An improved Context Adaptive Binary Arithmetic coder for the H.264/AVC standard

During the last years, the increment of video transmissions over wireless channels has created the need for increasingly-efficient coding algorithms that are capable of coding the video information with a reduced number of bits. Among them, the H.264 coder provides the best performance in terms of video quality and reduced bit rates thanks to many enhanced coding solutions that were included in it. One of the most effective is its adaptive arithmetic coder that estimates the probability of each syntax element via an elaborate structure of contexts. However, the coder can be significantly improved exploiting the statistical dependency in the transformed signal. The DCT coefficients of a single transform block or of a macroblock are correlated among each other. This statistical dependency makes it possible a better estimation of the bit probability with respect to the context counters defined by the standard. For this purpose, the probability mass function of the different bit planes in a block of coefficient can be estimated through a graphical model associated to a Directed Acyclic Graph (DAG). Experimental results report that the adoption of a DAG model leads to 10% reduction of the bit stream size for a given quality or, otherwise, a quality increment between 0.5 and 1 dB at the same bit rate.