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Understanding Dual Layer DVD Recording

An informational white paper by Robert DeMoulin

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When a dual layer recordable disc is inserted into a dual layer-compliant recorder, the optics will focus the laser at one of the dual layers to try and detect an “Address In Pre-groove” (ADIP) signal. From the ADIP signal, the recorder can detect whether the disc is dual layer and which layer it’s focused on. Once the media type and the layer are detected, the laser will be able to move its range of focus down or up to access any one of the two recordable layers. The drive will then focus on the Lead-In area of the disc to determine whether the disc is completely blank, partially recorded in Multi-session format, or Finalized (completed).

The two layers represent one contiguous address stream for recording as a Video Disc, a DVD-ROM, or even a packet recorded disc. When recording on dual layer media, the drive first records on the first recordable layer L0 from the inside hub area outward, just like a typical DVD recordable disc. When the end of information recorded in L0 is reached, Middle Zone 0 is added. Next, the drive focuses on the second recordable layer L1 to create Middle Zone 1 that over-wraps Middle Zone 0. The disc is then recorded from the outside rim inwards. Multi-session discs can be recorded with dual layer recordable media, so it’s possible to add data in “sessions” on a disc.


 

Reflectivity of both recording layers of a dual layer recordable disc is the similar: greater than 18 percent. The reflectivity between the L0 and L1 layers, however, is greater than 50 percent because the upper (second) recording layer absorbs and reflects some of energy that is directed at the lower (first) recording layer L0 in order for organic dye to be recorded. As a result, the organic dye formulation and shape of the pre-groove in dual layer discs must be optimized to provide the appropriate reflectivity for both layers. The spacer layer separates the two recording layers and prevents cross layer recording. It is transparent to allow the laser to easily focus on either recording layer by simply changing the position of the laser’s object lens.

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