Dynamic rate control for MPEG-2 bit stream transcoding

Most of the multimedia service over Internet uses pre-encoded video bit strewn for transmission. This requires a transcoder capable of performing on-line bit rate conversion according to bandwidth variations to prevent network from congestion and subsequently minimize cell data lose. In this paper, re-quantization of DCT coefficients is adopted and rate control is performed at both the frame and macroblock levels. At the frame level, we intend to prefer more 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 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.