【作者】 罗泽宙；

【导师】 黄桂金；

【作者基本信息】 华中科技大学 ， 通信与信息系统， 2004， 硕士

【摘要】 随着计算机网络技术和数据通信技术的飞速发展,在分组网络上进行媒体数据通信成为一种趋势。H.323协议是一个在分组网络上进行媒体通信的协议标准,描述了包括终端、网守、网关和多点控制器等在内的功能节点,以及包括呼叫接续、通道管理、媒体压缩传输等在内的通信规程。本文是在参与课题“基于H.323终端协议栈软件”中的视频编码传输模块的研究设计和开发的基础上完成的。本文首先介绍H.323系统结构与协议栈,并给出H.323终端协议栈软件总体设计方案,以及各模块的主要功能。视频编码模块是协议栈软件的重要组成部分。本文将介绍视频编码在H.323协议栈软件中的应用及相应的设计要点。视频编码器作为一个实体,通过不同的控制块映射不同的编码通道,通过消息传递机制与其他实体进行通信协作。并讨论H.263视频编码中的两个关键过程:离散余弦变换和帧间编码。离散余弦变换的实现采用行-列变换算法,将二维运算转换成为一维运算。对几种常用的基于块的运动估计算法进行比较。讨论H.263协议中的四种可选模式:非限制运动矢量模式、高级预测模式、PB帧模式和基于语法的算术编码,并介绍前三种模式在系统中使用情况。速率控制是视频编码中的重要技术。本文详细介绍和讨论基于DCT变换的视频编码的几种速率控制算法。并详细分析TMN8速率控制算法,该算法主要针对中低速率场合。根据大量的实验数据分析, TMN8速率控制算法在高速率场合,以及宏块方差波动较大的场合下有一定局限性。最后,提出一种以块组层速率控制代替宏块层速率控制的方案,通过更少的运算量获得较好的速率控制效果。通过实验验证该方案的可行性和有效性。更多还原

【Abstract】 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.更多还原