New entropy coding technology
The final stage in the compression process is entropy coding. Here, AVC adds two alternative methods for a more powerful entropy coding compared with the variable length coding, VLC used in MPEG-2. These are context adaptive variable length coding (CAVLC) and context adaptive binary arithmetic coding (CABAC).
Recall the VLC employed in MPEG-2 which provides an extremely effi- cient coding scheme based on a table indicating which symbols are much more likely to occur than others. The CAVLC process is a more refined version of this coding method employing a number of codeword tables to encode transform coefficients. The difference is that the best table is selected adaptively based upon statistics of already processed data.
The CABAC process offers substantial gains of about 20% compared with VLC at the expense of intensive computing power for both the encoder and decoder. It continually updates the statistics of the incoming data and adaptively adjusts the algorithm in real time.
