[mythtv] Trying to pinpoint the source of judder... help requested

[Viktor Avramov]

Hi there...

I have been struggling with a playback judder problem ever since I set up my 
mythbackend/remote frontend setup 6 months ago and have gone through all the 
usual suggestions on the wiki and the archives without any luck....

I watch DVB recorded material that it 25fps and interlaced (576i)... my tv is 
an  interlaced CRT that is fed a straight VGA signal to it's SCART (I use the 
VGA2SCART circuitry mentioned around the traps)... I have a custom modeline 
that puts out precisely 50Hz interlaced signal to the TV...

The issue I see is playback is sometimes nice and smooth and will then begin 
to judder and may or may not flick back into smooth playback by itself... 
this can cycle back and forth every few seconds like this forever....  what i 
have also noticed is that while it is juddering, I can sometimes stop it by 
pausing and unpausing the playback...

The other interesting thing is that it seems to mainly be a problem during 
playback of recordings and not live tv...

I have OpenGL vsync turned on and have confirmed in the logs that it is 
working... switching it off doesn't help... fiddling with nvidia-settings 
sync options doesn't seem to make any difference... running from the onboard 
geforce 6150 or from a PCIe 7300 makes no difference...  switching between 
ffmpeg, libmpeg2 and xvmc makes no difference... copying the recording to a 
local disk and playing back from there makes no difference...

I'm not sure how interlaced material is structured but to me it seems like 
myth is maybe getting the odd and even frames out of order sometimes... I've 
wondered whether during the recording process, glitches in reception caused a 
loss of a frame or frames which throws the odd-even-odd-even sequence out of 
whack... this wouldn't explain the better playback via live tv though....

Can anyone shed some light on some of this??

How tolerant is myth of stream errors in general and in interlaced materials 
in particular?
Does anyone else have myth playback running 100% on an interlaced display?
Is there a way of getting even more info than that which is provided by "-v 
playback"?  some sort of debug patch or something???

any other tips for debuging this??

Viktor

PS: Frontend: A64 3700+, Asus VM-CSM with built in GF6150, 2x512Mb DDR 400, 
SPDIF out via Chaintech AV710, hard disk DMA on, no irq conflicts, Gentoo 
2006.1 64 bit