[mythtv-users] fps drops with DRM sync and bob

Hamish Moffatt hamish at cloud.net.au
Wed Jun 6 13:32:38 UTC 2007

On Sun, Jun 03, 2007 at 10:44:13AM -0400, Doug Larrick wrote:
> Audio as timebase is the normal mode of operation for most folks.  The
> standard deviation is in usec, I believe, not fps, so these are not
> terribly huge standard deviation values.  On my desktop system (which is
> 60 Hz refresh, watching video that's 29.97 Hz refresh) I get std. dev.
> values in the 4000-ish range, which I attribute to the mismatch.
> If you could get your refresh rate to *exactly* 50 Hz I believe these
> would go down even further.
> How does the picture *look* with audio as timebase?

More experimentation;

I've tweaked my modeline to get ~50.002Hz, but 50.0 simply isn't
possible. I cut up myth's vsync.cpp to write a simple test program to
measure the DRI vblank interval, and it's reporting 50.002Hz.

When I start playing video with bob (or kerneldeint) there's not much
change in the result, though it does jitter a bit more than when just
sitting in the static MythTV menus.

However bob still goes nuts after a few seconds; dropping frames to
catch up with video. This is with audio as timebase. The picture is
rather jerky as frames are dropped.

I notice this seems to occur with video as timebase as well, but in that
cause shouldn't audio frames be dropped rather than video?

Maybe I need to measure my audio output rate too, though I don't see wy
bob and kerneldeint would differ in behaviour if audio was the cause of
the problem. My system has Intel HDA audio on board.

Before it goes nuts, the picture shimmers quite a bit vertically. 
Also I notice a lot of stretched frames and things when seeking which I
never saw on the old NVIDIA system.

Is the DRI sync well tested? If NVIDIA doesn't support it I wonder how
many people are actually using it.

Hamish Moffatt VK3SB <hamish at debian.org> <hamish at cloud.net.au>

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