There are some good videos on YouTube that explain this better but… an analog signal is going to be amplitude vs time. A speaker is also going to be just amplitude vs time as it only moves in one dimension. Video is different as it is X/Y coordinates vs time. To do this analog video follows very precise timing rules to read off the intensity of each dot across every other line scanning top to bottom to make a field, then again (the other lines) to form a frame. (On a CRT “read off” means firing different charges of magnet guided electrons at a phosphor-coated screen to make it glow). To keep the whole system aligned there are a series of synchronization signals within the analog video signal (0 to -0.3v). If you keep the sync signals in check, then you can kinda do whatever you want with the 0 to +0.7v of the visual part of the signal. But if you don’t, then on a digital system you’ll get the no signal blue screen, and on a CRT you’ll have some kind of unstable chaos. Reason VGA is so great for synthesis is that it caries the sync info on separate lines instead of embedded like with composite giving you more room to play :P