After testing a half dozen recievers, I noticed that none of them followed the 1960 video specification. They all have a different bias voltage, one even goes negative with respect to ground. One receiver was close enough to work, the others I was able rebias through a potentiometer.
The fix is to measure the offset with an ADC, then rebias the signal using a voltage from a DAC. Unfortunately the G071 chip I am using only has 2 DAC outputs, one is needed set the comparator voltage, and the other is needed as a voltage limiter.
I could include a reciever module or port the OSD to a different chip, but I only allocated 1 month for this project. I am not sure I want to extend this project, like I did for the OSD project. I ended up designing a flight controller to support the OSD.
Anyways I am pausing this project for now. The alpha boards work, but I need to add a comparator to regain video sync after it is lost. As it is now, if video sync is lost for over a few seconds, it will start outputing with a horizontal offset. It looks like the vertical hold of an old school TV, except hoizontal.
The fix is to measure the offset with an ADC, then rebias the signal using a voltage from a DAC. Unfortunately the G071 chip I am using only has 2 DAC outputs, one is needed set the comparator voltage, and the other is needed as a voltage limiter.
I could include a reciever module or port the OSD to a different chip, but I only allocated 1 month for this project. I am not sure I want to extend this project, like I did for the OSD project. I ended up designing a flight controller to support the OSD.
Anyways I am pausing this project for now. The alpha boards work, but I need to add a comparator to regain video sync after it is lost. As it is now, if video sync is lost for over a few seconds, it will start outputing with a horizontal offset. It looks like the vertical hold of an old school TV, except hoizontal.
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