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Video: How to Synchronize Streaming Playback

Learn more about stream synchronization at the next Content Delivery Summit.

Watch Peter Chave's keynote, The Future of TV Delivery, on the Streaming Media Conference Video Portal.

Read the complete transcript from this video:

Peter Chave: Synchronization. This is great, the drift problem. How can we keep the players from wandering away from each other? ABR players today fundamentally work on the principle of "I will show every frame of video at the frame rate I receive them, which is, I will wait if I'm missing something, and I will catch up."

That makes sense, because if I'm watching an episode, I don't want to see whatever happened last night, the critical moment where somebody comes down with the sword and then it cuts and comes back and then "Oh, what happened?"

Or somebody kicks the ball, and oh, look, the score went up by seven points! How do we know what happened? We missed the action.

ABR worked on the principle of, "I'm going to keep the bits in the buffer, and if I don't have enough buffer to play, I'm going to wait until the buffer's full again, so I show every frame of video."

Traditional TV doesn't work that way. Satellite doesn't work that way. WebRTC doesn't work that way, which is, if the frame of a video doesn't arrive within the time it has to be shown, because it's end-to-end synchronous, you just don't see it.

So there are a couple of ways we can solve this. I'm going to show you one way that we've done already, and there's another way we can do this, which is you can achieve synchronization today with players by just having a simple external time source, say an NTP server, and ability in the player to change the playback rate, very subtly, or very aggressively, or just skip, and a common latency target. If you put those all together, you can achieve synchronization.

What I have here is three players. One's on Android, two are on Mac, all of these are synchronized within a frame of video of each other. There's nothing magic about it. These are standard dash.js players, where there's a little bit of JavaScript on top, which tells it what latency target, what do I want the stream latency to be, it's looking at time.Akamai.com, to find a common clock reference. And it's then comparing the timestamp in the player to the timestamp from the clock, and it's putting those together to work out, do I adjust the playback rate to make it go faster, or go slower, so that my videos all synchronize?

You can be as aggressive as you want, or as, you know, as subtle as you want to bring these players together. But it is possible to get three completely separate devices on three totally different networks within a frame of video without any other special protocols, just a standard player and a common time source.

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