[mythtv-users] 720p HD content from WPVI in Philly periodic a/v sync issues

[Andrew Gallatin]

Andrew Gallatin [gallatin at cs.duke.edu] wrote:
> Taylor Ralph [tralph11 at yahoo.com] wrote:
> > --- On Fri, 12/11/09, Andrew Gallatin <gallatin at cs.duke.edu> wrote:
> > 
> > > From: Andrew Gallatin <gallatin at cs.duke.edu>
> > > Subject: Re: [mythtv-users] 720p HD content from WPVI in Philly periodic a/v sync issues
> > > To: "Discussion about mythtv" <mythtv-users at mythtv.org>
> > > Date: Friday, December 11, 2009, 10:54 AM
> > > Byron Poland [wpoland at gmail.com]
> > > wrote:
> > > > for the past couple of months anything I record from
> > > WPVI in Philadelphia
> > > > (delivered via clear qam from Verizon FIOS in
> > > Wilmington, DE), has had
> > > > playback problems where the video and audio fall out
> > > of sync, and myth then
> > > > speeds up the video and/or drops frames for a second
> > > or 2 to catch up.? I've
> > > > read the list and seen other threads describing a
> > > similar phenomenon,
> > > > including being isolated to one channel.? All
> > > other 720p and 1080 recordings
> > > 
> > > <..>
> > > 
> > > I think this is the same problem as reported here, nearly
> > > 2 years ago:
> > > ? http://www.gossamer-threads.com/lists/mythtv/dev/317736
> > > 
> > > As DanielK said "they are just using an encoding extension
> > > we don't
> > > support very well.....I believe they are broadcasting about
> > > 40 fps on
> > > a 60fps stream and marking some of the frames as
> > > repeatable"
> > > If this is true, you may have luck complaining to the
> > > broadcaster like
> > > the people in that thread did.? Unfortunately, that's
> > > not much of an
> > > option for me, as multiple broadcasters in my market have
> > > their
> > > encoders setup this way.? The only way I was able to
> > > play back these
> > > sorts of streams was by using a non-MythTV front end (sigma
> > > based
> > > SageTV HD100).
> > > 
> > > I'd love to come back to MythTV, and I'm eagerly looking
> > > forward to a
> > > fix to this.? I have a mythbox setup running trunk,
> > > and I'm happy to
> > > test patches, make snippets of these streams available to
> > > the devs,
> > > etc.
> > > 
> > > Drew
> > 
> > I've got a couple patches attached that might help. They are both based on 0.22-fixes but the latest av-sync changes to trunk will probably break the repeat_pict patch. Let us know if these help at all.
> 
> I checked out a fresh 22-fixes and applied the patch.
> Unfortunately, it actually seemed to make things worse.
> I've attached -v playback from a problematic file.
> This is using an Ion 330 using VDPAU, displaying to
> a 1080p panel.
> 

I have a very high-level understanding of what's going on here, so I
apologize if my assumptions are incorrect, but here is my theory: The
broadcaster is using the mpeg2 video repeat flags to save bandwidth.
So he's broadcasting, on average, about 54fps and marking about 10% of
them repeatable, which brings the frame rate up to 60fps.  But
mythtv's mpeg2 decoder is not counting these repeats, and the a/v sync
code thinks the frame rate is all messed up.  From what I can tell,
your patch adds a delay based on the repeat flags. Rather than adding
delays, wouldn't it be simpler to use the actual decoded frame count
(or rate, or whatever the code uses)? Also, is the video portion of
MythTV honoring the repeat flag?  Does it matter than I'm using VDPAU?

Again, sorry if I'm being dumb, and thanks for the interest!!

Drew