A hybrid coder for low bit rate image sequence coding based on two-dimensional DCT and motion compensated DPCM is presented.Adaptive quantization and efficient encoding of the transform coefficients is achieved by applying vector quantization (VQ) to the absolute values of the coefficients within a transform block.The blocks of DCT coefficients are pre-classified according to their energy distribution and for every class a suitable VQ codebook is applied.Motion compensated interframe prediction in the present coder is based on motion estimation by ierarchical block matching where a smoothing criterion has been incorporated to obtain more reliable displacement vectors.The coder achieves an agreeable image quality at a net data rate of 61 kbit/s.
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