摘要
视讯产业蓬勃发展,多媒体通讯技术应用日益广泛,诸如视频监控、可视电话、流媒体、IPTV等。在众多多媒体技术中,视频压缩编码技术是核心,是构建多媒体系统的关键模块。H.264/AVC是由ISO/IEC MPEG和ITU-T VCEG联合制定的最新的视频编码标准。在相同重构图像质量下,与H.263+和MPEG-4 ASP标准相比,H.264/AVC能节约50%的码流。由于其高压缩比、高图像质量和友好的网络特性,必将成为下一代视频压缩标准的主流。
本文对H.264视频编码器关键技术和基于DM642的H.264编码器的实时实现进行了研究工作。首先分析了编码复杂度以及不同的参数设置对编码器性能的影响,确立了应用于本课题的视频编码方案。然后从算法层面对比较耗时的运动估计和帧内预测模块进行优化。针对监控视频的特点,通过对H.264编码器运动估计部分的研究,提出了一种改进的菱形搜索优化算法,改进了跳过宏块预判技术和提前中止技术。研究了基于预测模式直方图的帧内预测快速算法。最后结合DM642嵌入式系统的硬件特性,讨论了基于PC机的T264编码器DSP化过程中需要注意的事项及移植的方法。然后从编码器编码流程、内存分配与调度、Cache的优化、EDMA等方面讨论了编码器系统级优化,分析了程序级代码优化与汇编级优化的方法。同时在这个过程中,总结了基于DM642的视频压缩编码软件开发的一些方法和技巧,可为今后基于DM642系统的软件开发打下基础。
Video monitoring can be directly perceived through the sense. Because of its richness and convenience, it is used widely by safety guard, product management, and so on. It plays an important role in the environment monitoring. The video compression is key technology of video monitoring, and video codec is one of the most important core modules for a multimedia application systems. H.264/AVC is newest video coding standard of the ITU-T Video Coding Experts Group and ISO/IEC Moving Picture Experts Group. The H.264/AVC video coding standard can achieve considerably higher coding efficiency than previous standards. Due to the advanced compression performance and the“network-friendly”nature, H.264/AVC will be the next generation of“industry-leading”video coding standard.
This paper will make an emphasis on research of key technologies and the implementation of H.264 video encoder based on DM642 platform. At the beginning of the paper , we analyse the complexisity of video encoder and different parameters which have an effect upon encoder performance. According to the fact that video scene is smooth , little motion, has many background blocks, method for detecting skipped macroblock ahead of discrete cosine transform, quantization and context adaptive variable length coding and early termination algorithm are improved. A fast mode decision algorithm for Intra prediction in H.264/AVC video coding is proposed based on Prediction Mode Histogram. Experimental results show that the algorithm increases the speed of intracoding significantly with negligible loss of peak signal-to-noise ratio. Secondly, we discussed the method that T264 encoder based on PC is transferred to DM642 platform, then the application optimization in DM642 embedded system was made by the following sides: application architecture,memory system,Cache optimization and EDMA. After those steps, not only the encoding performance is improved , but also many skills of software development about DM642 were summaried.
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