[gmpi] Re: Reqs 3.9. Time - opening arguments.1

  • From: Tim Hockin <thockin@xxxxxxxxxx>
  • To: gmpi@xxxxxxxxxxxxx
  • Date: Thu, 5 Feb 2004 09:08:03 -0800

On Thu, Feb 05, 2004 at 09:19:53AM -0500, Paul Davis wrote:
> >I think that if there is a way to say "What's the UST time for sample X?"
> >that should be enough, right?
> 
> yes, but you can't do this in general. there is no "mapping" between
> UST and a sample clock - all you can do is to ask what the UST time is
> at a certain time when you know the sample clock value. then you have
> to pair those two times together, forever. thats why *every* event
> needs UST time associated with it. you cannot infer UST time from any
> other piece of information.
> 
> remember, UST time is running on its own clock. you have no idea
> whatsoever how fast it moves.

All events are timestamped some amount of time into the future of the sample
timeline and the real time timeline.  If all you can do is get a UST time
'now', then what good is it?

If you send the same event to two different plugins, they will be run at
different real times (assuming a single thread, they must be serial) so by
the time they receive the event, the UST timestamp is either estimated or it
is in the past (and getting the current UST time would give a different
result).

Is UST time not linear?  If you know the UST time at t0 and the UST time at
t1 and it is linear, the rest is cake.  If not, I need a more in depth walk
thru of exactly how it is used

example:

event E is sent to plugin A with timestamp 123 and UST 456
plugin A receives an event from the video frobnicator with UST 456
...




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