[mythtv-users] Video playback glitches only when recording

Will Dormann wdormann at yahoo.com
Mon Dec 11 03:58:53 UTC 2006

Hi Folks,

I've got an Asus Pundit (non "-R") with a Celeron 2.0GHz, Hauppauge
PVR-250, and a PCI Geforce MX4000 video card.

Using Bob deinterlace XV and OpenGL Vsync in Mythtv, I've found that the
video playback is flawless.   Scrolling text is quite smooth.  No
dropped frames or anything.

That is...  only while nothing else is recording.   If I am recording
something else, I get something that looks like a dropped field every
3-10 seconds.  The symptoms are a bit subtle, but they're there.   It's
easiest to see when there is a thin horizontal line on the screen, and
it'll appear to jump.

I've tried playing around with PCI Latency values, but that didn't seem
to have much of an effect.  It seems that only the IVTV activity is what
causes the glitches.  I'm using IVTV driver 0.4.8.  Heavy disk or
network activity has is no problem.  I also tried using XVMC playback
instead of just XV, and it seemed to have the same sort of glitches when
recording.  The CPU usage of X dropped dramatically, though.

The question now is *why* is the IVTV activity interfering with the
smooth video playback?   Possible theories:

1) CPU too weak.  I know the P4-based Celeron chip is a dog, but it's
not being overtaxed as far as I can tell.  Watching a 720x480 recording
uses approx 40% CPU.  (25% mythfrontend, 15% X).   While recording,
ivtv-enc uses 2.3% CPU time.   So I think this one is out.

2) Limitation of having a PCI graphics card.   The Geforce card works
nicely for TV output, but the Pundit only has PCI and no AGP.  The
PVR-250 and Geforce don't share an IRQ, but they are sharing the same
PCI bus.

3) Limitation of PCI bus implementation of the Pundit.   Perhaps the SiS
chipset of the Pundit just isn't up to the heavy load of a combined
frontend/backend mythtv machine?

Anybody have any ideas on where the problem may lie?   I'm considering
the possibility of getting a new system that would be HD capable and
hopefully a tad quieter, and I'd hate for this problem to carry over.


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