Description

This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.

93 Why is there a special VLC for DCT_coefficient_first:? (MPEG-2)

Since the coded_block_pattern in NON-INTRA macroblocks signals every
possible combination of all-zero valued and non-zero blocks, the
dct_coef_first mechanism assigns a different meaning to the VLC
codeword (run = 0, level =+/- 1) that would otherwise represent EOB
(10) as the first coefficient in the zig-zag ordered Run-Level token
list.

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