CHAPTER 13

Figure 13.9 DVD disk types and construction.
DVD Chapter 13 727
Dual-layer discs can have the layers recorded in two ways: either OTP or parallel track path (PTP). OTP
minimizes the time needed to switch from one layer to the other when reading the disc. When reaching
the inside of the disc (end of Layer 0), the laser pickup remains in the same location—it merely
moves towards the disc slightly to focus on Layer 1. When written in OTP mode, the lead-out zone
toward the outer part of the disc is called a middle zone instead.
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.
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 13.10 shows the differences between PTP and OTP on a DVD:
DVDs store up to 4.7GB–17.1 GB, depending on the type. Table 13.19 shows the precise capacities of
the various types of DVDs.