Philips DVDR985 Technical Training Manual page 8

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Wobble
A Pre-groove is stamped on writable discs.
All recordable DVD media types feature a micro-
scopic wobble groove embedded in the plastic
substrate. This wobble provides the recorder
with the timing information needed to place the
data accurately on the disc. During recording,
the drive's laser follows this groove, to ensure
consistent spacing of data in a spiral track. The
walls of the groove are modulated in a consistent
sinusoidal pattern, so that a drive can read and
compare it to an oscillator for precise rotation of
the disc. This modulated pattern is called a wob-
ble groove, because the walls of the groove
appear to wobble from side to side. This signal is
only used during recording, and therefore has no
effect on the playback process. Among the DVD
family of formats, only recordable media use
wobble grooves.
Figure 10 - Wobble Pregroove
Dual Layer Discs
Two information layers are separated by a thin
transparent layer. Refer to Figure 11. The first
layer is partially transparent. This allows the sec-
ond layer to be read through the first layer. Both
layers are read by controlling the focus. There
are two methods for reading the data of a Dual
Layer disc, PTP and OTP. Refer to Figure 12.
PTP is Parallel Track Path. That means the Lead
In and Out Areas of the two layers correspond to
each other. Each Lead In Area is on the inner
portion of the disc, and the Lead Out Area is on
the outer portion of the disc. This is useful to link
data between the layers.
Figure 11 – Dual Layer DVD
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