Steinar H. Gunderson

Tue, 05 Oct 2010 - VLC latency, part 1: Introduction/motivation

The Gathering 2011 is as of this time still announced (there's not even an organizer group yet), but it would be an earthquake if it would not be held in Hamar, Easter 2011. Thus, some of us have already started planning our stuff. (Exactly why we need to so start early with this specific will be clear pretty soon.)

TG has, since the early 2000s, had a video stream of stuff happening on the stage, usually also including the demo competitions. There are obviously two main targets for this:

  1. People in the hall who are just too lazy to get to the stage (or have problems seeing in some other way, perhaps because it's very crowded). This includes those who can see the stage, but perhaps not hear it too well.
  2. People external to the party, who want to watch the compos.
  3. Post-party use, from normal video files.

My discussion here will primarily be centered around #1, and for that, there's one obvious metric you want to minimize: Latency. Ideally you want your stream to appear in sync with the audio and video from the main stage; while video is of course impossible, you can actually beat the audio if you really want, and a video delay of 100ms is pretty much imperceptible anyway. (I don't have numbers ready for how much is β€œtoo much”, though. Somebody can probably fill in numbers from research if they exist.)

So, we've set a pretty ambitious goal: Run an internal 720p50 stream (TG is in Europe, where we use PAL, y'know) with less than four frames (80ms) latency from end to end (ie., from the source to what's shown a local end user's display). Actually, that's measured compared to the big screens, which will run the same stream, so compared to the stage there will be 2–3 extra frames; TG has run their A/V production on SDI the last few years, which is essentially based on cut-through switching, but there are still some places where you need to do store-and-forward to process an entire frame at a time, so the real latency will be something like 150ms. If the 80ms goal holds, of course...

I've decided to split this post into several parts, once per day, simply because there will be so much text otherwise. (This one is already more than long enough, but the next ones will hopefully be shorter.) I have to warn that a lot of the work is pretty much in-progress, though β€” I pondered waiting until it was all done and perfect, which usually makes the overall narrative a lot clearer, but then again, it's perhaps more in the blogging spirit to provide as-you-go technical reports. So, it might be that we won't reach our goal, it might be that the TG stream will be a total disaster, but hopefully the journey, even the small part we've completed so far, will be interesting.

Tomorrow, in part 2, we're going to look at the intended base architecture, the latency budget, and signal acquisition (the first point in the chain). Stay tuned :-)

[23:19] | | VLC latency, part 1: Introduction/motivation

Steinar H. Gunderson <sgunderson@bigfoot.com>