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Existing rate-control research can be summarized into two approaches. One attempts to optimize the rate-distortion (R-D) characteristics of the input source before any decisions about the quantizer step-size assignment are made. As such, rate-control schemes out of this approach incur iterative procedures, high computational cost and encoding delays. The other approach is highlighted by direct buffer-state feedback techniques in predicting the R-D characteristics based on either probabilistic models or on historic information. Both approaches trade suboptimality in the prediction of R-D characteristics for reduced computational cost and delay. We combine the simplicity of the predictive models with the high-complexity R-D approaches to design effective rate-control algorithms to exploit the advantages of both approaches: low encoding delay and low complexity, yet optimized estimation of the R-D statistics. Extensive experiments show that improvements over the MPEG-2 scheme with gains of 0.5-1 dB per frame are achieved by the proposed schemes for a variety of test sequences when the same target bit rates are maintained.