In this work, the coding bit rate R is considered as a function of Rho which is the percentage of zeros among the quantized DCT coefficients. We discover that the rate function R(Rho) has some very interesting properties in the rho-domain. By introducing the new concepts of characteristic rate curves and rate curve decomposition, a novel framework for source modeling is proposed. Using the proposed source model, we can estimate the rate-quantization (R-Q) curve before quantization and coding with relative error less than 5%. Based on the estimated R-Q curve, the output bit rate of the video encoder can be accurately controlled. Our extensive simulation results show that the proposed algorithm outperforms TMN8 1 and VM7 5 rate control algorithms by providing more accurate and robust rate control.