场景切换下 MPEG-2 码率控制策略

Bit Rate Control Strategy for MPEG 2 under Scene Change

通过研究输出缓存器码率控制策略，分析了码率控制策略能精确地控制码率及合理地分配码字的机制．结果表明，码率控制通过监视“虚拟缓存器”状态调节量化尺度因子，能保持实际帧编码比特数与预分配帧编码比特数一致，从而在恒定码率下，获得均匀的图象质量．针对场景切换所带来的影响，根据人眼对场景切换时图象质量降质不敏感的特性，提出了一改进的码率控制策略，原则是保持正常的码字分配，而降低切换帧的图象质量，从而使得后续帧能较快地恢复图象的正常质量，特别是不影响下一个ＧＯＰ编码．结果表明，所提出的码率控制策略能够有效地解决场景切换引起编码比特数可能的剧烈变化所带来的困难．

By studying the MPEG 2 based output rate control strategy in detail, the mechanism of accurate bit rate control and rational bit allocation is analyzed. The simulation results show that the difference between actually picture coded bits and picture target bits can be minimized by monitoring the “virtual buffer” state to adjust the quantization scaling factor, and thus more stable picture quality can be achieved at a fixed rate. Considering the decrease in human eye's sensibility after a scene change, a bit rate control strategy under scene change is proposed, in which the basic ideas are to decrease the scene change picture quality and to keep a regular bit allocation. Thus, after the scene change, the follow up pictures qualities can be restored rapidly and the picture quality of the next GOP can not be influenced especially. The experiments show that the proposed rate control strategy can solve the problem of bit rate control from the drastic changes of coded bits under scene change efficiently.