Abstract: Rate control is an important module in video coding systems,which makes an encoder output specific bit rates,and minimizes the distortion of encoded videos.The R-λ model based rate control is recommended in high efficiency video coding (HEVC) international video coding standard,which mainly includes two schemes,i.e.,the fixed ratio bit allocation and the adaptive ratio bit allocation.In order to improve both the accuracy of rate control and the rate-distortion (R-D) performance,this paper proposed an improved R-λ model based rate control algorithm.Firstly,an accurate R-D model update method is designed according to the coding structure of group of picture (GOP).Secondly,the GOP-level bit allocation scheme is improved according to the relationship of R-D dependency.Finally,the calculation formulas of the dynamic Lagrange multiplier at GOP-level and the dynamic bit weight for the frame to be encoded are proposed.Experimental results demonstrate that the bit rate relative errors of the proposed method are only about 0.006% and 0.005%,and achieves average 1.2% and 1.3% R-D performance gains compared with the adaptive ratio bit allocation scheme under the low delay configuration of P and B frames,respectively.

Key words: Bit allocation, HEVC, Rate control, R-λ model, Video coding