Evan’s Techie Blog discusses making a small video circuit for taking sync and video signals and generating a composite video signal out, in this case for a vintage Northstar Advantage computer.

How to do this is mined from old original schematics.

This is my generic polarity selectable TTL logic to composite video generator. I based the circuit off the one in the ZRT-80 but I made each input individually polarity selectable like the advantage has on its outputs. That makes this design fairly generic and able to be used on a variety of computers to replace monitors that may have failed or video outputs that did not fir the composite video voltage levels. You could even hook this up to a raspberry pi pico or an FPGA and use it to generate a good solid composite signal as long as your video timings were correct.

See the details in the blog post here.