Rate-distortion optimized DCT-domain video transcoder for bit-rate reduction of MPEG videos

In this paper, we propose a rate-distortion optimized video transcoder which converts MPEG videos into a similar form at lower bit-rates. Our transcoder design is characterized by two features: 1) transcoding is performed in the DCT domain and the motion vector information is re-used; and 2) the rate-distortion relationship is optimized in both the frame-level rate allocation and macroblock-level rate control, thus leading to performances even better than the direct encoding and re-encoding methods based on the well-known TM5. In the proposed algorithm, the Lagrangian multiplier plays not only its traditional role in macroblock-level optimization, but also a variable to be optimized in frame-level rate allocation. These two levels of optimization process are highly linked. Experiments show that the R-D optimization is effective in getting better video quality, even the drift errors are ignored. Several speedy schemes were developed to make our transcoder design suitable for real-time video transmission over heterogeneous networks.