[mythtv] MythTV stuttering playback

Christiaan van Dijk dvbmail at xs4all.nl
Sat Jun 2 10:24:18 UTC 2007

Hi all,

recently I ran in a problem with playback in MythTV on a VIA CN700 based
board with XvMC. The problem started when I switched to DVI instead of
analog in combination with the Openchrome driver. Before playback was
smooth but after the switch playback is stuttering and not watchable
anymore :-( .

I tracked down the problem to /libs/libmythtv/videoout_xv.cpp. In this
file the refresh rate of the monitor is determined from the xorg
modeline. Openchrome returns the correct values when not using VBE modes
but returns zero's when using VBE modes (due to DVI usage). In the
calculation this results in a division by zero and the refresh rate is
set at NaN which results in stuttering playback.
I've patched the file to check for a zero result and use my monitors
real refresh rate (60.09Hz) in this case. Normally  myth should switch
to 60Hz, I did not check what a difference in real/myth refresh rate does.

Patch is attached, it also outputs the used value in the playback log to
see what's happening. I'm no Xorg, Openchrome or MythTV expert so I can
not say who's really at fault here. It would be nice to have a more
reliable way to get the refresh rate if this is really critical, ideas
are always welcome.

Christiaan van Dijk

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