DVD Demystified

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Chapter 3
which ones are worth worrying about. Many a shrewd marketer has capitalized
on the fears of consumers worried about jitter and sonic quality,
bestowing on the world such products as colored ink that supposedly
reduces reflections from the edge of the disc, disc stabilizer rings that claim
to reduce rotational variations, foil stickers alleged to produce “morphic resonance”
to rebalance human perception, highly damped rubber feet or hardwood
stabilizer cones for players, cryogenic treatments, disc polarizing
devices, and other technological nostrums that are intimate descendents of
Dr. Feelgood’s Amazing Curative Elixir.
Basically, there are five types of jitter that are relevant to DVD. 13
■ Oscillator jitter Oscillating quartz crystals are used to generate clock
signals for digital circuitry. The quality of the crystal and the purity of
the voltage driving it determine the stability of the clock signal.
Oscillator jitter is a factor in other types of jitter, since all clocks are
“fuzzy” to some degree.
■ Sampling jitter (recording jitter) This is the most critical type of jitter.
When the analog signal is being digitized, instability in the clock
results in the wrong samples being taken at the wrong time (see Figure
3.8). Reclocking at a later point can fix the time errors but not the
amplitude sampling errors. There is nothing the consumer can do about
jitter that happens at recording time or during production, because it
becomes a permanent part of the recording. Sampling jitter also occurs
when the analog signal from a DVD player is sent to a digital processor
(such as an AV receiver with DSP features or a video line multiplier).
The quality of the DAC in the receiving equipment determines the
amount of sampling jitter. Using a digital connection instead of an
analog connection avoids the problem altogether.
■ Media jitter (pit jitter) This type of jitter is not critical. During disc
replication, a laser beam is used to cut the pattern of pits in the glass
master. Any jitter in the clock or physical vibration in the mechanism
used to drive the laser will be transmitted to the master and thus to
every disc that is molded from it. Variations in the physical replication
process also can contribute to pits being longer or shorter than they
should be. These variations are usually never large enough to cause
13 One particular phenomenon is incorrectly referred to as jitter. When DVD-ROM drives and CD-
ROM drives perform digital audio extraction (DAE) from audio CDs, they can run into problems
if the destination drive cannot keep up with the data flow. Most drives do not have block-accurate
seeking, so they may miss or duplicate a small amount of data after a pause. These data errors
cause clicks when the audio is played back. This is colloquially referred to as “jitter,” and there
are software packages that perform “jitter correction” by comparing successive read passes during
DAE, but technically this is not jitter. It is a data error, not a phase error.