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White Paper Blu-ray Disc™ Format
General
1.2 Optimization of Cover-Layer thickness
photograph is corrected through estimations using adjacent image data. Through this method, errors
are prevented by diluting the influence of partial signal defects in a large-scale data block. It can be
said that this Error-Correction method is the electronic version of defocusing by the Cover Layer. This
further suggests that defocusing by the Cover Layer partially can be replaced by Error Correction (Fig.
1.2.4). However, there is a problem if the thickness of the Cover Layer is made nearly zero by raising
the Error-Correction capability. This is because the smallest unit of the Error-Correction calculation is
one byte, and a very small error such as one bit is practically magnified to a one-byte error. Therefore,
defocusing is still desired to prevent small dust from causing bit errors. The thickness of the Cover
Layer for that purpose is about several tens of micrometers.
1
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2
m
m
Converging light
Information Layer
Cover Layer
Cross section of light beam has
wide area on the disc surface.
Influence from a particle on the
disc surface is diluted in the
area:S.
(Optical Dilution)
Area:S
Dropout
Readout signal
Constraint length for Error Correction
Fig. 1.2.4 Optcal and Electronic dilution of burst error
Calculation for Error Correction is carried
out on data in long constraint length.
Influence from a dropout is substantially
diluted in this constraint length.
(Electronic Dilution)
Defocusing effect
Error correction
Cover Layer has a function of optically diluting influence from a particle on the disc surface.
Error Correction has a function of electronically diluting influence from a dropout. Both of
these functions are similar to each other.
In the DVD standard, it was decided that the thickness of Cover Layer could be as small as 0.6 mm.
This thickness could be made by the disc production facilities of those days, because the same
mechanical strength as a CD could be obtained by sticking two pieces together.
For the purpose of BDs, this thickness was reconsidered with the essential condition to realize a 23
GB capacity, which is necessary to record a bit stream of Japanese BS digital HD broadcast for two
hours.
At first, it was natural to consider using the same production facilities as DVDs, that is, to employ a
thickness of 0.6 mm. However, the capacity could only reach around 12 GB by changing the
wavelength from red (650 nm) to the blue (405 nm). As a further disadvantage, when there’s an incline
in the disc, the magnitude of aberrations increases in inverse proportion to the wavelength. To
suppress this aberration, the NA value of the objective lens must be reduced. In the mass-produced
record type DVD, NA is 0.65 the maximum, even if a tilt servo controlling the tilt angle of the optical
pick-up to the disc is used. As described before, the aberration when a disc is inclined is proportional to
the cube of NA. To cancel the increase in aberrations due to shortening of the wavelength, NA should
be reduced to around 0.55. In this case, the recording capacity is lowered to around 10 GB, far smaller
than the 23 GB allowed by one of the newest reading technologies such as PRML. Although the
narrowing of the Track Pitch was considered while suppressing crosstalk by recording in and between
the guide Grooves, the influence of heat conduction became relatively large when the Track Pitch was
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