Joint temporal-spatial rate control for adaptive video transcoding

A joint temporal-spatial rate control method is proposed for adaptive MPEG video transcoding in this work. The proposed rate control scheme adjusts both the frame rate and the frame quantization step size according to the available channel bandwidth to achieve good temporal-spatial quality tradeoff. In the proposed transcoding scheme, motion vectors are reused to facilitate the transcoding speed. Consequently, I and P frames are not skipped in order to maintain the prediction sequential order while some B frames containing more temporal redundancy may be skipped to save the bit rate for image quality enhancement. Skipped frames can be regenerated at the decoder to ensure the full frame rate playback. The TM7 quadratic Rate-Qtz model is adopted in the proposed scheme to calculate the re-quantization step size from a given target bit rate. Simulation results show that the proposed rate control scheme out-performs the MPEG-4 frame level rate control in the average PSNR value when applied to the context of MPEG2 to MPEG-4 video transcoding. The complexity of the proposed transcoder remains low so that it can be used in real-time applications.