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Video signals, resistors and capacitors

J

jamma-plusser

Jan 1, 1970
0
I've recently had a problem connecting trying to connect up a Nintendo
Wii console to my VGA monitor using a transcoder box and the Wii's
component output (which goes to the transcoder box) - I was finding
that the colours were very 'washed out' - turning down the brightness
just made the image darker, turning it up the colours lost their
vibrancy and contrast, and blacks were a bit grey.

After looking around the 'net it seemed there were two possible
solutions - add a 150 ohm resistor (or thereabouts) into each of the
Red, Green and Blue signals in the VGA cable, or use a 220uf
electrolytic capacitor. The resistors just made everything even
darker, however the electrolytics fixed the problem.

What I'm curious about is why this is the case - I can understand why
the resistors made the image darker, but why did the electrolytics fix
the problem?


Thanks
 
T

Tim Wescott

Jan 1, 1970
0
jamma-plusser said:
I've recently had a problem connecting trying to connect up a Nintendo
Wii console to my VGA monitor using a transcoder box and the Wii's
component output (which goes to the transcoder box) - I was finding
that the colours were very 'washed out' - turning down the brightness
just made the image darker, turning it up the colours lost their
vibrancy and contrast, and blacks were a bit grey.

After looking around the 'net it seemed there were two possible
solutions - add a 150 ohm resistor (or thereabouts) into each of the
Red, Green and Blue signals in the VGA cable, or use a 220uf
electrolytic capacitor. The resistors just made everything even
darker, however the electrolytics fixed the problem.

What I'm curious about is why this is the case - I can understand why
the resistors made the image darker, but why did the electrolytics fix
the problem?


Thanks
Technically, component video is supposed to have the black level at 0V,
with sync tips going negative. Some source devices cheat on this, and
have the sync tips going to zero, with black being above ground.

Some monitors can handle this with no problems, and others don't -- it
sounds like the Wii cheats a bit, and the transcoder box can't handle
it. Capacitively coupling the video would fix this somewhat or
entirely, depending on what other things the Wii and the transcoder did
with the signal.

Now I want to haul my O-scope up to the living room where my kid keeps
his Wii...

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
J

jamma-plusser

Jan 1, 1970
0
Technically, component video is supposed to have the black level at 0V,
with sync tips going negative. Some source devices cheat on this, and
have the sync tips going to zero, with black being above ground.

Some monitors can handle this with no problems, and others don't -- it
sounds like the Wii cheats a bit, and the transcoder box can't handle
it. Capacitively coupling the video would fix this somewhat or
entirely, depending on what other things the Wii and the transcoder did
with the signal.

Thanks very much. Just to show my further ignorance, what is happening
to the signal when a cap is applied as opposed to a resistor?
Now I want to haul my O-scope up to the living room where my kid keeps
his Wii...

Sounds like a great idea, let us know how it goes. :)


Cheers!
 
N

Nobody

Jan 1, 1970
0
Thanks very much. Just to show my further ignorance, what is happening
to the signal when a cap is applied as opposed to a resistor?

If you put a capacitor in series, so that the signal has to pass through
the capacitor, it will remove the DC component from the signal. This will
cause the black voltage to be lower.
 
J

jamma-plusser

Jan 1, 1970
0
If you put a capacitor in series, so that the signal has to pass through
the capacitor, it will remove the DC component from the signal. This will
cause the black voltage to be lower.

I see, thanks very much. :)
 
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