Last year, artist Kyle McDonald was trying to figure out ways to get high-frequency analog signals out of his computer, and noticed that nice VGA connector on the side of the computer. As an experiment, he built a custom cable to break the R, G, and B lines out of a cable, and hooked up the R and G lines to a stereo. He then wrote some custom software to translate audio signals into video frames, and played back the video to hear the sound. Surprisingly, this more or less worked, however the horizonal and vertical sync rates proved to be problematic.
I made an adapter for outputting audio via your video card. VGA has three analog color wires, and each has its own ground/return. If you connect a 75 ohm resistor across one of them (generally green), the computer recognizes your “external monitor”, and you’re free to use Red/Red-return and Blue/Blue-return for audio.
Unfortunately, while video can handle a DC bias (e.g.: white screens), about 28% of the signal is dedicated to the horizontal and vertical sync (at 800 x 600) — which is at ground. This means that if you scale audio from -1/+1 to 0-255 color intensity, you will get annoying hums at your refresh rate and horizontal sync rate. Instead, I just chopped off the bottom half of the audio signal.
Two apps are demoed: one for generating tones using ASCII values from the keyboard, and one for rendering a stereo audio file (the amen break) to Red/Blue components at 60 fps.