An OSD system - Clamping a video signal

Clamping a video signal

Hello,

I'm currently doing an OSD system. A microcontroller handles all the vertical/horizontal sync, and then outputs a high/low voltage regarding the bitmap to display and the timings.

Everything is working correctly with a DVD pal source, which is a full positive signal, but I have problems making it work with a small camera => the PAL video signal has a negative sync, that I can't directly handle with the pic - I'm triggering horizontal sync with a threshold -.

I have attached to this post a view of the signal directly connected to the scope.

How could I add a voltage offset (or clamp the sync - low levels) to 0V in order to avoid any negative voltages. An keep the right impedance matching (75ohms) of the whole system too... ?

Thanks,
Fabien

Re: Clamping a video signal

You are looking for a DC restoration circuit. The simplest one uses a diode. But your signal level has to be greater than what you have. So my suggestion is amplify the signal and then use a diode or opamp-based ideal-diode to do the DC restoration. Take a look at this article, too. **broken link removed**

The simple diode to GND will do the trick, if you have enough amplitude, like I said. After the DC restorer I would use a comparator to detect just the sync pulses and then use the output of the comparator for PIC input.

Re: Clamping a video signal

Right ! I tried the diode trick.
If I connect directly the camera to the scope, and add a diode, then the signal has a +600mV offset. Great !

But when I connect the camera (+diode) to the TV, then the signal goes back to its initial level... Can anyone explain me what happens then ?

Thanks,
Fabien

Re: Clamping a video signal

probably because the TV has 75ohm impedance, which makes the diode ineffective. So after the diode you should use a buffer.