| 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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Physical Sector
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A
Physical Sector is formed by splitting each row of a
Recording Sector into 2 parts of 91 bytes each. EFM+
is then applied to these bytes, providing 16 bits
for each converted byte, which are finally preceded
by a special EFM+ sync pattern of 32 channel bits.
By using alternately 8 different sync patterns,
decoders can easily synchronise (or re-synchronise
after clock loss) their timing.
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A
Physical Sector has 38,688 channel bits :
13 rows * 2 * (32 sync bits + 91 Recording Sector
bytes * 16 EFM+ bits).
These 38,688 channel bits form 26 Sync Frames
of 1488 channel bits each.
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1
Physical Sector consisting of 26 Sync Frames
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Channel
modulation
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Like
for DVD, SACD uses a modified version of the CD
eight-to-fourteen channel modulation scheme (EFM),
which is known as EFM+. This scheme converts 8-bit
data directly into 16 channel bits.
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The
16-bit channel values are chosen from the EFM+
Conversion Table. There are 256 possible Data Symbol
values and 4 possible run-length conditions, or
States. For each Data Symbol / State combination,
the conversion table contains a 16-bit EFM+ codeword
and the value of the run-length State for the next
Data Symbol to be encoded. This coding not only
allows unique mapping during demodulation, from one
EFM+ codeword to one Data Symbol, but also limits
error propagation.
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The
16-bit codeword represents a small but serviceable
saving of data capacity as compared with the 14-bit
codeword and 3 merging bits of EFM used in CD. This
enhanced version of EFM channel modulation is both
more effective and more efficient than its
predecessor, wothout compromising the performance of
servo and data extraction systems.
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All
EFM+ codewords are serialised into EFM+ Sync Frames
consisting of 91 converted Data Symbols (half a row
from a Recording Sector) and a Sync Word of 32
channel bits. There are 8 different Sync Words, all
consisting of a run-length of 14 channel-bits (which
does not appear in normal EFM+), plus a specific
channel bit pattern that identifies the Sync Word.
The Sync Word identify different positions in the
Physical Sector :
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SY0 |
= start of the
first Sync Frame of a Physical Sector |
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SY1...4 |
= start of an
odd Sync Frame |
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SY5...7 |
= start of an
even Sync Frame |
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