Rotation speed
In DVD applications, the disc may be made to rotate at a constant linear velocity (CLV), in the same way as a CD-ROM or at a constant angular velocity (CAV), similar to a hard disk. In the CLV technique, the track which the pickup head is reading must be moving across the head at the same speed regardless of the position on the disc. This means that the disc must rotate faster if the head is reading a part of the track which is nearer to the centre than if it was reading a part of the track that is nearer to the outer cir- cumference. As the head follows the spiral track, the angular velocity of the disc must continuously change to keep the linear velocity constant. In the CAV technique, the rotation of the disc is constant resulting in the track moving faster across the head when a data that is being read is further away from the centre than nearer to it. To keep the bit rate constant, pits nearer the outer circumference must be spread out compared with pits nearer the inner circumference. The result is a lower average data density compared with the CLV technique. Compared with CLV, CAV has the advantage of faster access to the various sectors on the disc surface. This is because, when the pickup head moves to the required part of the disc, it does not have to wait for the angular velocity to change before reading the sector, as is the case with CLV.
CLV is used where the data on the disc is normally read sequentially, as in the case of a movie. Conversely, where the recorded data is not sequential with the pickup head having to move in search of the required sector, CAV with its fast access is used.
While CAV provides fast access to the recorded data, it is inefficient in terms of its use of the available recording surface of the disc. To maintain the advantage of fast access of CAV while at the same time improving the efficient use of the disc surface, the zoned CLV disc layout is used. In the zoned CLV technique, the disc is divided into multiple concentric rings, known as zones in which the speed of rotation remains constant. The only change in angular velocity takes place when the pickup head moves from one zone to another.
In the same way as CD-ROMs, DVD discs may be driven at multiple speeds: single-speed X1, double-speed X2 and so on. Multi-speed drives spin the disc at multiples of the standard velocity to increase the rate at which data is read off the DVD disc.