Made a design for an OSD board using a genlocked AD724. When I attempted this previously, one issue seemed to be mismatched DC levels for the input video and the OSD video. To get them matched, both signals will need to be clamped. Normally video signals that are AC coupled are clamped to 0V for black level, which means sync tips are -0.3V. So I need a circuit to do that.
Design Note DN327 from Linear Technology has a clamp circuit using a diode and a capacitor-resistor network. Not sure what DC level it actually clamps to, it is probably optimized for the amplifier in the circuit which would have sync tips at >= 0V.
Wondering if this actually matters? Assuming the next stage down from the OSD (the monitor, or the vtx) is going to be AC-coupled, the DC level is going to be stripped out anyway. As long as the black level of both signals is equal it is probably OK.
If I do clamp to a DC offset with no negative excursion, maybe I won’t need a negative supply for the amplifier and pixel switch either.
My plan for the time being is to experiment with clamp circuits before continuing with the OSD board design.
Weka OSD 