For a little project of mine (FPV OSD for Quadcopter Camera) I need to detect every time the signal voltage goes negative. The Signal I get is NTSC. Here each line sync is marked by the voltage going to about -0.3V. The OSD itself will be realized using an ATTiny, which can't read negative voltages. Also, the ADC on the ATTiny is by far too slow to detect that short line sync time. For this to work I will need to use a digital interrupt.
So I nees some kind of circuit, that detects every time the Voltage dips below 0V and turns that into a HIGH (>0.7V), while it stays LOW if the voltage is above 0V. This should work fast enough for NTSC (so should be able to work at at least a few megahertz).
I am basing this on an existing open source circuit that currently uses an LMH1980 (for line sync detection) together with an ATTiny13A (to render the actual OSD), which (the LMH1980) is comparatively expensive and bulky. I think it should be possible to replace the LMH1980 wih a cheaper and simpler circuit to detect negative voltages (=line syncs) and have the existing ATTiny do the then trivial work of detecting line and frame syncs.
I would prefer the solution to use only standard components and be as small and simple as possible.
Edit: One design constraint that I forgot to mention is that the solution should impact the video signal as little as possible to not introduce image artifacts, since the video singnal will also be sent over a video transmitter and viewed by the pilot.