Hello, I want to build a circuit with trimmers to reduce the color
saturation of a signal going to a monitor. The signal is made up of
these wires: R, G, B, Sync, GND.

Do I need some IC? Are there any schematics to look at?

Thank you

--
TSM

"TSM" wrote in message
...
Hello, I want to build a circuit with trimmers to reduce the color
saturation of a signal going to a monitor. The signal is made up of
these wires: R, G, B, Sync, GND.

Do I need some IC? Are there any schematics to look at?

Thank you

--
TSM

You can't really alter the 'saturation' as such, of an RGB signal, once it's
in the outside world, in that format. Reducing saturation implies that at
some point, you could get back to a picture that had no colour in it at all
ie a black and white picture, and that is not possible. All you could do
would be to "pot down" each individual colour component, but the overall
effect of this would be exactly the same as turning down the monitor
contrast control. What exactly is it that you are wanting to do ? What is
the source of your RGB signal ? If it's a computer, most picture
manipulation software, allows for saturation and hue to be adjusted.

Arfa

"Arfa Daily" wrote:

You can't really alter the 'saturation' as such, of an RGB signal, once it's
in the outside world, in that format. Reducing saturation implies that at
some point, you could get back to a picture that had no colour in it at all
ie a black and white picture, and that is not possible. All you could do
would be to "pot down" each individual colour component, but the overall
effect of this would be exactly the same as turning down the monitor
contrast control. What exactly is it that you are wanting to do ? What is
the source of your RGB signal ? If it's a computer, most picture
manipulation software, allows for saturation and hue to be adjusted.

They are 2 arcade boards I'm powering with an AT PSU. They are
connected with a Commodore monitor through a SCART cable. The signals
directly come out from the board and the owner was supposed to adjust
the picture using the monitor's trimmers. One of the board gives a bad
picture. Since it is very difficult for me to explain, you can look at
these pictures:
http://spazioinwind.libero.it/vic20/service/zid1.bmp
http://spazioinwind.libero.it/vic20/service/zid2.bmp

In the second one, no matter what you do with the contrast and
brightness knobs, you will never be able to read the player's name on
the shirt.

I don't want to touch the monitor's internal trimmers, since they are
ok for most things, so I would like to build some device to alter the
signal before it arrives at the monitor.

Thanks

--
TSM

"TSM" wrote in message
...
"Arfa Daily" wrote:

You can't really alter the 'saturation' as such, of an RGB signal, once
it's
in the outside world, in that format. Reducing saturation implies that at
some point, you could get back to a picture that had no colour in it at
all
ie a black and white picture, and that is not possible. All you could do
would be to "pot down" each individual colour component, but the overall
effect of this would be exactly the same as turning down the monitor
contrast control. What exactly is it that you are wanting to do ? What is
the source of your RGB signal ? If it's a computer, most picture
manipulation software, allows for saturation and hue to be adjusted.

They are 2 arcade boards I'm powering with an AT PSU. They are
connected with a Commodore monitor through a SCART cable. The signals
directly come out from the board and the owner was supposed to adjust
the picture using the monitor's trimmers. One of the board gives a bad
picture. Since it is very difficult for me to explain, you can look at
these pictures:
http://spazioinwind.libero.it/vic20/service/zid1.bmp
http://spazioinwind.libero.it/vic20/service/zid2.bmp

In the second one, no matter what you do with the contrast and
brightness knobs, you will never be able to read the player's name on
the shirt.

I don't want to touch the monitor's internal trimmers, since they are
ok for most things, so I would like to build some device to alter the
signal before it arrives at the monitor.

Thanks

--
TSM

OK that looks just like the highlights are burning out due to excess level,
and would almost certainly be fixed by turning down the monitor's contrast
control. However, if you don't want to do this, the same effect can be
achieved externally, by three pots in the R, G, and B signal channels. You
are never going to preserve exactly the correct impedances by doing this,
but I doubt that it will be an issue. Get hold of three pots. I would
suggest somewhere in the region of 500 - 1000 ohms, as we don't know the
source impedance for sure, and you don't want to load this up too hard.
Looking from the top of these trimmers, or from the front if you use full
sized pots, connect all three track anticlockwise ends together, and then to
the ground lead from the source. This ground also carries on to the monitor,
as before. Now, break into the R, G and B wires, and connect each new cut
end coming from the source, to a separate pot clockwise end, and reconnect
the remaining cut ends that carry on to the monitor, to their corresponding
pot wipers.

Set all three pots to the middle of their range, and supply your test
picture. Adjust each pot in turn, until you have the desired picture
contrast level, with the best preserved neutral highlights - ie whites that
aren't tinted any colour, and likewise lowlights - mid to dark greys. It may
be a bit of a compromise depending on how linear the controller outputs are,
how linear the monitor amps are, and what condition the CRT is in, but
probably nothing you will notice in the end. Note that the SYNC lead does
not figure in the equation at all - it should just carry on straight to the
monitor, untouched, as it always did. Good luck !

Arfa

Arfa Daily wrote:

"TSM" wrote in message
...
"Arfa Daily" wrote:

You can't really alter the 'saturation' as such, of an RGB signal, once
it's
in the outside world, in that format. Reducing saturation implies that at
some point, you could get back to a picture that had no colour in it at
all
ie a black and white picture, and that is not possible. All you could do
would be to "pot down" each individual colour component, but the overall
effect of this would be exactly the same as turning down the monitor
contrast control. What exactly is it that you are wanting to do ? What is
the source of your RGB signal ? If it's a computer, most picture
manipulation software, allows for saturation and hue to be adjusted.

They are 2 arcade boards I'm powering with an AT PSU. They are
connected with a Commodore monitor through a SCART cable. The signals
directly come out from the board and the owner was supposed to adjust
the picture using the monitor's trimmers. One of the board gives a bad
picture. Since it is very difficult for me to explain, you can look at
these pictures:
http://spazioinwind.libero.it/vic20/service/zid1.bmp
http://spazioinwind.libero.it/vic20/service/zid2.bmp

In the second one, no matter what you do with the contrast and
brightness knobs, you will never be able to read the player's name on
the shirt.

I don't want to touch the monitor's internal trimmers, since they are
ok for most things, so I would like to build some device to alter the
signal before it arrives at the monitor.

Thanks

--
TSM

OK that looks just like the highlights are burning out due to excess level,
and would almost certainly be fixed by turning down the monitor's contrast
control. However, if you don't want to do this, the same effect can be
achieved externally, by three pots in the R, G, and B signal channels. You
are never going to preserve exactly the correct impedances by doing this,
but I doubt that it will be an issue. Get hold of three pots. I would
suggest somewhere in the region of 500 - 1000 ohms, as we don't know the
source impedance for sure, and you don't want to load this up too hard.
Looking from the top of these trimmers, or from the front if you use full
sized pots, connect all three track anticlockwise ends together, and then to
the ground lead from the source. This ground also carries on to the monitor,
as before. Now, break into the R, G and B wires, and connect each new cut
end coming from the source, to a separate pot clockwise end, and reconnect
the remaining cut ends that carry on to the monitor, to their corresponding
pot wipers.

Set all three pots to the middle of their range, and supply your test
picture. Adjust each pot in turn, until you have the desired picture
contrast level, with the best preserved neutral highlights - ie whites that
aren't tinted any colour, and likewise lowlights - mid to dark greys. It may
be a bit of a compromise depending on how linear the controller outputs are,
how linear the monitor amps are, and what condition the CRT is in, but
probably nothing you will notice in the end. Note that the SYNC lead does
not figure in the equation at all - it should just carry on straight to the
monitor, untouched, as it always did. Good luck !

Arfa


That Commodore 1084 monitor is dual mode RGB, TTL and analog. The
TTL mode attenuates the RGB inputs by about 4:1.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida