Automatic voltage scaling with op-amp?

I recently got a grab bag of parts from an electronics supplier. I was
going through the chips I got and I'm wondering if any of them might
be of any use in making a circuit that can automatically scale a 15khz
video signal from 0V-n (never more than 3V) to the 0V-0.7V that a
normal RGB monitor uses.

I think I have a whole bunch of LM311 op-amps. I found a circuit on
the net that shows how to use an op-amp as a peak detector. I thought
I could use the peak detector as some sort of input for the maximum of
the input video signal, and then somehow use 0.7 as the maximum of the
output, and then an op-amp could scale it. Obviously I'll have to
build 3 such circuits, one each for red, green, and blue.

But... I have no idea how to do it. I can't find any schematics on the
net which do anything similar.

 

Subject: Automatic voltage scaling with op-amp?

The LM311 is a comparator, not an op amp. It switches between logic 1 and
logic 0 depending on the inputs.

Good luck
Chris

 

I'm afraid your LM311s are not opamps but voltage comparators. You can
see the datasheet here
http://www.fairchildsemi.com/ds/LM/LM311.pdf
As for gain scaling of video, the video level is not a good way to do
it since its hard to tell what levels should be until you get a peak
white. The sync amplitude is very reliable because its always there at
300mV (OK, 286mV to be spot on). Continuously variable AGC would be a
little harder than range detecting but certainly doable. If you don't
have sync on the video, it will get more interesting. Personally, I
just fix the problem with the video since widely varying levels
implies faulty transmission lines/terminations which will muck it up
anyway. Fixing reflections from faulty cables automatically is
possible but way beyond simple opamps.
GG

 

Not exactly sure what the difference is. I think one of these other
chips might be an op-amp, it's an LM324.

It isn't a problem with the video really. I'm hooking up arcade PCBs
to an old RGB monitor which is able to sync down to the 15khz. Some of
the boards I have output up to 3V, some output less than 0.5V. They
all work great hooked up to the monitor that is in their dedicated
cabinet. I just wanted something that could automatically adjust the
levels while it's on my test bench.

I guess what I was hoping for was a schematic that showed how to have
a input voltage, a reference to the input's max, and a reference to
the output's max, and it would do the scaling. Can that be made with
an LM324?

 

LM324 is an opamp on Valium. You need something much faster for video.
National, TI, Maxim, Analog devices and other have good amps for
reasonable prices. Check the MFRs websites and Digi-Key, Mouser ...

For your purposes I think I would do it as a multi-input switch with
the proper gain scaling for each input.
GG

 

I second this. I've worked with video games before, and there are
only a couple of attenuations you'd need.

And 15 KHz is the sync rate, but the video goes up to 6 MHZ. Lowering
the bandwidth spreads out the pixels - their edges are rise time, you
know.

It's cool watching live TV video on a scope - it's like an edge-on
view of the screen, where amplitude is intensigy - it's almost 3D,
with the proper medication, of course. ;-)

Good Luck!
Rich

 

Even if you use a video opamp it will not be able to *automatically* correct
the gain. you will need a VCA with the appropriate bandwidth and a circuit
to determine the level of the video signal. This is not a trivial task.
If you do not need amplification, a passive adjustable damping pad could be
manually adjusted, so the following stage is not overloaded.

 

They've been paying me 28 years to look at scopes of video of almost
every variety, from the lowly VHS up to HD film to video (telecine).
What is real cool is to see a frame rate background, light at the top
of the screen and dark at the bottom. Superimposed text is readable on
both the scope and the pix monitor. I've never seen it but I have it
on good authority that the guys at NBC varied the chroma and hue to
say 'Merry Christmas' on the vectorscope one year.
GG