A new classified VQ scheme in the DCT domain, named DCT-CVQ is proposed. DCTsub-blocks are classified simply into six classes by two DCT coefficients which well represent edge characteristics and the distribution of ac energy in a DCT sub-block. To reduse the complexity of VQ, ac transform coefficients in a sub-block are partitioned into several vectors, and adaptivity is provided by varying codebook size of vectors for each class. Because of the adaptivity which makes the codebook size large for high ac energy class which is edge class, DCT-CVQ well reconstructs edge area which is important for visual quality. Simulation results show that this DCT-CVQ reveals good subjective quality at low bit rates, and the mismatch effect is much reduced. SNR is increased about 1.5-3.0 dB for rates in the range 0.2-0.5 bpp compared with other DCT-VQ coding schemes.
