A Rate Control Algorithm for MPEG-2 to H.264 Real-time Transcoding

The demand of transcoding MPEG-2 to H.264 will be raised as the H.264 promises higher coding efficiency over previous video compression standards, while MEPG-2 has been widely adopted in many existing systems. This paper presents a rate control scheme for MPEG-2 to H.264 transcoder with the consideration of peak signal-to-noise ratio (PSNR) fluctuation control and re-quantized quality loss reduction. Our main idea is to reuse information extracted from the incoming MPEG-2 video stream as efficiently as possible. An analytic formula is constructed to set an appropriate initial quantization parameter (QP) for different encoding settings via extensive experiments. The mean absolute difference for current basic unit to be coded is predicted with that of MPEG-2 coded units, which thus avoids complicated parameter estimation. Furthermore, a logarithmic model is established to describe the relationship of quantization parameter between MPEG-2 and H.264 under the same PSNR level. The QP derived from the well-known quadratic model is further revised by a lower bound obtained with this proposed model. Therefore the overall coding performance is improved by setting reasonable QPs for those macroblocks which had been substantially quantized in original MPEG-2 video stream. According to experiment results, the transcoded video quality is improved significantly and the fluctuation of PSNR is restrained remarkably with the rate control algorithm.