摘要
随着无线网络技术与流媒体技术的快速发展,流媒体技术越来越多的应用于无线网络环境中,人们对流媒体系统的稳定性与质量也提出了更高的要求。无线网络通常具有传输速率低,传输速率变化较大,丢包率较高等特点,而流媒体是面向实时的应用,对网络的带宽、时延、时延抖动等有较高要求。因此在无线网络环境中如何提高流媒体播放的稳定性与质量成为一个研究热点。流媒体系统的优化策略主要分为两个方向,一是在发送端根据信道的状态、接收端的反馈信息等对信源编码、信道编码、发送策略等进行控制,提高数据的可靠性与信道的利用效率等;二是在接收端通过自适应播放技术对媒体播放过程进行控制,提高媒体播放过程中的质量与稳定性。本文主要对接收端的自适应播放技术进行了较全面的讨论与分析。
本文首先简要回顾了流媒体技术与无线网络技术的发展历史,分析了流媒体应用在无线网络环境中可能遇到的主要问题,介绍了自适应媒体播放技术的发展与研究现状。本文对流媒体系统中视频编码与网络传输两大关键技术进行了介绍。视频编码方面,本文介绍了当前广泛使用并具有代表性的H.264/AVC视频编码标准,包括H.264/AVC的编解码基本流程与主要特点。网络传输方面,本文介绍了当前IP网络中主要使用的流媒体传输协议RTP协议,讲解了H.264/AVC的NAL单元在RTP数据包中的封装方法。
本文提出了一种使用场景感知技术的自适应播放算法,该算法在接收端提取已接收视频图像的运动强度信息,在进行播放速率控制时,根据视频图像的运动强度信息来调整接收端的视频播放速率,使播放速率在快速运动的场景中变化较小,而在慢速运动的场景中变化较大,在减少接收端缓冲下溢出概率的同时保证视频可视质量。在文章的仿真部分,通过实验对本文提出的自适应播放算法的控制特性进行了仿真与分析;在不同信道条件下,将本文提出的算法与其他几种典型自适应播放算法进行了对比与分析。从仿真实验可以看出,本文提出的算法相比其他几种算法更好的控制了在视频播放过程中由于视频播放速率的变化产生的失真。
As the rapid development of the wireless network technology and the streaming media technology, streaming media system is widely used in wireless networks. The interruptions caused by the fluctuation of the transmission rate, low bandwidth and high packet lost rate in the wireless networks will significantly downgrade the perceptive quality in streaming media systems. Many literatures propose schemes to improve the stability and smoothness in the streaming media systems. Optimization strategies for streaming media system are mainly divided into two directions. At the server side, the streaming media server adaptively controls the sending strategy and adjusts the source encoding parameters according to the channel state or the feedback from the client to improve the efficiency of the channel and transmission robustness. At the client side, the client controls the playback process to improve the smoothness and the stability of the streaming media. This thesis mainly analyses and studies the client side based technique called adaptive media playout.
At first, this thesis briefly reviews the history of the streaming media technology and wireless network technology, discusses the problems usually encountered when applying the streaming media application in the wireless networks. The two key technologies in streaming media system are introduced including H.264/AVC video coding standard and RTP protocol. H.264/AVC which is current widely used video coding standard in many areas such as streaming systems, DVB, Blu-ray Discs is described in this part including the coding progress of the encoder and decoder and its major features. The RTP defines a standardized packet format for delivering audio and video over IP networks. RTP is used extensively in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applications.
This thesis proposed a novel scene-aware based adaptive media playout algorithm. In the algorithm, the client computes the motion intensity by the motion vectors in each video frame, and then adjusts the video frame rate according to the estimation of the buffer state and the motion intensity of the frame. Generally, in fast moving scenes, the frame rate will change slowly to ensure not affect the perceptive quality too much. And in slow moving scenes, the frame rate will change fast to reduce the probability of the buffer underflow. In each control period, the playout distortion value which is introduced in this algorithm can be controlled by the video quality threshold to limit the adjustment speed of the frame rate. The simulation results show that the proposed AMP algorithm significantly reduces the video distortion during the playback process.
引文
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