Re: [AVT] Syncronization of co-located clients?

[Stephen Casner <casner at acm.org>]

On Sun, 3 Jul 2005, Greg Herlein wrote:

> I found this as well - might provide a mechanism for sync based
> on SMPTE time code:
>
> http://www.ietf.org/internet-drafts/draft-singer-smpte-rtp-01.txt
>
> Assuming all clients controlled the playout buffer using the
> SMPTE time code, this could likely do it.  My particular
> application is on a local LAN (a controlled VLAN actually) so I
> am not expecting the network to present significant problems.

I don't think the SMPTE timecodes help unless they are representing an
absolute time in the future, where the sender has made a prediction
about the amount of time needed for propagation to all receivers and
buffering time within each receiver.  That might work in a controlled
LAN environment, but it is not a general solution.

The BBN Synchronization Protocol that I mentioned is based on using
synchronized clocks at the receivers (e.g., using NTP).  Each receiver
relates its earliest possible playout time for a media timestamp to
real time, and communicates that back to the controller.  The
controller finds the latest of these times and informs all the
receives so they can choose the same playout time.  There's more to it
than that, of course.  Sync "epochs" were used to deal with media
clock drifts, etc.

[I state the above without having read the paper in a long time.  I
seem to remember a centralized controller, but it could be done in a
distributed manner as well.]

                                                        -- Steve

_______________________________________________
Audio/Video Transport Working Group
avt at ietf.org
https://www1.ietf.org/mailman/listinfo/avt