Adaptive Streaming in the Field
自适应流听起来很容易:生成具有不同质量相关配置的多个文件, 从匹配观看者带宽和播放平台的流开始, 然后适应不断变化的吞吐量条件和查看器平台上不同的CPU负载. As with all things streaming, however, the devil is in the details-how many streams, what resolutions, what keyframe interval, should you encode using VBR or CBR; what audio parameters should you use (and should they change per stream); and what's the most efficient way to address those pesky iOS devices?
为了回答这些问题(以及许多其他问题)，我们联系了几个使用不同技术的组织，并询问了有关所有这些问题的详细问题. Producers who participated included MTV, Turner Broadcasting, NBC (through Microsoft), Indiana University (IU), Harvard University, and Deutsche Welle, 我想首先感谢他们分享他们的经历. These companies use a variety of technologies, including Adobe Flash, Microsoft Silverlight, and Apple HTTP Live Streaming. 这支持了我的目标，即呈现与技术无关的编码和实现细节集合.
How Many Streams?
正如您所期望的那样，我看到了流的数量及其配置的显著变化. Table 1 (下)包含我们的受访者使用的配置列表. 请注意，该表并不包含所有活动的所有配置. 例如，哈佛处理现场活动和点播活动的方式不同. 相反，它显示了具有最多配置选项的替代方案. 我没有把特纳广播公司包括在表格中，因为尽管它共享数据速率和其他信息, Turner didn't provide resolution, frame rate, and other information referred to later.
At Indiana University, I spoke with Matthew Gunkel, 凯利商学院的多媒体专家, who advised that all streams are produced at 29.97 fps. 德国之声的配置基本上遵循苹果公司在其产品说明书中给出的方向 Technical Note (这应该是所有考虑或实现HTTP直播的人都需要阅读的. I spoke with multimedia developer Jan Petzold, 谁分享了前两个流是以全帧速率(25fps)制作的?, the 200Kbps stream was produced at 12.5 fps, and the 100Kbps stream was produced at 8 fps.
At Harvard, Larry Bouthillier, 大学CIO办公室的媒体技术架构师, explained that for live events, 它们都是通过Flash媒体服务器(FMS)和HTTP直播分发的, he uses three streams, 800x600@1152Kbps, 400x300@564Kbps, and 320x240@132Kbps, 仅使用最高质量的流FMS，其他两个使用两种技术. 哈佛大学以全帧速率编码其点播视频，除了低于150Kbps的流, which are encoded at 15 fps.
温哥华奥运会和NBC的周日橄榄球之夜, I spoke with Alex Zambelli, 他是微软媒体技术的传道者，为这两部作品设计了实时平滑流工作流程. Zambelli介绍说，温哥华奥运会的所有流都是全帧速率的.97)而“周日橄榄球夜”则在29点播放除最后一个以外的所有直播.97, while the 112x64 resolution stream, which is used only for thumbnail previews, is produced at 15 fps.
在这两种情况下，NBC都分发了高清源和标清源视频. For the Olympics, 有两个纯hd流配置和两个纯sd流配置, 包括来自SD交错源的848 × 480 SD流. With Sunday Night Football, NBC在纯SD流(848 × 480)上生成两个纯HD流(720p和960x540)，然后使用所有其他配置用于SD和HD.
在MTV电视网，我采访了媒体技术战略副总裁格伦·戈尔茨坦. As Goldstein explained, MTV creates two groups of files for adaptive delivery, one for "broadband" clients, 包括高速连接的台式电脑吗, and one for "constrained" clients, 哪些设备或计算机受解码CPU或连接的限制. MTV在连接时将观众分配到一个组，并在连接期间将他们保持在该组中. In Table 1, the top configurations are for the broadband group; the bottom two are for the constrained group.
Goldstein also advised that MTV Networks uses the native frame rate for all encodes; that encompasses 23.98, 24, 25, and 29.97 footage. 戈尔茨坦还提到，MTV会在必要的时候，在编码之前，将电视电影片段反转到原始帧率.
注意，Adobe提供了动态流的配置和编码建议 "Dynamic Streaming on demand with Flash Media Server 3.5," by Abhinav Kapoor and "Flash平台上HTTP动态流媒体的视频编码和转码建议," by Maxim Levkov 11. 微软的Ben Waggoner也为Silverlight提供了类似的信息 "Expression Encoder 2 Service Pack 1; Intro and Multibitrate Encoding.我将参考这三篇文章以及相关的苹果技术说明
in this article.
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