冰上学校通告(石膏)
The streaming needs of schools are modest relative to those of many other institutions, especially when it comes to live streaming. The highest-profile live streams a school would expect to see are likely commencement ceremonies, 政策的演讲, 还有几位杰出的演讲者. In my many years supporting educational video, the largest audience for a live stream was 1,400 concurrent viewers (the awards ceremony for the Science Olympiad nationals from 2010), an audience that was easily handled by a cluster containing just two edge Flash Media Servers. 虽然观众不多, they're important: We don't want a parent who couldn't make the trip to graduation to miss their student's name being called or a donor to miss 他们的认可度因为一条差的小溪.
除了这些事件, there are surprisingly few uses for traditional live streaming in support of K–12 schools' core educational missions. 然而, I recently consulted on an interesting one: the morning announcements at an elementary school. Rather than using the venerable intercom system, this school has students provide a newscast-style live stream that is projected into all of the classrooms. This used to be done via closed-circuit television, 但是当这个系统老化的时候, the teachers improvised technologies and made very smart choices in the transition to a streaming media solution.
等我参与进来的时候, they were using OBS Studio to publish the stream to a commercial host that would stream to classroom smartboards. The teacher running the show wanted to explore an on-prem media server to eliminate the non-trivial hosting costs, 虽然小溪很安全, the teacher was uncomfortable having the video feed of elementary school children sent off-prem and back. The teacher also wanted to see if the latency in the stream could be reduced: The streaming vendor had followed industry trends and adopted a fairly traditional HTTP 在线直播 (HLS) model that pushed latency well past 30 seconds—long enough that it was causing confusion for the students.
理想的解决方案是没有成本, on-prem, 低延时, adequately robust for a broadcast to a maximum of 30 clients without last-mile concerns, 而且足够简单，可以可靠地工作, both for the students publishing the broadcast and the classroom teachers playing it.
At least one of these features would need to be compromised: After weighing the performance benefit of deploying a WebRTC-based solution such as Janus or Kurento against the difficulty of maintaining and troubleshooting an open-source product that's primarily being developed for live interactive services rather than as a broadcast streaming server, it was clear that latency was not going to be as low as with the school's CCTV solution. I recommended perhaps the oldest media server still in widespread use: Icecast. Icecast is typically thought of as an audio-only streaming server, but it can also stream WebM video to HTML5 video elements natively in modern browsers.
Tuning Icecast for 低延时 requires essentially the same considerations as tuning HLS for 低延时: You need to minimize video stored in buffers, with the trade-off that overall streaming reliability will be less robust. The only thing you can do is to adjust the burst size for your broadcast's mount point. Burst size is roughly the same as fragment size in HLS and directly relates to latency since it defines how much video data needs to accumulate before being sent to the client. Icecast的默认值是64Kbps, a very small amount of server-side data buffering before sending to the clients. The real trick for 低延时 in Icecast is to use a short GOP length at the encoder. In my tests using a range of school-budget hardware, I could consistently get latency of around 4 seconds with tolerable picture quality by setting the keyframe interval in OBS Studio to twice the frame rate: Firefox's
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