velocity (CLV) describes the motion of a body with a constant linear velocity. This type of motion can occur in certain methods of writing or reading information from a rotating data disk.
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[edit] Constant Linear Velocity
Constant Linear Velocity means that the read/write head traverses the disk surface at a constant linear rate, which requires an increase in rotational velocity (rpm) as the head moves towards the center, and a decrease in rotational velocity as it moves towards the rim of the disk. This distinguishes the speed control of a compact audio disc (CD) from an LP record. An LP spins at a constant angular velocity (CAV) of 33⅓ rpm. Because the linear distance travelled by the phonograph stylus per revolution of the record decreases as it moves closer to the center of the disk, the data rate increases and the data becomes more compressed. When LPs were introduced in 1948, motor speed control was not nearly as accurate as it is today, and the more desirable constant linear velocity would have been more difficult to achieve. Thus a record cut and replayed with CLV would have suffered increased wow and flutter distortions. The concept of constant linear velocity was patented in 1886 by phonograph pioneers Chichester Bell and Charles Tainter.
An audio CD employs constant linear velocity to maintain a constant data rate. The motor speed decreases from 495 to 212 rpm as the read head moves away from the center, to keep the disc moving past the read head at a CLV of 1.2 m/s. For higher speed data transfer rates, constant angular velocity systems are used for some CD-ROM drives.
[edit] Zoned Constant Linear Velocity
Zoned Constant Linear Velocity (ZCLV) is a modification of CLV for high speed CD and DVD recorders. Early model recorders were CLV drives. The recording speed on such drives was rated in multiples of 150 KiB/s; a 4X drive, for instance, would write steadily at around 600 KiB/s. The transfer rate was kept constant by having the spindle motor in the drive vary in speed and run about 2.5 times as fast when recording at the inner rim of the disc as on the outer rim. At higher speeds, ZCLV offers a compromise between constant angular velocity which enables faster seek times, and CLV, which enables greater storage capacity. High-speed recorders use the zoned CLV method (ZCLV), which divides the disc into stepped zones, each of which has its own constant linear velocity. A ZCLV recorder rated at "52X", for example, would write at 20X on the innermost zone and then progressively step up to 52X at the outer rim. DVDs have as many as 24 ZCLV zones.
[edit] Constant Angular Acceleration
Constant Angular Acceleration (CAA) is a variant of CLV that was used on the Laserdisc format. The initial specification of CLV (as it applied the laserdisc) resulted in several playback artifacts to be present in the audio/video portion as well as compatibility problems with laserdisc players as produced by different manufacturers.
In the mid 1980's, Pioneer electronics, introduced the CAA scheme where the speed in rotation of the laserdisc was lowered in steps and eliminated most playback artifacts and compatibility issues. Since its introduction, most manufacturers of laserdisc discs adopted the CAA format but still referred to their CAA-encoded product as CLV.
