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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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Understanding Dual Layer DVD Recording 