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

  • From: Paul Davis <paul@xxxxxxxxxxxxxxxxxxxxx>
  • To: gmpi@xxxxxxxxxxxxx
  • Date: Thu, 05 Feb 2004 13:35:27 -0500

>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

plugin A knows that sample 128 (not 123) should be audible at the same
time as the video frame its about to deliver to the frobnicator. it
knows that in fact, sample 123 is being played at the same time as
this frame, because the UST stamps match. it can now adjust either the
audio stream or video stream in some way that it chooses to bring them
back into sync.

the alternative is to watch the speed of the two clocks (GMPI
timestamp, video frame count), and then implement a PLL to try to keep
them in sync. throw in backward motion, varispeed etc. and it all gets
pretty hairy pretty quickly.

with the matching UST stamps, you can always determine the exact
relative alignment of the two clocks whenever you need it (in either
the plugin's GMPI process callback or its retrace interrupt
handler-thingy). 

--p


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