OTT技术培训.ppt

* * 基于HTTP的渐进下载(Progressive Download)流媒体播放仅是在完全下载后再播放模式基础上做了一些小的改进。与下载播放模式中必须等待整个文件下载完毕后才能开始播放不同，渐进下载客户端在开始播放之前仅需等待一段较短的时间用于下载和缓冲该媒体文件最前面的一部分数据，之后便可以一边下载一边播放。 在正式开始播放之前的这一小段缓冲应使得后续即使在网络较为拥塞的情况下媒体数据也能够得以不间断地连续播放，通常需要几十秒甚至上百秒的时间。在这种模式下，客户端以自己以及Web服务器和网络所能允许的最大速度尽可能快地从服务器索取数据，而不考虑当前所播放压缩码流的实际码率参数。只有满足特定封装条件的媒体文件格式才支持这种类型的渐进下载播放，例如用于初始化解码器的编码参数必须放置在媒体文件的起始部位，音视频数据完全按照时间顺序进行交织等。 渐进下载流媒体播放采用标准HTTP协议来在Web服务器和客户端之间递送媒体数据。 * A Simple Case of Adaptive Streaming Figure 1 illustrates a simple example of on-dEVAnd dynamic adaptive streaming. In this figure, the multimedia content consists of video and audio components. The video source is encoded at three different alternative bitrates: 5M, 2M and 500K bits/sec. Additionally, an I-frame only bitstream with low frame-rate is provided for streaming during the trick mode play. The accompanying audio content is available in two languages: Audio 1 is a dubbed English version of the audio track and is encoded in surround, AAC 128K and AAC 48K bits/sec alternatives, while Audio 2 is the original French version, encoded at AAC 128K and AAC 48K bits/sec alternatives only. Assume that a device starts streaming the content by requesting segments of the video bitstream at the highest available quality (5M) and the English audio at 128K AAC since for instance the device does not support surround Audio (1). After streaming the 1st segments of video and audio, and monitoring the effective network bandwidth, the device realizes that the actual available bandwidth is lower than 5M bits/sec. So, at the next available switching point, it switches the video down to 2M bits/sec by streaming the next segments from the mid-quality track while it continues streaming of the 128K AAC English audio (2). The device continues to monitor the actual network bandwidth and realizes that the network bandwidth has further decreased to a value lower than 2M bits/sec. Therefore, in order to maintain continuous playback, the device further switches the streams dow