picture level rate control systems , apparatuses and methods are described which indirectly control bit rate through peak signal to noise ratio , thereby providing a stable psnr while maintaining a given target average bit rate . in a preferred implementation , encoding is performed in conjunction with macroblock qp modulation . in this way , the picture level rate control provides stable temporal quality and the macroblock qp modulation provides stable spatial quality . by way of example , the number of bits generated in a group of pictures is controlled by modulating the target bit allocation for a group of pictures based on coded picture buffer fullness . macroblocks are encoded for each picture based on a rate - distortion model , which is modified in response to the relative fluctuation of psnr of pictures within a group of pictures .