Dual-Layer DVD

A dual-layer DVD has two layers of data, one of them semi-transparent so that the laser can focus through it and read the second layer as needed. Since both layers are read from the same side, a dual-layer disc can hold almost twice as much as a single-layer disc, typically 4 hours of video (depending on compression). Initially, only a few replication plants could make dual-layer discs, but most plants now have the capability. The second layer can use either a PTP (parallel track path) layout or an OTP (opposite track path) layout. One of the advantage of two layers is that long movies can use higher data rates for better quality than with a single layer.