MediaStreamRecording, DataChannel and Time Synchronisation

[Leighton Carr]

Hi WebRTC,

As many of you may know - the good people doing Chromium have started an early implementation of the MediaStreamRecording API:

https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/2l_G_apqk30

https://code.google.com/p/chromium/issues/detail?id=273277

One of the issues that we are looking at is time synchronising video, audio and data from multiple sources and then recording it to disk for synchronous playback.  At the moment we are writing a native WebRTC app to perform this, but the prospect of having the MediaStreamRecorder in a verson of Chrome or Canary soon (?) could really simplify this.  

So I have a few questions:

1)  Is there any way to sync the data sent via a DataChannel on a peer connection with the video stream attached to that peer connection (I think no?) ?  Either with frame number or time-stamp?  Is there any plan to allow this at some point in the future of WebRTC?  

2)  Is anyone from the WebRTC side discussing the possibility of allowing time-stamping of video frames from source through to recording?  Especially considering how the standard for MediaStreamRecording is still being fleshed out?

Thanks,

Leighton.

[bryand...@gmail.com]

RTCP Sender Reports are sent regularly for audio and video.  The SR associates NTP time (objective time) with RTP timestamp, which is in a scale particular to the media stream.  So, I think that you have absolute synchronization for every packet:

http://tools.ietf.org/html/rfc3550#section-6.4.1

I've not worked with the data channel, but I believe the same thing applies.

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[Leighton Carr]

Are these accessible at the Javascript layer though?  Part of the work for MediaStreamRecording is defining the API, and there's definitely some use for being access the time-stamps at this higher level.

[V]

Did you by chance figure out if this is possible or in the pipeline?  I would like to also have the ability to sync the video and datachannel.

[Justin Uberti]

We are working on making getStartDate work in HTMLMediaElement. Once that is present, you can send wall time in the DC and compare against getStartDate() + currentTime.

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