DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

For a more elaborate description,
and the quantization tables, see
Symes, cited on page 496.
This scheme is described as threepass;
however, the first pass is trivial.
tion is merely a binary shift. The lookup and assembly
of VLE-coded bitstream need not be performed at this
stage; it suffices for now to accumulate the bit count.
The final QNO for the segment is the one that produces
the largest number of bits not exceeding the capacity of
the segment – for DV25, 500 bits for luma AC coefficients
(including four 4-bit EOBs), and 340 bits for
chroma AC coefficients (including two 4-bit EOBs).
Once the segment’s QNO is determined, VLE coding
takes place, and the CMs of the segment are assembled.
Each CM starts with one byte containing its QNO
and error concealment status (STA) bits. Each block
includes its DC coefficient, its mode (8-8-DCT or
2-4-8-DCT), and its class. Finally, the VLE-coded AC
coefficients are distributed in a deterministic three-pass
algorithm – first to the associated block, then to unused
space in other blocks of the same CM (if space is available),
and finally to unused space in other CMs of the
segment. QNO has been chosen such that sufficient
space for all coefficients is guaranteed to be available:
Every bit of every coefficient will be stored somewhere
within the segment.
Each CM comprises 77 bytes, including by a 1-byte
header. In DV25, a CM includes four coded luma blocks
and two coded chroma blocks:
• A coded luma block totals 14 bytes, and includes a
9-bit DC term, one mode bit, and a 2-bit class number.
One hundred bits are available for AC coefficients.
• A coded chroma block totals 10 bytes, and includes
a 9-bit DC term, one mode bit, and a 2-bit class
number. Sixty-eight bits are available for AC
coefficients.
For 4:2:2 subsampling in DV50, a CM has four blocks,
not six; space that in 4:1:1 or 4:2:0 would be allocated
to luma blocks is available for overflow data. For 3:1:0
subsampling (used in SDL, to be described in
a moment), a CM has eight blocks, not six: Each luma
block has 10 bytes; each chroma block has 8 bytes.
CHAPTER 39 DV COMPRESSION 467