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In this paper, we propose efficient techniques and architectures for realizing spatial-downscaling transcoders in the DCT domain. We present efficient DCT-domain methods for arbitrary-size downscaling and upscaling. We show that, by integrating the downscaling process into the DCT-domain motion compensation (DCT-MC) operation for B-frames, the computation of DCT-MC and downscaling can be significantly reduced, leading to a simplified cascaded DCT-domain downscaling transcoder (CDDT) without introducing extra quality degradation. We also propose another scheme to further reduce the computation and storage cost which may introduce drifting errors. Experimental results show that the two proposed schemes can achieve significant computation reduction when compared with the original CDDT without any degradation or with introducing acceptable quality degradation, respectively.