Why is the DC value always divided by 8 ?
Clarification point: The DC value of Intra coded blocks is quantized by a constant stepsize of 8 only in MPEG-1, rendering the 11-bit dynamic range of the IDCT DC coefficient to 8-bits of accuracy. MPEG-2 allows for DC precision of 8, 9, 10, or 11 bits. The quantization stepsize is fixed for the duration of the picture, set by the intra_dc_precision flag in the picture_extension_header(). 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 leng
