Has anyone here tried creating a video sync generator using discrete logic on a breadboard? I'm looking for tips on how to get a sync signal that a TV can properly recognize. I have an 8 MHz clock, a bunch of 74-logic chips, and an RCA/composite video connector. So far, I've managed to get some stripes on the screen and a flickering 'no signal' indicator, but I'm aiming for a stable PAL signal with a full-screen black or dark gray image.
I've been thinking about using GAL chips for this project since they were used in some other systems I found, but I've never programmed them before. Currently, my circuitry outputs hsync when the horizontal counter reaches 480-511 and vsync when the vertical counter hits 272-275, but I'm running into issues. Both signals are XOR'ed and inverted before they reach the video output, and the counters reset at 511 and 311. What am I missing here?
2 Answers
Just a heads up, using XOR for Vsync and Hsync isn't going to cut it. You need shorter sync signals, except during vertical sync when they should be longer. This is key for getting PAL to work. Try creating different H sync pulse lengths controlled by your H counter and do similar for the vertical counter with some combining logic. Remember, they figured this stuff out using older tech, so it’s definitely possible!
You should check out Ben Eater's video series on building a video board. It might not be exactly what you're after, but he covers generating sync signals and timing, which could help direct you. If you haven't seen it yet, definitely give it a shot!
Thanks for the suggestion! I actually watched that series before. I ended up taking a different approach and created my own logic for drawing, but I did borrow some ideas about the sync from his work.
Interesting! I took inspiration from an existing design, so if they got it working, I guess I need to revisit how they structured their sync logic.