Jump to page content

CAV vs CLV capacity

The space on a computer disc is arranged into individually addressable areas called sectors. There are two basic methods for arranging these sectors on a disc: one is to have them placed in concentric rings (called tracks) of equal angle per sector, and the other is to have them in an Archimedean spiral with the physical length of sectors along the disc kept constant instead of the angle. All sectors themeslves have identical capacity regardless of their physical size (area or length), although the density of the sector (the size of the individual bits) can vary.

Discs arranged into discrete tracks (including floppy discs, DVDs, and hard drives) are constant angular velocity (CAV) discs: the disc spins at a fixed rate. This means that sectors at the outside of the disc pass under the head much faster than those at the centre, and thus the data is more spread out. This wastes physical space on the disc.

CD-ROMs have a single, spiral track and are constant linear velocity (CLV) discs. CD-ROM drives change the speed at which the disc spins such that the amount of disc surface passing under the laser unit is constant; sectors at the outside and inside of the disc’s surface are the same size (same length). This results in increased capacity at the expense of a more complex format. (Vinyl records also have a spiral track but are nevertheless CAV.)

The question is, how much extra space does CLV provide? What is the ratio of capacity of a CLV disc compared to a CAV disc? This question, I do not have the answer to; I am curious to see what you maths freaks make out of this one.

Modern hard drives, however, are zoned-CAV – the drive’s surface is divided into zones with differing numbers of sectors per track, with more sectors per track for the outer zones. This is a sensible compromise between wasteful CLV and the overly complex CLV.)


Return to Maths questions