| Sectors
and ECC |
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Super
Audio CD |
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Sector format,
error correction and channel modulation (cont.)
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Low-frequency
suppression
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Suppression
of the low-frequency components is very important to
ensure stable tracking and accurate RF-signal
detection. For this reason, the running Digital Sum
Value (DSV) of the modulated data is maintained
close to zero by using a Substitute Conversion Table
in addition to the Main EFM+ Conversion Table.
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Taking
into account the rules for a minimum run-length of 3
channel bits and a maximum run-length of 11 channel
bits, there are 351 source words allowed by the
run-length encoding. For historical reasons, seven
of these are discarded, leaving 344 available
codewords. Of these, 256 are needed by the Main
Table, leaving 88 for the Substitute Table.
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The Substitute Table,
with different DSVs, is applied to the 88 lowest
Data Symbols. Codewords are chosen from one or other
table as necessary to obtain the lowest absolute
value for the running DSV.
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The
higher rate of EFM+ encoding (8 -> 16 instead of
8 -> 14+3) results in only a small power increase
of about +4dB in the low-frequency part of the data
spectrum. Because of the higher channel bit rate
(26.16MHz instead of 4.3218MHz), the power within
the same servo bandwidth is actually reduced by
about -12dB. For 2kHz, a typical servo bandwidth,
the net reduction is about -8dB (2.5:1).
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For
information, the following table shows the relative
encoding efficiencies of SACD high-density layer and
normal Compact Disc :
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| CD
Format |
SACD
Format
(high-density layer) |
Data Sector
|
| User
Data |
2048
bytes |
| Sync&Header |
16
bytes |
| EDC+ECC |
288
bytes |
| Total |
2352
bytes |
| Efficiency |
87% |
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|
Data Sector
|
| User
Data |
2048
bytes |
| Identification
Data |
12
bytes |
| EDC |
4
bytes |
| Total |
2064
bytes |
| Efficiency |
99% |
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Error correction
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| CIRC |
24
data + 8 parity |
| Efficiency |
75% |
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Error
correction
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| RSPC |
2064
data +
302 parity |
| Efficiency |
87% |
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Modulation
|
| EFM |
32*8
data bits |
| per
frame |
588
EFM bits |
| Efficiency |
44% |
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Modulation
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| EFM+ |
91*8
data bits |
| per
frame |
1488
EFM bits |
| Efficiency |
49% |
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Lead-in
Area
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Except
for some blocks at the end, the Lead-in Area is
filled with ECC blocks, the Main Data of which are
set to '00h'. The Lead-in ends with 2 ECC blocks
containing Reference code data, 30 ECC blocks with
'00h' data, 192 ECC blocks containing Control data
(physical format information, disc manufacturing
information, and contents provider information) and
another 32 ECC blocks with '00h' data. Next to the
Lead-in Area starts the Data Area.
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The
reference code contains a specific fixed data
pattern and can be used for instance for the setting
of adaptive equalizer circuits.
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Physical
format information concerns the book type and
version, disc size and minimum read-out rate, disc
structure, recording density, Data Area allocation,
BCA descriptor, and a number of reserved bytes.
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Disc
manufacturing information is text or code data
written by the disc manufacturer in free format, and
contents provider information is text or code
written by the contents provider, again in free
format.
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In
the Lead-out Area, all Main Data are set with '00h'.
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