Correcting video offset and amplitude

Discussion in 'General Electronics Chat' started by RobeeJ, May 10, 2013.

  1. RobeeJ

    Thread Starter New Member

    Mar 19, 2013
    24
    1
    Hi all!

    Okay, short short story, I'm trying to work out the best way to correct a composite video signal, but I'm sort of snowed under by options and unsure of which direction I should be taking or if indeed any of them would actually work.

    The signal is AC (about 1.1v pp) with a DC offset of around +1.8v. I need to set the DC offset to 0, with a 0.7v pp.

    One way I thought of doing this was AC coupling, then an op-amp to correct the output level. Is that the route I should be investigating, or should I be looking at something else?

    I'm not really looking for a "here's the circuit you need" kind of answer, more "do X and Y" as I want to understand it all, not copy it. :)

    Any replies gratefully received.
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,669
    804
    AC coupling will be needed anyway. But since you have such nice numbers and want the amplitude to go down, you can use a capacitive divider made of two same caps. This will give you AC coupling and half the voltage in one go.
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,220
    By "set the DC offset to 0" do you mean clamp the negative peak to zero, as a DC restore circuit would do, or do you mean the average DC value should be zero?
     
  4. RobeeJ

    Thread Starter New Member

    Mar 19, 2013
    24
    1
    Yes, I mean clamp the negative peak to zero, sorry should have made that a bit clearer.

    Composite video signals should run from 0v - 0.7v, well that's the best range I can find anyway!
     
  5. chuckey

    Well-Known Member

    Jun 4, 2007
    75
    10
    I thought that composite video went from -.3 -> 0 for the sync pulses then 0 -> +.7 for luminance. just AC couple it then DC restore it with a diode, should put the sync tips at -.6V (Si diode) or - .3v for a Ge diode.
    Frank
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You can hardly find Ge (germanium) diodes anymore. Instead, use a Schottky diode; a 1N5817 should work just fine for the sync tip. Use a 1N914/1N4148 for the luminance.
     
  7. RobeeJ

    Thread Starter New Member

    Mar 19, 2013
    24
    1
    According to the Video Demystified book, unless I'm mis-reading it, the blank level is 0.321V, the sync level is 0.02V, and the white level is 1.02V. So I guess that's what I need to make it, well for PAL anyway, it's slightly more complicated for NTSC as the black and blank levels differ, but I'm not sure that makes any real difference for these purposes.

    I guess also this is a more generic question, of how do you take an AC signal with a DC offset that runs between A and B volts pp, and make it run between X and Y volts pp.

    (thanks for the replies so far!)
     
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