I would like to generate CVBS with the AD724 or AD725, with the colour subcarrier locked to an existing video signal, for an OSD application. Is this possible?
I'm thinking to use an MC44144 to lock to the colourburst, then buffer the 4FSC output using a comparator to convert the AC sinewave to a TTL square wave. Will this work, or alternatively can anyone suggest a better method?
U5 will be a fast switch used for pixel switching between the source video signal and the output of the AD724. The microcontroller will most likely be one of the STM32F4 family, converting 8-bit pixel values to RGB using a resistor ladder. The AD724 is working at the moment using a crystal and driven by an STM32F407. Interlaced video is not a problem.
The main issue I'm concerned about is whether the colour subcarrier of the AD724 and the source video will be in sync if I generate the clock as shown in the schematic, or at least with a constant phase relationship that can be brought into sync. Otherwise the colours in my overlay will be off. It would be great if you are able to confirm that.