[mythtv-users] Stupid S-Video Problem Continues

[Will Constable]

Yea-I was very aware of the 'rule breaking' I was doing. I origionally
planned on putting the computer by the TV and running network to it.  Lets
just say that wouldn't fly for aesthetic reasons-the computer is a piece of
junk with noisy drives and fans, and an ugly case-it belongs out of sight
and the room the TV is in has been decorated nicely and everything. So I
decided to try the video and audio-I really hope I can make it work-before I
terminated the cable as it is now, I put s-vid ends on the two ends of the
spool of cat 5 and ran the whole 100 ft from my main myth server to the TV
and found the picture to be fine; that's a Geforce 4 ti4200 while my new box
uses an 'el cheapo' mx420 that was lying around-it could be anything you
mentioned regarding ground loops/vga signal differences, but my first try
will be a friend's geforce fx that's lying around at work.. if that fixes
things, I'll just be happy at that.

 

-will

 

 

  _____  

From: mythtv-users-bounces at mythtv.org
[mailto:mythtv-users-bounces at mythtv.org] On Behalf Of Brian Wood
Sent: Thursday, June 08, 2006 10:57 AM
To: Discussion about mythtv
Subject: Re: [mythtv-users] Stupid S-Video Problem Continues

 

 

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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