In the course of the CGA debugging I talked about yesterday, I found it useful to be able to accurately visualize signals with a period much smaller than a frame. The setup I showed in yesterday's video was rather less than ideal for that since the TV adjusts the DC offset of the incoming signal so that the blanking period is at the normal (black) level.
I realized I could get around this by taking the "display enable" signal from the CGA card and ANDing it with the signal I wanted to visualize. Then a logic "1" signal would give me a white screen, signals with frequencies that were integer multiples of the horizontal sync frequency would give me vertical lines and so on.
When I tried it out I got a bit of a surprise - strange interference patterns were appearing on the TV that I hadn't seen without the AND gate. A moment's thought told me why this was - the AND gate was acting as an amplifier - picking up whatever electromagnetic radiation was flying around and turning it into 1s and 0s.
Here's some video of the effect.
This was much more interesting than actually debugging the CGA card, so I started poking around at all kinds of signals and visualizing them. Unfortunately I forgot where the -12V (or possibly +12V) signal on the bus was and touched it, frying my one remaining 74HC08. I finished the debugging with a 74LS08 which doesn't show such nice interference patterns, then wired up two NAND gates from a 74HC00 as an AND gate so that you could get to see the effect.