自适应流在现场
Adaptive streaming sounds easy enough: Produce multiple files with different quality-related configurations, start with a stream that matches the viewer's bandwidth and playback platform, then adapt to changing throughput conditions and the varying CPU load on the viewer's platform. 就像所有的东西都在流动一样, 然而, 细节决定成败——有多少条小溪, 什么决议, 什么是关键帧间隔, 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?
To answer these questions (and many others) we contacted several organizations using different technologies and asked detailed questions about all this and more. 参与其中的制作人包括MTV, 特纳广播公司, NBC(通过微软), 印第安纳大学(IU), 哈佛大学, 和德国之声, and I want to thank them up front for sharing their experiences. 这些公司使用各种各样的技术, 包括Adobe Flash, 微软的Silverlight, 和Apple HTTP 在线直播. This supports my goal of presenting a technology-neutral collection of encoding and implementation details.
有多少流??
As you would expect, I saw significant variability in the number of streams and their configurations. 表1 (below) contains a list of configurations used by our respondents. Note that this table doesn't contain all configurations for all activities. For example, Harvard handles live and on-demand events differently. Rather, it shows the alternative with the most configuration options. I didn't include 特纳广播公司 in the table because although it shared data rate and other 信息, 特纳没有提供解决方案, 帧率, 其他信息稍后会提到.
在印第安纳大学, 我和Matthew Gunkel谈过, 凯利商学院的多媒体专家, 谁建议所有的流都是在29岁产生的.97帧/秒. Deutsche Welle's configurations generally follow the direction given by Apple in its 技术报告 (that should be required reading for all considering or implementing HTTP 在线直播. 我采访了多媒体开发人员Jan Petzold, who shared that the first two streams were produced at full 帧率 (25 fps), 200Kbps的流是在12点产生的.100Kbps的流以8fps产生.
在哈佛大学, 拉里Bouthillier, media technology architect for the Office of the University CIO, 为现场活动解释, which are distributed both via the Flash Media Server (FMS) and HTTP 在线直播, 他用了三条小溪, 800年x600@1152kbps, 400年x300@564kbps, 和320年x240@132kbps, with the highest quality stream FMS only and the other two using both technologies. Harvard encodes its on-demand videos at full 帧率 except for streams less than 150Kbps, 哪些是15 FPS的编码.
温哥华奥运会和NBC的周日橄榄球之夜, 我采访了Alex Zambelli, the Microsoft media technology evangelist who architected the live Smooth Streaming workflows for the two productions. Zambelli related that all streams from the Vancouver Olympics were produced at full 帧率 (29.97) while Sunday Night Football produces all streams except the last at 29.97, 而112x64分辨率的流, 哪个只用于缩略图预览, 是15 FPS制作的吗.
In both instances, NBC distributed both HD source and SD source video. 为了奥运会, there were two HD-only stream configurations and two SD-only configurations, including an 848x480 SD stream from an SD interlaced source. 周日橄榄球之夜, NBC produces two HD-only streams (720p and 960x540) on SD-only stream (848x480) and then uses all other configurations for both SD and HD.
At MTV Networks, I spoke with Glenn Goldstein, VP, media technology strategy. 正如戈尔茨坦解释的那样, MTV为自适应交付创建了两组文件, 一个是“宽带”客户端, which includes desktop computers with high-speed connections, 还有一个是针对“受限”客户的, which are devices or computers constrained by decode CPU or connectivity. MTV assigns viewers to a group upon connection and keeps them within that group throughout that connection. In 表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 帧率 for all encodes; that encompasses 23.98 24 25 29.97视频. Goldstein also related that MTV will inverse telecine movie footage to return to the original 帧率 before encoding whenever necessary.
Note that Adobe supplies its configuration and encoding recommendations for Dynamic Streaming in 动态流媒体需求与Flash媒体服务器3.5阿比纳夫·卡普尔和Video Encoding and Transcoding Recommendations for HTTP Dynamic Streaming on the Flash Platform马克西姆·列夫科夫著. Microsoft's Ben Waggoner weighs in with similar 信息 for Silverlight in "Expression Encoder 2 Service Pack 1; Intro and Multibitrate Encoding." I'll refer to all three 文章 and the Apple 技术报告 where relevant
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