1. A method of video rate control for a video encoding system for encoding a video signal comprising: a) determining, using a processor, a variance of pixel values over a time t of the video signal; and b) calculating, using a processor, a quantization parameter for the time t using the pixel variance and a number of output bits over the time t and a quantization parameter used for an immediately preceding time t−1, a pixel variance over the immediately preceding time t−1, and a number of output bits over the immediately preceding time t−1; wherein the variance is a modified variance being a function of the calculated variance over the time t and a similarly modified variance over the immediately preceding time t−1, and wherein the modified variance V(t) is defined as: V(t)=(1−α)V a (t)+αV(t−1) where V a (t) is the calculated variance over a time t, V(t−1) is the modified variance over the immediately preceding time t−1 and α is a constant such that 0<α<1.