[FFmpeg-devel] [PATCH] ALSA: fix timefilter divergence

Michael Niedermayer michaelni at gmx.at
Mon Feb 20 02:15:32 CET 2012

On Sun, Feb 19, 2012 at 09:00:15PM +0100, Michael Niedermayer wrote:
> On Sat, Feb 18, 2012 at 01:33:38PM +0100, Nicolas George wrote:
> > L'octidi 28 pluviôse, an CCXX, Michael Niedermayer a écrit :
> > > Also theres another issue, if theres a significnat correction from
> > > NTPD that may mean the previous hours of recording used wrong
> > > timestamps. And if there are 2 streams (audio & video) which use
> > > seperate timefilter, they may with either implementation recover at
> > > different rates makeing the whole issue even worse
> > 
> > Indeed. But we can not do anything about it, timefilter or no timefilter. If
> > someone wants to sync streams with different time sources, it is their
> > responsibility to keep the time sources in sync.
> iam not sure if we misunderstand each other here.
> but if we have audio and video both from the same time source (wall
> clock) and this gets shifted by a few hours due to NTP then the 2 will
> recover very differntly because each has a seperate timefilter
> one run per video frame and one run per audio packet.
> the AV sync errors between the packets after such NTP discontinuity
> should be in the order of minutes and could oscilate around both audio
> and video being late before converging to the correct value

a simple solution for this seems to be to let the user provide a limit
on the maximal correction, that is the maximum amount of noise in
a single value.
With this a discontinuity has nicely bound effects while a classic
filter would double in error when the discontinuity doubles.


Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

When the tyrant has disposed of foreign enemies by conquest or treaty, and
there is nothing more to fear from them, then he is always stirring up
some war or other, in order that the people may require a leader. -- Plato
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