[mythtv] AVSync Problem on entering recorded playback

[Stephen Williams]

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":

2006-03-28 12:49:33.919 Using OpenGLVideoSync
2006-03-28 12:49:33.929 Using video as timebase
2006-03-28 12:49:33.929 Video timing method: SGI OpenGL
2006-03-28 12:49:33.929 Refresh rate: 40000, frame interval: 40000
2006-03-28 12:49:33.948 NVP: Waiting for prebuffer.. 0
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2006-03-28 12:49:34.046 OSDImgCache: Can't find
'/usr/local/share/mythtv/themes/Titivillus-OSD/edit'
2006-03-28 12:49:34.128 NVP: Waiting for prebuffer.. 1
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2006-03-28 12:49:34.129 OSDImgCache: Can't find
'/usr/local/share/mythtv/themes/Titivillus-OSD/edit'
2006-03-28 12:49:34.176 NVP: ClearAfterSeek()
2006-03-28 12:49:34.177 VideoOutputXv: ClearAfterSeek()
2006-03-28 12:49:34.177 VideoBuffers::DiscardFrames(0):
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2006-03-28 12:49:34.177 VideoBuffers::DiscardFrames(0):
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA -- done
2006-03-28 12:49:34.177 detectInterlace(Detect Scan, Interlaced Scan,
25, 576) ->Interlaced Scan
2006-03-28 12:49:34.177 Interlaced: Interlaced Scan  video_height: 576  fps: 25
2006-03-28 12:49:34.308 NVP: Waiting for prebuffer.. 2
UUUUuUULAAAAAAAAAAAAAAAAAAAAAAA
2006-03-28 12:49:34.650 NVP: Audio is 3.2878 frames ahead of video,
			doubling video frame interval.
2006-03-28 12:49:34.690 NVP: Audio is 3.54085 frames ahead of video,
			doubling video frame interval.
2006-03-28 12:49:34.750 NVP: Audio is 3.73062 frames ahead of video,
			doubling video frame interval.
2006-03-28 12:49:34.810 NVP: Audio is 3.7167 frames ahead of video,
			doubling video frame interval.
2006-03-28 12:49:34.870 NVP: Audio is 3.58127 frames ahead of video,
			doubling video frame interval.
2006-03-28 12:49:34.930 NVP: Audio is 3.3422 frames ahead of video,
			doubling video frame interval.
2006-03-28 12:49:34.990 NVP: Audio is 3.0379 frames ahead of video,
			doubling video frame interval.
2006-03-28 12:49:35.730 NVP: Audio is 3.0963 frames ahead of video,
			doubling video frame interval.
2006-03-28 12:49:35.790 NVP: Audio is 3.20972 frames ahead of video,
			doubling video frame interval.
2006-03-28 12:49:35.850 NVP: Audio is 3.17602 frames ahead of video,
			doubling video frame interval.
2006-03-28 12:49:35.910 NVP: Audio is 3.02575 frames ahead of video,
			doubling video frame interval.
2006-03-28 12:49:36.590 NVP: Audio is 3.0866 frames ahead of video,
			doubling video frame interval.
2006-03-28 12:49:36.630 NVP: Audio is 3.17745 frames ahead of video,
			doubling video frame interval.
2006-03-28 12:49:36.690 NVP: Audio is 3.25183 frames ahead of video,
			doubling video frame interval.

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