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Dual-Layer DVD explained.

Q: “What’s a dual-layer DVD?”

A: Today’s TTOTD is a compilation of three WFTDs, Dual Layer DVD, PTP & OTP, which together answer this question.

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.

PTP
An abbreviation for Parallel Track Path, which is a technique for data organization on dual layer DVD discs. Using the Parallel Track Path (PTP), as opposed to OTP (Opposite Track Path), on a dual-layer DVD, both layers have a Lead-in and a Lead-out area as part of the information area. For each layer, the Lead-in area is located at the inner radius of the disc, and the Lead-out area is located at the outer radius of the disc. This layout structure is comparable with the layout of Compact Disc. Reading of the data is done, like CD, from the inner radius of the disc to the outer radius, for both information layers. The PTP method, for example, can be used for providing artist profiles and other background information in one track along with a movie in the other track, or quick-responding game structures achieved by means of quick access from layer to layer.

OTP
An abbreviation for Opposite Track Path, which is a technique for data organization on dual-layer DVD discs. With movies that have long playing times or very high picture quality, the storage capacity of one information layer on a DVD might not be sufficient so dual layer DVD’s are used. In many cases, such as with long movies, the need for seamless continuation of the playback from one layer to the other is important. The method used for this is usually Opposite Track Path (OTP) method, as opposed to PTP (Parallel Track Path).

With OTP the first information layer starts with a Lead-in area at the inner radius of the disc and ends at the outer radius. The second information layer starts at the outer radius and ends with a Lead-out area at the inner radius of the disc. Reading of the data stored on the disc will start at the inner radius of the first information layer and proceeds until the Middle Area of this layer (found at the outer radius) is reached. Then a switch-over to the Middle Area (found at the outer radius) in the second information layer is made, in order to continue reading of the data from the outer radius up to Lead-out area at the inner radius of the second information layer.

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