I have a composite PAL/NTSC video signal. One problem I am having is getting a reliable sync signal from it; picture brightness causes the sync pulse (normally 0.3V) to vary; so much that my scope won't trigger on it reliably (with the video trigger.) I've heard I need a DC restoration or DC clamp circuit, but I can't find any information on this, except occasional references in books and on the internet, but no detailed information. Does anyone know what circuit I need?

I have previously used an LM1881 or LMH1980 to extract sync because they build the DC clamp/restoree parts in. However, the problem with these chips is that they are expensive and often bigger than a discrete solution.

  • \$\begingroup\$ Are you sure it's not your scope? Have you checked for bad connections, etc...? \$\endgroup\$ – Connor Wolf Mar 12 '11 at 13:32
  • \$\begingroup\$ @Fake Name, certain, because an output from a comparator at 300mV also has the same problem. \$\endgroup\$ – Thomas O Mar 12 '11 at 15:58
  • \$\begingroup\$ IC bigger then the discrete components required to do the same stuff? I find that hard to believe. Must be a bunch of features you are stripping out. \$\endgroup\$ – Kellenjb Mar 12 '11 at 18:17
  • \$\begingroup\$ @Kellenjb, AFAIK, a SOT363 diode+0603 components would be smaller than a SO8 or MSOP10. Besides; it's not just the IC's surface area you have to compare against. The LMH1980 requires three caps (ac coupler, plus two filter caps and a resistor - all of these take space up on top of the small surface area. \$\endgroup\$ – Thomas O Mar 12 '11 at 20:10
  • \$\begingroup\$ @Thomas O, I don't know anything about those chips, my point is just that it seems odd for them to put a diode + some 0603 components on a chip, too basic of thing, they must have more they are doing. If they require 2 filters caps and a resistor then when don't you? \$\endgroup\$ – Kellenjb Mar 12 '11 at 23:24

Many video devices are AC coupled on the inputs and outputs; it might be nice if that weren't the case, but in practice unless one knows one will only be accepting signals from devices with DC-coupled inputs, one has to accept AC-coupled signals. Further, in many cases, if one will have to deal with AC-coupled signals, it's often helpful to AC-couple one's own input stage.

If one has positive and negative supply rails available, the easiest approach is probably to have the input (past the coupling capacitor) pulled weakly to the negative supply and have a clamp diode to a point about 0.35 volts above ground. The effect will be a signal where the lowest parts of the signal are at -0.35 volts. This will make it easy to detect sync signals, and will provide a usable (though not wonderful) black reference for superimposing opaque video. If negative supply isn't available, one could use a pulldown to VSS and clamp to 3 volts or so and expect all levels to be shifted. The sync detection from this approach is fine; the biggest weaknesses are:

  • If the pulldown sinks much current, the end of each line will be made a little darker than it should be; if it doesn't, the circuit may be slow to deal with shifts in the input DC bias;
  • One has to 'guess' the difference between the input signal's sync level and black level. Ideally the sync voltage will be 0.34 volts below the black level, but not all video output devices are perfectly "to spec".

If one wants broadcast-quality video, one should decode the sync signals in a way that won't disrupt the original video (e.g. by passing it through a buffer and then AC coupling it to the described clamp circuit), and then ground an AC coupled output briefly after each rising edge of the sync signal (about 500ns if one doesn't have a 3.579MHz notch-reject filter, or 4.5us if one does).

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