JVET-Q0436 CABAC zero word threshold [A. Browne, K. Sharman, S. Keating (Sony)]
This document presents an analysis of the need for padding with cabac_zero_word. Following on from JVET-P0395, where CTU-level statistics were analysed, in this contribution statistics are provided for tiles of CTUs, to examine the impact of averaging. It is concluded that padding is required on tiles for some sequences within both “High” and “Main” tiers of the current specification. In addition, figures are given which indicate that these results are not “averaged out” as tiles become larger. Two possible changes to the cabac_zero_word threshold are proposed if the impact of padding is to be reduced.
The analysis shows that the usage of CABAC zero word becomes less when the formula in terms of slope (beta) is changed. The same formula had been used since AVC, but may in particular not be as appropriate for high tier bitrate limits.
Decision: Adopt JVET-Q0436, only for high tier, “candidate 1”, change beta from 4/3 to 3/2.