Research On Rate Control Algorithm In Video Communication

With the development of communication and network technology, video communication is more widely used in various fields. In video communication, to transfer a video sequence over limited channel bandwidth, effective video compression is essential. Rate control is one of the most important parts in video compression coding. The aim of rate control is to achieve a balance between compressed video quality and channel bandwidth to satisfy special requirements of specific application. In specific application, rate control usually need to do trade-off between objective video quality, subjective video quality, actual bit rate, hardware limitation, channel transmission delay restriction and special needs of application. An effective and reasonable rate control algorithm can not only improve compressed video quality, but also improve the channel efficiency.H.264/AVC is the most popular video compression coding standard in video communication, and JVT-G012 is the state-of-the-art rate control algorithm designed for H.264/AVC. In JVT-G012, a good balance between compressed video quality and encoder buffer fullness is achieved. However, there are still some flaws in JVT-G012. To improve the performance of JVT-G012, various improved algorithms is developed from different angles, such as Rate-Distortion model, Rate-Distortion optimization, encoding complexity estimation, encoder buffer control, channel bandwidth estimation, detection and disposal of scene change, joint rate control of multiple video sequences and Region-Of-Interest based algorithm and so on. In this dissertation, with a top-down view point, considering the features of video communication, three aspects are studied. They are target bits allocation between P frames, detection and disposal of scene change and joint rate control of multiple video sequences in video communication.Firstly, to improve target bits allocation between P frames in same GOP, a frame layer rate control algorithm based on channel bandwidth sufficiency and encoding complexity estimation is proposed. Channel bandwidth sufficiency is introduced to evaluate the relationship between channel bandwidth and the encoding complexity of video sequence. At the start of encoding a video sequence, the channel bandwidth sufficiency is judged, and the control parameters used in rate control are initialized according to the judgement. Macro-block level luminance histogram difference (BH) is applied to measure the encoding complexity of each P frame, and the target bits of each frame are adjusted according to the relative complexity. Comparing with JVT-G012, proposed algorithm achieves different improvements under different relationship between channel bandwidth and video sequence. When channel bandwidth is sufficient to video sequence, proposed algorithm improves the objective video quality and subjective video quality of compressed video sequence. When channel bandwidth is insufficient to video sequence, the actual bit rate controlled by proposed algorithm is more close to the target bit rate, and the encoder buffer fullness is improved.Secondly, to reduce the influence of scene change on compressed video quality, channel transmission and encoder buffer fullness, detection and disposal of scene change is studied. The features of video communication is analyzed, real-time processing restriction, channel transmission delay restriction and encoder buffer size restriction are confirmed as design principles. To satisfy real-time processing restriction, a BH-based scene change detection method is introduced. To satisfy channel transmission delay restriction and encoder buffer size restriction, a scene change disposal method based on forwards frame skip and variable length GOP is proposed. Proposed algorithm avoid transmission delay and encoder buffer overflow caused by scene change, and improve the video quality of new scene effectively.Finally, to solve the encoding resource allocation problem between multiple video sequences, a novel joint rate control algorithm is proposed. Existent joint rate control algorithms are designed for encoding multiple television programs at the same encoder side. In video communication, directly applying existent joint rate control algorithms will result in an unacceptable control cost. To reduce the control cost, a joint rate control algorithm with low computational cost is proposed. Firstly, the channel bandwidth allocated to each user, instead of target bits allocated to each frame, is chose as control object. Secondly, based on the slowly change of encoding complexity in the same scene, the channel bandwidth of each user is adjusted according to the average video quality of encoded frames. Thirdly, to deal with scene change and call initiation and call termination in video communication, a parameter update scheme is applied. Comparing with existent joint rate control algorithm, video sequences controlled by proposed algorithm can also achieve more uniform video quality. At the same time, proposed algorithm has an extremely low computational cost, and scene change, call initiation and call termination are effective disposed. Proposed algorithm is more suitable used in video communication.