[mythtv-users] Stupid S-Video Problem Continues

[Brian Wood]

On Jun 8, 2006, at 7:56 AM, Will Constable wrote:

>
>
> Yea- my current setup is s-video via homemade cable—40 ft of cat 5  
> with s-video and audio connectors at each end, running through the  
> walls- I know, sounds like a recipe for disaster.  But my laptop  
> drives a perfect signal from one end to the other while I have this  
> problem with the myth box.  So I am thinking its either the card or  
> the settings/driver..  I have been using nvidia instead of nv, but  
> have not tried a whole lot of options for resolution on my vga  
> port.  I was thinking it would be good to try identical settings  
> and do a clone, but I don’t know how to write xorg.conf for that— 
> mine does dual screens not using xinerama.  It does occur to me  
> though, that the setup I have upstairs is 1024x768 on s-video and  
> 1600x1200 on vga—while the monitor downstairs cant run 1600, maybe  
> this setting would make the interference go away.  Do you know how  
> to set up xorg for clone?  Also, can I set the horizsync and  
> vertrefresh of my vga monitor to match what my tv uses, or might  
> this cause a problem?
>
>
>
>

Some things to try:

Since you get a good signal with your laptop, try moving your TV set  
next to your Myth box and connecting it up via a short S-Video cable,  
if you still see the problem this will confirm that the problem is  
not your home-made cable.

It is possible, almost probable, that the problem *is* your cable,  
and something is different between your laptop and your Myth box, for  
example your laptop probably has only one video signal, the S-Video,  
and not a differing VGA floating around.

As you seem to realize, you have broken just about every rule about  
video cabling :-) But this doesn't necessarily mean you won't be able  
to get it to work.

It could also be something like your laptop having a different  
grounding scheme, in fact it may well be more resistant to common- 
mode interference because the power supply is not referenced to your  
house ground, or the exact VGA rates of your laptop are just enough  
different as to render the problem invisible.

Changing the resolution of the VGA signal may well help your problem,  
the interference will still be there but the rate difference will be  
different and thus might well cause it to move through the picture  
fast enough to not be noticeable. Changing the rates of your VGA  
signal will have the same effect, though setting them to be the same  
as your TV may well cause an interfering bar to simply be static,  
unless you can "genlock" them to a fixed phase relationship,  
something beyond most PC systems.

The README included with the nVidia driver explains how to set up  
clone mode, but having the same image on both screens is not the same  
as having the same video parameters.

In short, your cabling arrangement is in all probability the source  
of your problems. Rules and standards exist for a reason. The type of  
interference you are having is very sensitive to the precise  
frequencies involved, if they are close enough to each other (or  
their harmonics or beat products are) then you will see visible  
artifacts, if they are far enough away from each other the products  
will move through the picture fast enough to not be a bother, but  
they are still there, and could be measured with the right gear.  
Differences on the order of fractions of a percent of the sync rates  
can make the difference, and these are well within the tolerances of  
modern video cards. Simply trying another "identical" card might make  
a difference.

You could always use that cat-5 for what it was meant for - ethernet,  
and just put a MediaMVP at your set for about the same cost as a new  
video card :-)

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