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MPEG-2到H.264视频转码技术分析及码率控制研究
【作者】 张翼；
【作者基本信息】 电子科技大学 ， 通信与信息系统， 2008， 硕士
【摘要】 随着数字视频及多媒体技术的发展,针对不同的数字视频应用,出现了许多不同的视频编码标准,如面向低码率视频通信的H.263视频编码标准,面向数字广播,家用DVD等高质量视频应用的MPEG-2视频编码标准,面向流媒体和多媒体交互应用的MPEG-4视频编码标准以及由ITU-T视频专家组和ISO/IEC运动图像专家组联合开发的JVT/AVC/H.264标准。多种视频编码标准的广泛应用使得在不同视频标准之间进行格式转换的需求不断出现,视频转码技术使得在异质网络和不同终端设备间进行视频传输和处理成为可能。将经过压缩的视频数据进行远程传输时,需考虑网络拥塞、信道带宽等情况,码率控制策略可以通过调整转码器编码端的编码参数,使转码器输出的码流符合信道可以提供的带宽,保证信道正常传输,并获得尽可能好的图像质量。本文所研究的目的主要是进行MPEG-2到H.264转码器的设计,并在带宽有所变化的网络之间的网关上使用,同时在转码器中引入一种新的流量控制机制,使其能够根据输入和输出的带宽条件对要传输视频码流进行控制,以符合相应的网络传输条件。本文的主要研究内容以及研究成果如下:1.对视频转码的结构进行了介绍,对不同视频转码结构的漂移误差进行了分析,并具体指出MPEG-2到H.264转码结构建立时所需注意的问题,以此为基础进行MPEG-2到H.264转码器的设计。2.对目前已有的码率控制的各个关键技术进行了总结分析,包括:帧层比特分配,即根据目标码率要求给不同类型帧分配不同编码比特数;目标量化参数的确定,即确定量化所要采用的量化步长;跳帧算法,即控制编码时的跳帧数量;复杂度估计方法。3.提出一种帧层比特自适应的码率控制方案。该方案采用基于序列而不是以往那样基于图像组(GOP)的码率控制策略,根据输入的视频流比特率和输出端网络带宽条件来进行目标比特分配,能够解决基于GOP的码率控制方案造成的视频序列质量剧烈波动的情况。在完成目标比特分配后,使用一种新的模型来计算目标量化步长,并通过统计已编码帧的信息来对模型的参数进行更新。4.通过仿真实验,我们发现本文算法与JM10.2的码率控制方法相比,在保证计算复杂度和图像质量相当的情况下,能够更有效的对码率进行控制,解决了JM10.2中码率控制算法对部分视频序列无法准确控制的问题,使得输出的视频流更符合设定的目标码率,能够适应各种网络传输过程中的带宽要求。
【Abstract】 With the fast development of multimedia communication and video coding technologies, many video coding standards have been established for various video applications, such as H.263 for low bit-rate video communications, MPEG-2 for digital broadcasting, DVD and general high-quality video applications, MPEG-4 for streaming video and interactive multimedia applications, and H.264 which is developed by ITU-T and ISO/IEC. As digital video applications become more popular, there are increasing needs to dynamiclly convert the video between different formats. Video transcoding is one of the key technologies for video delivery over heterogeneous networks and various terminals. When the video data is transmitted for a long distance, network congestion, channel bandwidth and disposal ability of terminal should be considered. While the rate control mechanisms could ajust the bit rate of output video stream to the bandwidth provided by channel, ensure the normality of channel transmission and obtain better image quality by adjusting the parameters of the transcoder.The goal of the research of this thesis is to design a transcoder which is used in gateway between two networks with different bandwith. A new rate control technique was introduced to regulate the bitrate of video stream so as to transmit the video stream between different networks.The main contents of this thesis are given as follows.First, we introduce the framework of transcoding process, analyze the drift-error of different architectures, and point out the issues that should be paid close attention to when building the MPEG-2-to-H.264 transcoding architecture. Those researches form the theoretical basis for designing a transcorder with satisfying performance. Second, we introduce the rate control mechanism in existing ethods, including bit allocation in frame layer, namely, different bits are allotted to different frames; and the target QP computing, which is used for quantization; frame skipping and complexity computing.Third, we propose a new method of rate control, which is based on sequence but not the existing GOP schemes. In the new method, the number of bits of the frame to be transcoded is allotted according to the bit rate of input video stream and the output bandwidth of network. This design can reduce the fluctuation of video quality caused by GOP based rate control method. After the number of bits is allocated, our method computes the QP of the frame to be transcoded according to a new computing model, the parameters of which are updated by information of encoded frames.Fourth, the results of our experiments show that the new method can help rate control mechanism to work more efficiently while keeping the complexity low and ensuring the video quality better. Besides, our design can also achieve better rate control precision which can adapt the bit rates to various bandwidth requirements in different network environments.