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disc |
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Super
Audio CD |
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Hybrid disc structure
(cont.)
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As
shown in the following diagram, the two layers of an
hybrid disc are separated by 0.6mm from each other,
since the high-density layer must have a substrate
thickness of 0.6mm while the CD density layer must
have the standard substrate thickness of 1.2mm.
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Hybrid
disc readout principle
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The
SACD laser (bigger NA) reads data directly from the
high-density layer through a 0.6mm polycarbonate
substrate. This intermediate layer presents indeed a
reflectivity coefficient which is high enough for
the 650nm wavelength.
The hybrid disc can also be used in a conventional
CD player, the laser of which has a bigger
wavelength (780nm) and a smaller NA (0.45). In this
case, the intermediate layer appears
semi-transparent for the laser which can directly
focus on the usual CD reflective layer.
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Difficulties
to retrieve information from these two layers did
not actually consist in making the laser focus on
the proper layer, but they were more a reflectivity
problem.
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Layers
reflectivity
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The
high-density layer should have a reflectivity of 25%
with the 650nm wavelength laser which is used in
SACD players, while it has to let 100% of the 780nm
laser light go through, in order to fulfil the 70%
reflectivity specification of CD.
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A
single dielectric layer has consequently been
designed to
fulfill
these transmission and
reflectivity properties. However, such a layer could
not reach a 100% transmission efficiency for a 780nm
laser, and less than 70% of laser light would have
been reflected on the aluminum
layer corresponding
to the CD information. Hence, another metal is used
for CD information in hybrid discs, in order to have
a better reflectivity than with the
aluminum
layer
of conventional CDs.
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