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).