Hardware Reference
In-Depth Information
disc is 45%-85%, and for a dual-layer disc the reflectivity is 18%-30%. The automatic gain
control (AGC) circuitry in the drive compensates for the different reflective properties.
Figure 11.11
shows the construction of each of the DVD disc types.
Figure 11.11. DVD disk types and construction.
Note that although
Figure 11.11
shows two lasers reading the bottom of the dual-layer discs, in actual
practice only one laser is used. Only the focus is changed to read the different layers.
Dual-layer discs can have the layers recorded in two ways: either opposite track path (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 toward the disc slightly to focus on Layer 1. When written in OTP
