To date I have built a proof concept that shows colour overlay is possible and within reach of the hobbyist. I believe the hardware problems are solved, now I am working on the software.
I’m calling my proof of concept a “Video Experimenter’s Board”. It consists of two video ASIC chips from Analog Devices: the ADV7184 which converts a composite video signal such as an FPV camera would output, into a digital byte stream. This chip is connected back-to-back with an ADV7341 digital-analogue converter which turns the byte stream back into composite video. On its own this back-to-back arrangement is not very useful: why would you want to convert analogue to digital, then back to analogue?
The reason is that the ADV7184 also supports SCART overlay.
Among other things, the European SCART standard allows a device such as set-top box to perform pixel switching. By switching rapidly between its own video and that of the TV set, it can build up a text or graphic overlay. Exactly what what we want for an OSD system! Using a microcontroller we can generate RGB video which the ADV7184 will combine with the composite video from an FPV camera. As long as our RGB video is synchronised with the timing signals from the ADV7184, we will have a full-colour OSD.
My Video Experimenter’s Board is not a stand-alone device. As well as a microcontroller, it also needs a dual-voltage power supply and a resistor network to convert the digital pixel video into analogue RGB. However any microcontroller should be able to interface with it, even an Arduino! Shortly I will provide schematics and design files for the VEB and its supporting modules.
By using a modular approach, I will be able to experiment with different components without committing to a final design. Once I do have a final design I will produce another board with all components, small and light enough to mount on an RC aircraft.
Weka OSD 
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