Opposite / Parallel Track Path Mastering
If the extra capacity of a DVD-9 is being used to increase the playing time of a single program, it is important that the change over time between layers should be short enough to allow seamless playback. To allow for this, layer 1 of a DVD-9 can be mastered with the spiral running either from inside to outside, the same as layer 0, or from outside to inside. These are refered to as Parallel and Opposite Track Path respectively.
In order that DVD-9 discs should meet the specifications for DVD playback, each layer needs to be mastered in a subtley different way from a standard DVD-5. Through manipulation of the pit geometry it is possible to achieve results well within the specification, despite the lower reflection from layer 0 due to the semi-reflective metalised layer and the interference from layer 0 when reading layer 1.
DVD-9 discs are pressed in the same way as a normal DVD-10, but the bonding layer between the two substrates has to be both optically transparent and of a very precise and consistent thickness.