In optical storage, constant linear velocity (CLV) 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 (i.e. rpm) varies during an operation, as contrasted with CAV modes. The concept of constant linear velocity was patented in 1886 by phonograph pioneers Chichester Bell and Charles Tainter. History Laserdiscs, the first consumer optical discs, used constant linear velocity to double playback time (CLV / "extended play" discs can hold 1 hour per side; CAV / "standard play" discs can only hold 30 minutes). As the motor's speed decreases from 1,800 to 600 rpm when the read head moves away from the center (which is the start of the recording), the disc moves past the read head at a constant speed. Later optical formats such as the audio CD also employ CLV to maintain both a constant data rate and a constant bit density. Their rotation gradually decreases from 495 to 212 rpm to keep the disc moving past the read laser at (assuming 1:1 playback speed and Red Book encoding). To accommodate the higher data transfer rates and random access requirements of modern CD-ROM drives, CAV systems are used. This is because seek performance would be greatly affected during random access by the requirement to continually modulate the disc's rotation speed to be appropriate for the read head's position. Dimensions In case of a 12 cm standard diameter disc, data at the inner edge of the so-called program area, the area containing the data (2.5 cm from disc center) is accessed at 2.4 times the angular (rotation) speed of the disc compared to at the outer edge (6 cm from disc center). For a miniature disc with a diameter of 8 cm (radius of 4 cm), the angular (rotation) speed ratio of outer to inner data edge is 1.6 if accessed at a constant linear velocity. This means that, for example, at a constant linar velocity of ×10, the equivalent angular velocity of the disc is ×24 while the being accessed at the inner data area, while being ×10 during access at the outermost edge. Zoned Constant Linear Velocity Zoned Constant Linear Velocity (ZCLV or Z-CLV) is a modification of CLV for high speed CD and DVD recorders where a constant linear velocity is maintained until the next zone, when the