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Automatic voltage scaling with op-amp?

Discussion in 'Electronic Basics' started by Chris Osborn, Sep 19, 2004.

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  1. Chris Osborn

    Chris Osborn Guest

    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.
     
  2. CFoley1064

    CFoley1064 Guest

    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
     
  3. 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
     
  4. Chris Osborn

    Chris Osborn Guest

    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?
     
  5. 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
     
  6. Rich Grise

    Rich Grise Guest

    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
     
  7. Ban

    Ban Guest

    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.
     
  8. 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
     
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