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Rate conversion of MPEG coded video by re-quantization process

We propose rate conversion method by re-quantization in which MPEG coded video at high bit rate is converted into the MPEG bitstream at a lower bit rate without decoding to obtain the reconstructed picture. The quantization step required for re-quantization is determined by the local and global quantization steps which are closely related to the activities in the pixel domain. The simulation results show that very similar rate distortion curves to those of transcoding have been obtained, and the difference in SNR is relatively small, about 0.4 dB at 1 Mbit/s in MPEG1 using a master bitstream at 4 Mbit/s, and 1 dB at 3 Mbit/s in MPEG2 using a master at 9 Mbit/s. Since the proposed method is very simple and requires much less hardware implementation cost than transcoding, it has a significant advantage as a rate conversion tool.