How the Nintendo NES Zapper gun worked (and why it doesn't on today's TVs)

My 10-year-old son Lux is a retro videogame historian who collects and studies 1980s consoles and games with the gravitas of a PhD student working on his thesis. Last year he acquired Nintendo's NES Zapper gun controller from 1984 that was used to play shooting games like Duck Hunt. (Below, a TV commercial for the NES Deluxe Set including the Zapper and R.O.B. The Robotic Operating Buddy.) Unfortunately, the NES Zapper doesn't work with modern LCD televisions. The video above from "Today I Found Out" explains the clever technology behind the NES Zapper gun. And here's a great text explanation from How-To Geek about why it doesn't work on non-CRT screens, something my son already knew but, of course, wanted the Zapper anyway for, er, display purposes:

First, it requires extremely precise timing between the trigger pull on the Zapper and the response on the screen. Even the slightest difference (and we're talking milliseconds here) between the signal sent to the NES and the signal displayed on the screen can throw it off. The original timing sequence was based on the very dependable response time of a CRT hooked up to the analog NES signal. Whether the old tube TV was big, small, cutting edge or 10 years old, the speed of the signal via the CRT display standard was reliable. By contrast, the latency in modern digital sets is not reliable and is not the same as the old consistent delay in the CRT system. Now, this doesn't matter in most situations. If you have your old VCR hooked up to the coax jack on your new LCD display, it doesn't matter one bit if the audio and video are delayed by 800 milliseconds because you'd never know (the audio and video would play in sync and you'd have absolutely no way of knowing that the entire process was lagging by a fraction of a second). However, this latency completely destroys communication between the Zapper, the NES, and the events on the screen.

This extremely precise timing was possible (and consistent) because Nintendo designers could count on the refresh rate of the CRT being consistent. CRT displays use an electron gun to activate phosphors in screen hidden behind the display glass. This gun sweeps across the screen from the top to the bottom at a very dependable frequency. Even though it happens faster than the human eye can detect, every single frame of every single video game or television broadcast is displayed as if some hyperactive robot is drawing it line by line from the top to the bottom.

By contrast, modern digital displays make all the changes simultaneously. This isn't to say that modern televisions don't have progressive and interlaced video (because they most certainly do), but the lines aren't rendered one at a time (however quickly). They are displayed all at once in their respective standards. As for why this matters to the Zapper, the software driving the Zapper's detection algorithm needs that line-by-line refresh to pull off the timing tricks which make it possible to have 5 ducks on the screen and successful hit detection all within 500 milliseconds or so.

Of course, there are hacks to take care of the problem.

And here's an original TV commercial for the NES Deluxe Set: