How Pong's inventor gave Woz a hack to bring color to the Apple II

In 1977, Steve "Woz" Wozniak used a neat hack to bring color to the Apple II computer. According to IEEE Spectrum, the obscure trick, called NTSC artifact color, "allows digital systems without specialized graphics hardware to produce color images by exploiting quirks in how TVs decode analog video signals." That hack later was employed by the IBM PC, Radio Shack TRS-80, and other early home computers. But how did Woz learn about it? Turns out, videogame legend Al Alcorn, inventor of Pong, turned Woz onto the hack. From IEEE Spectrum:

Stephen Cass: Analog NTSC televisions generate color by looking at the phase of a signal relative to a reference frequency. So how did you come across this color test tool, and how did it work?

Al Alcorn: When I was 13, 14, my neighbor across the street had a television repair shop. I would go down there and at the same time, I had my father sign me up for an RCA correspondence course on radio and television repair. So, by the time I got to Berkeley, I was a journeyman TV repairman and actually paid my way through college through television. In one repair shop, there was a real cheap, sleazy color bar generator [for testing televisions]. And instead of doing color properly by synthesizing the phases and stuff like that, it simply used a crystal that was 3.58 megahertz [the carrier frequency for the color signal] minus 15.750 kilohertz, which was the horizontal scan frequency. So it slipped one phase, 360 degrees, every scan line. You put that signal on the screen and you've got a color bar from left to right. It really was really the cheapest, sleaziest way of doing it! [...]

SC: So, in the home version of Pong, the graphics would simply change color from one side of the screen to the other?

AA: Right, the whole goal for doing this was just to put on the box: "Color!" [...]

SC: How does Wozniak get wind of this?

AA: In those days, in Silicon Valley, we didn't keep secrets.

"Al Alcorn, Creator of Pong, Explains How Early Home Computers Owe Their Color Graphics to This One Cheap, Sleazy Trick" (IEEE Spectrum)

image credit: Evan-Amos (CC BY-SA 3.0)