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Super
Audio CD |
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Hybrid disc structure
(cont.)
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As shown in the
previous comparison table, the storage capacity of
the high-density layer is 6.9 times higher than the
storage capacity of a conventional CD. Such an
increase in storage capacity is possible thanks to
several technology's improvements. The following bar
chart of disc capacity gain shows how the overall
capacity gain from a CD layer to a high-density
layer can be expressed as a number of steps :
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Density
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Starting
with a CD reference of 100, the use of a shorter
wavelength achieves a capacity gain of 1.44 times
(proportional to 1/l2).
Using a pickup lens with a higher Numerical Aperture
(NA) adds a further factor of 1.78 times
(proportional to NA2). Indeed, most of
the physical (channel bit) density of the
high-density layer is on account of a better optical
resolution, caused by the smaller size of the laser
spot focused on the information layer. The area of
the laser spot is proportional to (l/NA)2.
For the high-density layer, l
is reduced to 650nm, while NA goes up to 0.6
(instead of 0.45), together accounting for an
improvement of the optical resolution by a factor
1.6, resulting in a density increase of 2.586
without sacrificing margins.
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In
the case of a single layer disc of 1.2mm substrate
thickness, such a high NA would have reduced the
disc tilt margin to unacceptable values. For that
reason, substrate thickness for the high-density
layer is reduced to 0.6mm. The CD layer, which is
added to the high-density layer, provides a total
thickness of 1.2mm to ensure a good rigidity of the
disc.
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Cross section of a
SACD hybrid disc
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