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

In March, we published a Hands On article about Matt Sarnoff's modern homebrew computer that uses a very old hack: NTSC artifact color. This hack allows digital systems without specialized graphics hardware to produce color images by exploiting quirks in how TVs decode analog video signals.

NTSC artifact color was used most notably by the Apple II in 1977, where Steve "Woz" Wozniak's use of the hack brought it to wide attention; it was later used in the IBM PC and TRS-80 Color Computers. But it was unclear where the idea had originally come from, so we were thrilled to see that video game and electrical engineering legend Allan Alcorn left a comment on the article with an answer: the first color computer graphics that many people ever saw owe their origin to a cheap test tool used in a Californian TV repair shop in the 1960s. IEEE Spectrum talked to Alcorn to find out more:

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?

Photo: vonguard/Wikipedia

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: How did that idea of not doing NTSC "by the book" enter into your own designs?

Nolan said, "Oh, by the way, it has to be colored" part of the way into the design. But I knew he was going to pull this stunt.

AA: So, I learned the cheap, sleazy way doing repair work. But then I got a job at Ampex [a leader in audio/visual technology at the time]. Initially, I wanted to be an analog engineer, digital was not as interesting. At Ampex, it was the first time I saw video being done by digital circuits; they had gotten fast enough, and that opened my eyes up. [Then I went to Atari]. Nolan [Bushnell, co-founder of Atari] decided we wanted to be in the home consumer electronics space. We had done this [monochrome] arcade game [1972's Pong] which got us going, but he always wanted to be in the consumer space. I worked with another engineer and we reduced the entire logic of the Pong game down to a single N-channel silicon chip. Anyway, part of the way into the design, Nolan said, "Oh, by the way, it has to be colored." But I knew he was going to pull this stunt, so I'd already chosen the crystal [that drove the chip] to be 3.58 MHz, minus 15.750 kilohertz.

Near the end of making Pong, Nolan said, "Well, where's the sound?" I said "What do you mean, sound?"

SC: Why did you suspect he was going to do that?

AA: Because there never was a plan. We had no outline or business plan, [it was just Nolan]. I'm sure you've heard that the whole idea behind the original arcade Pong was that it was a test for me just to practice, building the simplest possible game. But Nolan lied to me and said it was going to be a home product. Well, at the end it was kind of sad, a failure, because I had like 70 ICs in it, and that was [too expensive] for a home game. But [then Nolan decided] it would work for an arcade game! And near the end of making [arcade] Pong, Nolan said, "Well, where's the sound?" I said "What do you mean, sound?" I didn't want to add in any more parts. He said "I want the roar of the crowd of thousands applauding." And [Ted] Dabney, the other owner said "I want boos and hisses." I said to them "Okay, I'll be right back." I just went in with a little probe, looking for around the vertical sync circuit for frequencies that [happened to be in the audible range]. I found a place and used a 555 timer [to briefly connect the circuit to a loudspeaker to make blip sounds when triggered]. I said "There you go Nolan, if you don't like it, you do it." And he said "Okay." Subsequently, I've seen articles about how brilliant the sound was! The whole idea is to get the maximum functionality for the minimum circuitry. It worked. We had $500 in the bank. We had nothing and so we put it out there. Time is of the essence.

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!" Funny story-home Pong becomes a hit. This is like in 1974, 75. It's a big hit. And they're creating advertisements for television. Trying to record the Pong signal onto videotape. I get a call from some studio somewhere, saying, "We can't get it to play on the videotape recorder, why?" I say, "Well, it's not really video! There's no interlace" Treat it as though it's PAL, just run up through a standard converter."

I remember showing Woz the trick for the hi-res color, explaining, sitting him down and saying, "Okay, this is how NTSC is supposed to work..."

SC: How does Wozniak get wind of this?

AA: In those days, in Silicon Valley, we didn't keep secrets. I hired Steve Jobs on a fluke, and he's not an engineer. His buddy Woz was working at HP, but we were a far more fun place to hang out. We had a production floor with about 30 to 50 arcade video games being shipped, and they were on the floor being burnt in. Jobs didn't get along with the other guys very well, so he'd work at night. Woz would come in and play while Jobs did his work, or got Woz to do it for him. And I enjoyed Woz. I mean, this guy is a genius, I mean, a savant. It's just like, "Oh my God."

When the Apple II was being done, I helped them. I mean, I actually loaned them my oscilloscope, I had a 465 Tektronix scope, which I still have, and they designed the Apple II with it. I designed Pong with it. I did some work, I think, on the [audio] cassette storage. And then I remember showing Woz the trick for the hi-res color, explaining, sitting him down and saying, "Okay, this is how NTSC is supposed to work." And then I said, "Okay. Now the reality is that if you do everything at this clock [frequency] and you do this with a pulse of square waves"" And basically explained the trick. And he ran with it. That was the tradition. I mean, it was cool. I was kind of showing off!

SC: When people today are encouraged to tinker and experiment with electronics, it's typically using things like the Arduino, which are heavily focused on digital circuits. Do you think analog engineering has been neglected?

AA: Well, it certainly is. There was a time, I think it was in the '90s, where it got so absurd that there just weren't any good analog engineers out there. And you really need analog engineers on certain things. A good analog engineer at that time was highly paid. Made a lot of money because they're just rare. So, yeah. But most kids want to be-well, just want to get rich. And the path is through programming something on an iPhone. And that's it. You get rich and go. But there is a lot of value in analog engineering.