Research And Implementation Of Video Coding System Based On Adaptive Network Bandwidth

Currently, the applications based on Internet video have been gaining more and more widely. However, video contains large amount of information, while the Internet still provides best-effort service basically, and has no guarantee of video quality. Video system for transmission needs to provide good visual quality based on current network conditions. Besides coding efficiency, IP network-oriented video communication has higher requirements, such as packet loss treatment, encoding rate and flow control. This paper focuses on how to effectively adjust the video transmission rate and the code rate real-timely in application-layer, to make full use of the available bandwidth resources, and provide good images to terminals under the condition of changing network environment.First of all, for a live broadcast system, the transmission bit rate is very high, and therefore needs a large transmission bandwidth. Also, it must be smooth when the transmission bit rate changed. A transmission rate control algorithm is consequently designed based on RTP/RTCP protocol. The central idea of the algorithm is: According to the information of the RTCP receiving report, calculate the link's Round-Trip Time, Retransmission Time-Out and packet loss rate, and then calculate the current transmission throughput of the network by using the Padhye Throughput Model. With this throughput, the sending rate of video service can be adjusted dynamically.Next, through analyzing MPEG-4 Video Standard, this paper proposes a method of dynamically adjusting video compression ratio, which is based on seamless switch of video stream, and then realizes an MPEG-4 Encoder. The main idea is: when starting to adjust the video compressed rate, a suitable switch point can be found in the video stream according to the characteristics of I, P, B-VOP and stream structure, and a new video stream can be inserted at this point. During the process of seamless switch, not only the correctness of decoding and the fluency of playing should be considered, the continuity of images should also be guaranteed.Finally, an adaptive IP network bandwidth video coding system, which includes the server and the client, is developed based on DirectShow platform. The main function of the server is calculating the effective bandwidth according to network parameters, and adjusting the video coding quality dynamically. The main function of the client is sending the network parameters back to the server, and real-time playing.Base on this paper, a further research can be done, such as how to improve the accuracy of the available bandwidth measurement and the stream regulated algorithm, to get better effect of video transmission.