Dynamic rate control for MPEG-2 bit stream transcoding

Most of the multimedia service over the Internet uses a pre-encoded video bit stream for transmission. This requires a transcoder capable of performing on-line bit rate conversion according to bandwidth variations to prevent network congestion and subsequently minimize cell data loss. In this paper, re-quantization of the DCT coefficients is adopted and rate control is performed at both the frame and macroblock levels. At the frame level, we prefer greater bit allocation for I-frames for video quality consideration. At the macroblock level, a feedback mechanism is devised to keep the transcoded output bit rate approximate to the varying channel capacity. Experiments show that our algorithm behaves well in its rate control capability and besides, keeps a nearly constant PSNR degradation of only 2 dB relative to the MPEG-2 encoder when a bit rate conversion ratio from 100% down to 50% is requested