Efficient Requantization Method for INTRA MB in Heterogeneous Transcoding

摘要

In this paper, we propose an efficient requantization method for INTRA Macroblocks (MB) in heterogeneous transcoding MPEG-1 to MPEG-4 simple profile. The quantizer for MPEG-1 INTRA MB usually uses a quantization matrix while the quantizer for MPEG-4 simple profile doesn't. As a result, the quantization step sizes of the two quantizers may not be the same even for the same quantization parameter. Due to this mismatch in the quantization step size, the transcoded MPEG-4 sequence suffers serious quality degradation and the number of bits produced by transcoding increases from the original MPEG-1 video sequence. To solve these problems, we propose an efficient method to find a near-optimum reconstruction level in the transcoder. We also present a PDF (probability distribution function) estimation model for the original DCT coefficients of MPEG-1 video sequence, which is required for the proposed requantization. Experimental results show that the proposed method gives 0.3～0.7dB improvement in PSNR over the conventional method, even at the reduced bit-rate about 5～7% from the conventional method.