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Constant Linear Velocity (usually known as CLV) is the way CD-ROMs are nominally written. See also Constant Angular Velocity for the alternative system.

In a CLV disk, the idea is that the head passes over the disk at a constant linear speed. However, as the outside of the disk is "longer" than the inside of the disk, this means that the disk has to spin quicker when the head is towards the middle and slower at the end. For example, traditional 1x CD-ROM drives spin at about 600 RPM when the head is in the middle, and about 200 RPM when the disk is right at the outside. Older Apple Macintosh floppies used a similar system.

Rather than having concentric tracks, the data is written to a CLV disk as a continuous spiral. This particularly lends itself to data which is played continuously - hence the use for music CD-ROMs. However, it makes for an inefficient system when it comes to searching. It means, though, that a sector of data is always written in the same amount of space on the disk surface, which can be optimised to the smallest amount.

However, many modern CD-ROM drives use a form of hybrid system. This is where the idea of, say, a "40x max" speed CD-ROM comes in. A drive of this speed spins at 8000 RPM all the time - wherever it's reading. This is why it's "max" - 8000 RPM is 40x when in the middle of the disk (when a 1x drive would be at 200 RPM) but only 13x when in the edge of the disk (8000 / 600 = 13.333). A drive at this speed takes a (relatively) long time to spin up and down, so easiest to keep the drive at the same speed and do clever stuff to read everything off it as necessary.

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