[I]There are two modes for dual-layer orientation, parallel track path (PTP) and Opposite Track Path (OTP). In PTP mode, used for DVD-ROM, both layers start recording at the inside diameter (ID) with the lead-in and end at the outside diameter (OD) with the lead-out. Sectors are sequenced from the beginning of the first layer to the end of the first layer, then the beginning of the second layer to the end of the second layer. In OTP mode, used for DVD-Video, the disk is recorded in the same physical way, but the sectors are sequenced from the beginning of the first layer to the end of the first layer, then the end of the second layer to the beginning of the second layer. In both modes, the layers share one lead-in and one lead-out.
A common mistake that people make is to think that the disc spins first in one direction, and then another, either for PTP or OTP recording, when in fact DVD-Writers always spin a disc in the clockwise direction. A simpler way to understand what's written above is to think of the little hole in the centre of the DVD as the "inside" and the rim of the DVD as the "outside". Since dual-layer DVDs have two data layers, placed one on top of the other â€“ Layer 0 (L0) and Layer 1 (L1), there are two ways in which these two layers may be written to - L0, inside to outside and then L1 inside to outside again (PTP), or L0 inside to outside and then L1 outside to inside (OTP). OTP is usually used for DVD-Video, to prevent the inherent delay that PTP involves - laser head moving from the outside edge of the DVD to the inside to start reading the next layer when it reaches the end of L0. This would result in your video skipping, or freezing up for some time as the laser head repositions itself and the system waits to start receiving data again.[/I]
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