Implementation And Research On Video Coding In H.323 Protocol Stack

With the rapid development of the technologies of computer networks and digital communications, multimedia communications over packet-based networks is becoming more and more popular. H.323 is a protocol that illustrates multimedia communications on networks that provide a non-guaranteed quality of service. The definitions of functional nodes in H.323-based system, such as terminal, gatekeeper, gateway, and multi-point controller are included in H.323 protocol, and the call process, logical channel management, and media coding are also illustrated. This paper is based on the design and implementation of the video-coding module in an H323-based video communication system.First, the framework and protocol stack of H323-based system are introduced. And then a brief description of our system is given, including some functional modules and the general design scheme.Video coder is one of the most important modules in the system. Some important features of the video coder in our system are given. As an independent entity, video coder uses different control blocks for different coding channels, and communicates with other entities through a 'message delivery' mechanism. Two key processes in video coding are introduced, which are transform and inter prediction. A row-column method is adopted in our system to accomplish Discrete Cosine Transform, which is the transform algorithm in H.263 protocol. Several block-based fast search algorithms are introduced and compared with each other by simple experiments. Four optional modes in H.263 are mentioned, which are listed in Annex D, E, F and G of H.263 protocol. These optional modes are implemented in our system except Syntax-based Arithmetic Coding mode, and the performances of these optional modes are described.Rate control method is an important aspect of video coding. Several popular rate-control algorithms in DCT-based video coding are studied, especially the one adopted in TMN8. With the observation and the analysis of experimental results, it is found that in scenarios of high-rate or sharp fluctuation of macroblock's variance, the control algorithm in TMN8 is not efficient enough. Finally, a new scheme is proposed in which a GOB-layer control measure is taken, instead of MB-layer control. A similar performance to TMN8 rate-control scheme is attained by using our scheme, with less computational complexity compared with the one in TMN8. Experiments prove that our scheme is applicable.