Simplifying Live Streaming at Scale with DASH-IF
Along with a number of other partners, Unified Streaming一直致力于一个DASH-IF项目，以形成一个实时摄取协议的新规范. It began as an internal project with a few other vendors a couple of years ago, 此前，一家欧洲广播公司强调需要记录实时编码器应该产生的内容. The specification has now been published and is ready to use. 在本文中，我将概述新协议的必要性和开发过程中需要考虑的一些主要因素.
这个项目的合作伙伴是从整个流频谱中招募的，以确保在设计规范时对工作流有一个完整的视图. They include Microsoft, Hulu, AWS Elemental, Akamai, Comcast, Bitmovin, Qualcomm, Media Excel, Harmonic, Ateme and CenturyLink.
The project addresses live OTT streaming at scale, 这是出了名的难以支持的，因为有太多的活动部分.
In particular, there were four areas that we felt could be improved:
- Timed metadata: 我们希望简化获取编程信息和其他元数据到fMP4表示. 我们还想考虑信号拼接信息，以实现及时包装和广告插入.
- Reliability: To improve fault tolerance and redundancy, we wanted OTT workflows to be able to use multiple live encoders, origins or CDNs to achieve end-to-end failover support.
- Low latency: This is really something that builds up across a workflow. 拥有正确的规范和避免额外的转换将有助于OTT提供商达到他们的延迟目标.
- Better than broadcast quality: 支持高质量视频流，质量与广播相当或更好.
By addressing these areas, 我们已经创建了一个适合现代流媒体需求的协议，并创建了一个更有效的工作流程, which ultimately saves on cost and resources. 低延迟已经是业界的一个大话题，它将为吸引观众和最大化广告收入开辟宝贵的新机会. 但重要的是，工作流要尽可能高效地实现这些潜力.
Rather than just standardizing a player format, we set out to create a more efficient overall workflow. We began by identifying some interfaces that would benefit from standardization, 我们特别关注自适应比特率编码器和原始服务器的输出, or packager, that are pushing towards a content delivery network (CDN).
活动编码器通常会将其内容推出，因为它需要将其百家乐软件集中在对内容进行编码上. 换句话说，它没有能力处理来自客户机的大量请求. Instead, 实时编码器倾向于生成帧或片段并将其推送到原始服务器, which in turn makes it available to clients. Both MPEG-DASH and HLS are great protocols for this use case.
但是在DASH和HLS中没有指定的推送内容的行为:它主要是关于HTTP GET将内容拉出. 因此，我们为fMP4和CMAF开发了一个配置文件，以便稍后进行包装.
We have developed two profiles, or interfaces. 如果你有一个工作流程，其中有一个单独的编码器和包装或源服务器，那么你可以只是发送CMAF轨道没有清单和做即时包装, then push that or link it to a CDN. The second interface would allow you just to post, or push, DASH directly, so in this case you would be pushing segments and manifests. Both cases use HTTP POST.
This is of course a very simplified overview of CMAF ingest. In the first interface, without a manifest, 你只是使用不同的TCP连接并发布一个CMAF音轨，所以它实际上是一个fMP4文件, but there is no multiplexing, so each track is a separate file and you're just posting them. 我想明确的是，它不是强制性的使用长时间运行的帖子:你可以使用长时间运行的帖子与块传输，或者你可以单独发布每个片段.
重要的是接收器在接收到碎片的时候能够检测到它们. 它是由这个触发的，所以这在低延迟的情况下也很有效，因为你可能有小块. 接收器可以检测到数据块并动态地生成输出清单. 你在这里看到的是不同的视频和音轨被发布到原始服务器. We use the CMAF tracks for video and audio, but we also use it for program metadata, which is something that is still relatively new.
该协议的一个优点是，断开连接并稍后重新连接非常容易. If, for example, 源下降，然后再次启动，它将在预定义的段边界恢复post. A second dual redundant encoder can be used, so if the CMAF header and the last fragment cannot be sent, 接收端仍然可以从冗余编码器获得完整的存档流.
我们希望通过新规范实现的另一件事是低延迟的即时封装. One thing you can do in a CMAF specification is insert top level boxes, called DASH event message boxes, which can carry metadata. To achieve fast packaging before our new protocol, the entire media file would need to be scanned to find those boxes, 但是我们定义了一个单独的轨道，这样一个支持ISO bmff的设备可以很容易地看到元数据轨道并找到正确的信息.
CMAF容器格式的使用允许使用最新的视频编解码器，如支持高动态范围(HDR)和分辨率高达4K的高效视频编码. CMAF容器格式很流行，许多新的编解码器都绑定到CMAF容器格式. 因此，CMAF摄取协议可以无缝地支持新的和新兴的编解码器, without any modifications to the protocol, as well as high quality video. 这种对新编解码器的支持是我们发现其他不基于CMAF的协议所缺乏的.g. WebRTC, RTMP etc.).
通过这些方式，我们实现了项目的三个主要目标:改进定时元数据, reliability, low latency and video streaming quality.
随着同时观看人数的增加，需要提供更好的服务，直播将变得越来越难以大规模交付, more engaging streams for viewers. As competition for viewer attention increases, features that rely on timed metadata and low latency will become more important. 支撑这些特性的是对更好的可靠性的需求，而新的摄取规范应该在解决这一问题上走了很长的路.
[Editor's note: This is a contributed article from Unified Streaming. 流媒体接受供应商署名完全基于它们对我们读者的价值.]
Related Articles
Companies and Suppliers Mentioned