2010年12月31日
By 1月时特约编辑
专题文章

自适应流在现场

VBR或CBR?
My knee-jerk reaction here was to use constant bitrate encoding because the consistent stream should be easier to distribute, particularly in an adaptive setting where the viewer is often retrieving 2- or 3-second chunks of video at a time. 这当然是微软的赞贝利的观点, 谁说, “The reason I prefer CBR for adaptive streaming is because it exhibits less oscillation in data rates, making fragment sizes more consistent and consequently making it easier for client heuristics to accurately estimate bandwidth. VBR可以工作, but one has to be careful with setting average/peak/buffer values in order not to cause issues on playback.”

在实践中, 所有的奥运直播都是带有5秒vvb缓冲的CBR, while on-demand streams were VBR constrained to 10% over the target bitrate. 周日橄榄球之夜，使用与奥运会相同的CBR配置.

Deutsche Welle is exclusively CBR, while Harvard is CBR except for the highest quality stream. 印第安纳大学使用了受限VBR, 将1500Kbps的流限制为2Mbps, 将750Kbps的流调到1Mbps, 将250Kbps的流调到500Kbps(参见表2).

Ozer自适应表

戈尔茨坦认为, MTV Networks constrains the peak data rate to be no more than two times the average and monitors the buffer in the player to make sure that it can tolerate a brief data spike. 虽然这显然对MTV网络很有效, Harvard’s Bouthillier cautions that many of the encoding tools that he’s tested don’t honor VBR constraints, which is why he opts for CBR in all but the highest data rate connections.

什么是关键帧间隔?
对于单个文件流, I recommend a keyframe interval of 10 seconds with keyframes inserted at scene changes. 使用自适应流，规则会发生变化，如中的结果所示表3.

Ozer自适应表

检查我们的技术百家乐软件, Adobe and Microsoft recommend using a single keyframe interval for all files produced for an adaptive streaming package. 苹果也做同样的事情，你可以在表4, where the keyframe stays at a consistent 3 seconds, irrespective of the file frame rate.

Ozer自适应表

尽管特纳没有为今年的PGA提供流配置, it did supply last year’s HTTP 在线直播 configurations for an article that I wrote last year, 作为表5. 如你所见, it also changed the keyframe setting for the lowest quality stream from 2 to 3 seconds. (欲了解更多关于特纳直播PGA锦标赛的信息，请参见“流媒体焦点:彼得·斯科特.")

Ozer自适应表

I’d be remiss if I didn’t mention another valuable resource for those attempting to stream adaptively, Bouthillier的如何使用Flash媒体服务器实现动态流媒体.Bouthillier在他的文章中说:

Keyframe Interval: When the server gets a request to switch to a different stream, it will try to make a smooth switch by lining up keyframes in the two files. 如果你的关键帧间隔太远, 服务器可以强制切换, 但它不一定是优雅的. The best bet is to set a fixed keyframe interval which is the same for all streams to guarantee that there are enough keyframes to facilitate stream switching.

很明显, 用于Silverlight和Flash, choose a relatively short keyframe setting such as 2 seconds and apply it consistently across all streams. 当使用HTTP直播时, I’d follow the recommendations in the Apple Technical Note in the absence of a very good reason not to.

关于关键帧还有其他几点. First, you should disable keyframes at scene changes if it’s an option in your encoding tool. You want keyframes at regular intervals and at the same location in all files, 禁用此选项有助于确保这将发生.

第二个, for those encoding tools that allow you to select which keyframes will be IDR keyframes (Telestream Episode comes to mind), 使所有关键帧IDR关键帧. I don’t have the space (or time) to define an IDR keyframe, so just humor me on this one.

最后, for technologies that divide files into chunks or segments (such as Apple’s HTTP 在线直播), 关键帧间隔应该与片段大小相同.g.(两者均为2秒)或可分为段大小. 例如, 在HTTP直播编码白皮书中, Akamai recommends a segment duration of 10 seconds and keyframe interval of 5 seconds. 这样，每个片段都从一个关键帧开始.

音频数据速率
The common wisdom regarding audio is best summarized by this quote from Adobe’s Dynamic Streaming article: “Keep the audio bitrates and sampling rates the same to provide a seamless switch between them as the stream plays. Switching between two streams with the same sampling and bitrates will allow seamless transition. 可以在不兼容的比特率或采样率之间切换, but could result in a slight audible ‘pop’ sound at the time of the transition.”

沿着同样的脉络, Apple recommends using a single 40Kbps bitrate in all adaptive encodings, 表达式编码器, 微软的主要平滑流编码器, 限制您对组中的所有文件进行单一音频配置.

不足为奇的是, 我们的大多数受访者，包括微软, MTV, 和德国之声使用了一致的音频参数, 尽管有两位受访者, 印第安纳大学和哈佛大学, 改变流的数据速率，没有报告有害影响. 更具体地说, Indiana used 128Kbps audio for its two highest quality streams and 96Kbps for the lowest quality stream, 而哈佛则实行三级制——32级, 64, and 96Kbps—on its five on-demand streams and in the live streams used 32, 64, 和128Kbps(见表6).

Ozer自适应表

如表5所示, 制作2009年PGA锦标赛时, Turner Broadcasting used 16Kbps audio with its 126Kbps video stream and 40Kbps with its two higher quality streams, 同样，没有报告在流切换时发出爆裂声. 回到我们现在的桌子, MTV produces audio using two different parameters but doesn’t switch between them adaptively.

潜在的爆破, varying audio quality to match video quality in the adaptive streams provides the best possible viewing experience. Several of our respondents are successfully using this strategy now; if you want to go this route, 试一试, 但一定要在一系列相关的观看平台上进行测试.

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