Well, sort of.
It’s a colour test pattern overlay, but there are a bunch of issues:
- The colours are wrong. I had to change the order of the red, green and blue inputs to get even this result, because the phase delay from the MC44144 is 60 degrees. The colours are also poorly saturated, and look worse in real life than in the picture.
- The image flickers, due to noise. This is most likely due to the use of a breadboard.
- The source video appears washed out. I believe this may be partly the result of clamping which seems to compress the waveform by 200mV.
On the positive side, I did solve some problems:
- I’m using an SN74LVC2G53 analog switch. This one is unbuffered. Since both the AD724 and the camera are both outputting at full-drive 2V p/p, there should be no need for buffering. The switch is very fast (<10ns) and seems to be performing as advertised.
- Originally the 2 video signals were mismatched by around 200mV. This was enough to cause an unstable picture, as the overlay signal dropped below sync level. Worse, it even appeared to contaminate the source video in the LM1881, causing it to lose sync.
In order to continue with this approach, I will need to solve the subcarrier phase problem, and also find out why the colours look so terrible.
Another problem that has been bugging me is that the MC44144 often starts up without generating a subcarrier. It seems worse when using the USB power supply which is very noisy, although the filter on the board reduces it to < 5mV p/p. Both the chip and the crystal came from AliExpress, could it be a quality issue?