[Mp4-tech] On cabac_zero_word in H.264

[Shevach Riabtsev]

Dear experts 
According to AVC standard (section 7.4.1), one or more cabac_zero_word 16-bit syntax elements equal to 0x0000 may be present in some RBSPs after the rbsp_trailing_bits( ). 
The section 7.4.2.10 says:
    When entropy_coding_mode_flag is equal to 1, BinCountsInNALunits shall not exceed    ( 32 ÷ 3 ) * NumBytesInVclNALunits+ ( RawMbBits * PicSizeInMbs )  ÷ 32. 
Here NumBytesInVclNALunits means the sum of the values of NumBytesInNALunit for all VCL NAL units of a coded picture and BinCountsInNALunits is the number of times that the parsing process function DecodeBin( ).
If the actual number of bins exceeds the value specified by the above formula, then cabac_zero_words should be added (in this way NumBytesInVclNALunits is incremented). 
I don't understand completely why BinCountsInNALunits is restricted ? 
Why additional stuffing should be added to bit-stream to result BinCountsInNALunits shall not exceed    ( 32 ÷ 3 ) * NumBytesInVclNALunits+ ( RawMbBits * PicSizeInMbs )  ÷ 32, it might hurt video quality?
Regards, Shevach
Broadcom
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