Hardware Reference
In-Depth Information
mode, the lead-out zone toward the outer part of the disc is called a middle zone. Discs written in
PTP have both spiral layers written (and read) from the inside out. When changing from Layer 0 to
Layer 1, PTP discs require the laser pickup to move from the outside (end of the first layer) back to
the inside (start of the second layer), as well as for the focus of the laser to change. Virtually all discs
are written in OTP mode to make the layer change quicker. OTP recording is also used by dual-layer
(DL) DVD rewritable drives.
To allow the layers to be read more easily even though they are on top of one another, discs written in
PTP mode have the spiral direction changed from one layer to the other. Layer 0 has a spiral winding
clockwise (which is read counterclockwise), whereas Layer 1 has a spiral winding
counterclockwise. This typically requires that the drive spin the disc in the opposite direction to read
that layer, but with OTP the spiral is read from the outside in on the second layer. So Layer 0 spirals
from the inside out, and Layer 1 spirals from the outside in.
Figure 11.12
shows the differences between PTP and OTP on a DVD.
Figure 11.12. PTP versus OTP.
DVDs store up to 17.1GB, depending on the type.
Table 11.10
shows the precise capacities of the
various types of DVDs.
Table 11.10. DVD Capacity
