[mythtv-users] [ G400 ] Hotwo enable flicker filters

[Cory Papenfuss]

On Fri, 18 Mar 2005, Peter van Gils wrote:

> Hi Bernd,
>
> Just a tip: you can save some money next time you need such
> a cable. I use Cat5 UTP for a 13m VGA cable and it works
> great! (Running 1024x600 @ 60Hz on a 32" LCD.)
> You have to keep signal and ground together in one
> twisted pair and add 22 ohm resistors in series with
> the signal wires at the TV end. (The resistors are
> needed to prevent ghost images.) The shell is left unconnected.
>
> My $0.02.
>
 	What you are describing can be done, has been done, and there 
exist commercial products for doing so:
http://www.svideo.com/coaxbalun.html
... and many others.

 	There are some issues there, though.  The one you propose fixing 
is the impedance one... video signals typically are run on 75 Ohm cable, 
the and nominal impedance of Cat5 is 100 Ohms.  It's also \pm 15 Ohms, I 
believe, so that's not so good.

 	There's also the balanced/unbalanced issue.  Coax is unbalanced 
but shielded.  Cat5 is balanced (differential), but unshielded.  If you 
put the unbalanced signal on it directly, you don't get the benefit of 
differential signalling and it radiates a lot of power.  This guy says it 
better:
http://hardware.mcse.ms/message159780-2.html

 	There's also the attenuation.  The high frequencies of baseband 
video (6MHz) will be attenuated 5dB or so over a 100 ft runk, but the low 
frequencies not as much.  It will soften the image, and possibly even 
cause other high frequency (read: chroma) artifacts.
http://www.chem.ox.ac.uk/it/staff/Network%20Documentation/standards.html

 	That said, what you propose will probably work OK for short runs, 
but coax is definately better.

-Cory
*************************************************************************
* Cory Papenfuss                                                        *
* Electrical Engineering candidate Ph.D. graduate student               *
* Virginia Polytechnic Institute and State University                   *
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