Trigger digital interrupt on negative voltage

Question

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.

Answer 1

I am no video expert but I do understand the negative sync pulse and, as far as I can see, your understanding is correct too.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. A comparitor can be made to work on signals outside its rail-to-rail limits by use of potential dividers.

Comparators are better than op-amps for this type of job as they are designed to switch rail-to-rail whereas op-amps can take some time to recover when the output hits either rail.

How it works:

• R2 and R4 set a reference voltage of 0.5 V.
• With R6 at minimum resistance R1 and R3 would do the same and the comparitor would switch when the video signal crosses 0 V.
• The resistor values chosen drop \$ \frac {5}{100k} = 50 \mathrm {mV/k} \$, To change the switching point to -0.2 V, therefore, we need to add about 4k to the negative leg. A 4k7 pot should give you enough adjustment to get reliable detection.
• Since you want an output high when A goes low then A must connect to the '-' input.
• Many comparators have open-collector outputs (check the one you've chosen) so R5 may be required if your micro's pull-up is not sufficient.
• Check the specs of your chosen comparitor and make sure it can work with input signals as low as 0.5 V.
• C1 is required to decouple the comparator from the supply.

The circuit will put a 5 V bias onto your video signal but with > 100k source impedance which the video driver should be able to handle. If this proves to be a problem you could scale up R1, 6 and 3 by a factor of ten. You could find out easily by connecting your video to +5 V through a 100k resistor.

Finally, 90k is an odd resistance value but I chose it just to make the maths easier. 100k will be fine and R2 / R4 will give \$ \frac {5}{11} \ \mathrm V \$ rather than \$ \frac {5}{10} \ \mathrm V \$.

Answer 2

Simplest is to use a Schmitt Trigger. You can find the details at Wikipedia using that name. You also find schematics there for transistors and Op-Amps. (I would use an Op-Amp). You also find the formulas there. There was a remark about negative voltage but you don't need that. If you take a resistor divider from e.g. a 3.3V supply and connect it to the sync, the voltage over the resistor will vary between 3.3 and about :-) 3.6V. For example, if you use two resistors in the ratio 3:7 your voltage swing will be ~200mV. Adjust your Schmitt trigger resistors to detect the difference between those voltages.