97 What is meant by modified Huffman VLC tables: (MPEG-2)
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
97 What is meant by modified Huffman VLC tables: (MPEG-2)
The VLC tables in MPEG are not Huffman tables in the true sense of
Huffman coding, but are more like the tables used in Group 3 fax. They
are entropy constrained, that is, non-downloadable and optimized for a
limited range of bit rates (sweet spots). A better way would be to say
that the tables are optimized for a range of ratios of bit rate to
sample rate (e.g. 0.25 bits/pixel to 1.0 bits/pixel). With the
exception of a few codewords, the larger tables were carried over from
the H.261 standard drafted in the year 1990. This includes the AC
run-level symbols, coded_block_pattern, and macroblock_address_increment.
MPEG-2 added an "Intra table," also called "Table 1". Note that the
dct_coefficient tables assume positive/negative coefficient PMF
symmetry.
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