Let Rho be the percentage of zeros among the quantized transform coefficients. We discover that, in any typical video coding systems, there is always a strictly linear relationship between Rho and the actual coding bit rate R. This linearity leads to a novel and unified source model for different types of source data and different coding systems, such as H.263 1, MPEG-2 2, and MPEG-4 3. The proposed linear source model is much simpler, but much more accurate than other source models reported in the literature. Based on this source model, a unified rate control algorithm is proposed for the above three video coding systems. Despite its extreme sim- plicity, the proposed algorithm outperforms other rate control algorithms by providing more accurate and robust rate control.