Writing Quality - Page 14

Jitter - Page 2

- What causes Jitter?

There are three possible causes of Jitter:

a) The recorded pits are not perfectly accurate in terms of size. The cause of that are the laser noise and the recording strategy of the recorder. Even if the pits were perfectly recorded and replicated, there would still be jitter. This is because of the limited resolution of the pickup in the player.

b) There is the influence of other pits nearby in the same track. The readout spot is broad enough that when the centre of the spot reaches the beginning of a short pit, the end of the pit lies within the fringes of the spot. So the apparent position of the one pit end is slightly dependent on where the other end is. The same applies to short lands. This is called inter-symbol interference. The jitter which arises from this is not truly random, but is associated with the pattern of recorded pit and land lengths.

c) There is crosstalk between pits in adjacent tracks, because the readout spot does not fall wholly on one track. It is a largely random contribution. It is worse at lower recorded velocities, because the highest frequency components of the readout signal in the wanted track, with which the crosstalk is competing, are weaker.

- Why Measure Jitter?

Jitter is important because the CD information is carried in the edges of the pits. A transition between pit and land (either pit to land or land to pit) is a one, and everything else is a zero. Since the data is self-clocked at a constant rate, if the edges of the pits are not in the correct places, errors will be generated.

If a transition between pit and land comes as little as 115 ns early or late, an error will result. A standard deviation of less than 35 ns will result in only about 1% probability of the effect lengths varying by more than 115 ns. In addition, any types of disc defects will affect the jitter, making this a sensitive test of disc quality. Pit distortion, crosstalk from adjacent tracks, intersymbol interference, pit wall steepness, low signal-to-noise ratio, and LBR (or CD-R writer) instability can all cause jitter on the disc.