[mythtv] AVSync Problem on entering recorded playback

[Steven Adeff]

On 3/28/06, Mark Buechler <mark.buechler at gmail.com> wrote:
> On 3/28/06, Stephen Williams <stephen.gw at gmail.com> wrote:
> > I'm seeing stuttering playback on entry to recorded program playback
> > (DVB source), a pause/unpause clears this up.
> >
> > Having performed some debugging I have determined that the AVSync code
> > is never managing to get a sync. The following excerpt is from the
> > frontend log with "-v playback":
> >

<snip>

> > We continually get the above "doubling video frame interval." messages
> > until playback has been paused / unpaused. It appears that the system
> > is never able to sync in the case where audio is ahead of video.
> >
> > Having added more debugging to NuppelVideoPlayer it appears that
> > something it deliberately 'correcting' for the doubled video frame
> > interval used to regain sync. This has been determined by timestamping
> > calls to NuppelVideoPlayer::DisplayNormalFrame, the
> results are as
> > follows:
> >
> > 1. During normal (correctly AVSynced playback)
> > NuppelVideoPlayer::DisplayNormalFrame is called at a very
> regular rate
> > every 40ms (25fps).
> > 2. When attempting to regain AVSync by doubling the video frame
> > interval this increases to 60ms (this is expected and temporarily
> > brings the AVSync to within 3 frames).
> > 3. However, as soon as we stop doubling the video frame interval this
> > timing is reduced to 20ms, it appears that something is trying to
> > compensate for the 60ms frames.
> >
> > I'm looking for some further guidance on how to debug this issue. Is
> > anybody able to tell me what is controlling the rate at which
> > NuppelVideoPlayer::DisplayNormalFrame is called during
> normal
> > playback, i.e. what mechanism is causing those 20ms frames that are so
> > effectively counteracting the attempt to gain AVSync?
> >
> > Some further system information is supplied below in case it is required:
> > - Using OpenGL Vsync to a PAL TV (using an interlaced modeline).
> > - I've tried with and without OpenGL Vsync, there is no difference.
> > - Running 0.19-Fixes branch.
> >
> > If I can provide any more data then please let me know.
> >
> > Thanks in advance,
> > Stephen
>
>
> Question, are you seeing this ONLY with recordings? A few people have
> reported seeing this with LiveTV until they pause/unpause. I have the same
> issue but mine seems to loose sync at times with 720p content and eventually
> regains it. I'm wondering if all of these could be symptoms of the same
> problem.
>
> - Mark.

I offer this up to help diagnose the problem as I don't think it is an
issue with MythTV directly....

I haven't noticed any audio sync issues on my machine with 1080i or
720p (I am not using XvMC). I do notice a 1-3sec stutter when starting
playback of recordings, and sometimes after pausing. For live tv the
stutter lasts a little longer usually and sometimes after changing
channels it looks like its dropping frames but the audio remains in
sync. Pausing and unpausing clears all these issues up (I just wait
for the 1-3sec stutter period since its easier and goes away on its
own).

I've been running my own builds off the fixes branch.
--
Steve