[mythtv-users] HTPC OT: Is this tearing, bad sync, bad modeline or bad hardware?

[David]

Hi

A little OT... but maybe some of the HTPC / video experts might know 
(Cory, Jarod?)

I'm trying to make Myth work on my projector and I'm seeing 'glitches' 
that I'm trying to get rid of.

I once 'captured' one using the 'freeze' function on the projector but 
they're not screengrab-able.
They appear to be 10 or so pixels high and the top line is offset to the 
right by 20 or so pixels, the next is offset less, then less again 
forming a tiny curve along the right edge of the picture. Dynamically 
they just appear as a messy horizontal line flashing on the screen. I'll 
try and illustrate:

goodimagehere
andhereisfine
butthenitstartstoget
abitoddandtheline
sappearanditloo
kshorridalthou
ghitgetsbackt
onormalandall
iswellagain!!

They appear in different places on the image and I get them every second 
or so - sometimes less, sometimes more. They get more frequent after a 
period of time (30 mins) watching.

I see them mostly when playing video but they also appear *very* 
occasionally when the static Myth menu is displayed. If I hold down the 
down arrow in a menu it cycles very quickly between the options and I 
get a lot of these glitches.
This suggests it's not XV related, more to do with changes on the X display.

So, what are they and why do I get them?

I don't get them at all when using a 'standard' mode (640x480, 800x600 
etc) but the image quality is much worse.

 From the manual I have this spec for the projector:
856x480, 30.057kHz hfreq, 60.114Hz vfreq, 31.5MHz dot clock

and this modeline:
Modeline "856x480"   31.50   856 912 920 1048   480 484 485 500  +hsync 
+vsync

which according to the calculator here 
http://www.hut.fi/Misc/Electronics/faq/vga2rgb/calc.html

gives: Horizontal sync frequency is 30.06 kHz and vertical sync 
frequency is 60.11 Hz. Field rate is  60.11 Hz. That matches pretty well 
so I'm thinking the modeline is good.
I've tried many many variations and they don't really affect the 
glitches much at all.

The card is an NVidia GeForce2 MX/MX 400

I'm using the latest nvidia drivers but I've tried the 5xxx and 4xxx 
ones too. I've looked at pci latency, I've changed the AGP settings in 
the BIOS, I've tried loads of modelines. I've tried +ve/-ve sync (+ve is 
better). I've disabled my PVR350's output (and capturing) in case there 
were DMA/irq issues. I've used the NV AGP aswell as AGPPART.

I can't try a new projector so right now I'm wondering (hoping) if the 
card (an old Gladiac) can't cope with the custom modeline. I'd like 
opinions before buying another card. I was thinking of a FX5200  as it's 
the cheapest NV I can see - but I don't know whether a 4000MX may be 
better at handling custom modelines...

I'd appreciate any help :)

David