Abstract: Context-based adaptive binary arithmetic coding (CABAC) is the major entropy-coding algorithm employed in H.264/AVC. In this paper, we present a new VLSI architecture design for an H.264/AVC CABAC decoder, which optimizes both decode decision and decode bypass engines for high throughput, and improves context model allocation for efficient external memory access. Based on the fact that the most possible symbol (MPS) branch is much simpler than the least possible symbol (LPS) branch, a newly organized decode decision engine consisting of two serially concatenated MPS branches and one LPS branch is proposed to achieve better parallelism at lower timing path cost. A look-ahead context index (ctxIdx) calculation mechanism is designed to provide the context model for the second MPS branch. A head-zero detector is proposed to improve the performance of the decode bypass engine according to UEGk encoding features. In addition, to lower the frequency of memory access, we reorganize the context models in external memory and use three circular buffers to cache the context models, neighboring information, and bit stream, respectively. A pre-fetching mechanism with a prediction scheme is adopted to load the corresponding content to a circular buffer to hide external memory latency. Experimental results show that our design can operate at 250 MHz with a 20.71k gate count in SMIC18 silicon technology, and that it achieves an average data decoding rate of 1.5 bins/cycle.

Key words: H.264/AVC, Context-based adaptive binary arithmetic coding (CABAC), Decoder, VLSI

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