Why is there a special VLC for DCT_coefficient_first:?
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. What’s the deal with End of Block ? Saves unnecessary run-length codes. At optimal bitrates, there tends to be few AC coefficients concentrated in the early stages of the zig-zag vector. In MPEG-1, the 2-bit length of EOB implies that there is an average of only 3 or 4 non-zero AC coefficients per block. In MPEG-2 Intra (I) pictures, with a 4-bit EOB code in Table 1, this estimate is between 9 and 16 coefficients. Since EOB is required for all coded blocks, its absence can signal that a syntax error has occurred in the bitstream. What’s this “Macroblock stuffing,” and why do people hate it?: A genuine pain for VLSI implementations, macroblock stuffing
