Writing Quality - Page 17

Jitter at DVD

For the CD format Philips maintains industry's Reference Standard measurement system, "RODAN," or "Read-Only Disc Analyzer." RODAN is kept in Paris. In the DVD industry, there has been no appointment of a "DVD Paris Meter." Instead, a collection, or sub-group, of fundamentally standard Pulstec DDU-1000 analyzers has been used to produce the multi-point calibration disc sets whose values are based on the sub-group averages for the key parameters. Both approaches, if executed properly, can be effective. Most testing equipment vendors using Pulstec SDP-1000 drive for DVD Jitter measurements.

The DVD specification includes a standard for jitter, which is not found in the CD specification. The logical format specifies disc mechanical and optical characteristics, but specifications for sources of degradation, like inter-symbol interference introduced in the disc cutting step, are not included in the current CD specification.

There are also items which cannot be expressed by parameters listed in the current CD specification, such as degradation introduced by the mastering machine or unevenness in pit replication. These effects can't be ignored in DVDs with their higher recording density, and thus it is necessary to specify such factors. There was an error rate specification in the CD specification, but this is impossible to measure unless there are defects or degradation due to the playback device.

The disc specification for Pioneer's Karaoke System specified an error rate using a tilted pickup, but this was a difficult measurement to make, and certainly not efficient. In the DVD specification it was decided to add a jitter specification, as jitter is a parameter where degradation can be measured numerically. Jitter is measured in the absence of tilt, which is an ideal disc specification, but it isn't practical to compensate for tilt at all points. Since the effect of jitter due to the varying component of surface tilt is small, it was decided to measure across one full revolution and measures the average value of tilt variation. As a result, it is only necessary to compensate for the average radial tilt when taking measurements.