In Nintendo's "Duck Hunt," how does the TV know when you've hit a duck?

May 11, 2001

Dear Cecil:

A friend and I were playing Nintendo, the original eight-bit system, and we played Duck Hunt, a game that requires a "light gun." I was wondering: How exactly does the Nintendo game "know" where you are pointing the gun on the screen when you shoot ducks?!? Very mind-boggling!

You're going to give yourself such a smack when you hear this one. Stripped of the high-tech accoutrements (by 1980s standards, anyway), it's the oldest trick in the book. You think you're using the gun to shoot at the TV, right? But really the TV is shooting the gun.

Here's what happens. You shoot at a duck, which appears on an ordinary TV screen. The gun is connected to the game console; pressing the trigger blackens the screen, then causes a duck-shaped white target to appear momentarily. If your aim is true, a photo sensor in the gun detects the shift from dark to light, and bingo--dead duck. In short, the TV emits the light pulse and the gun detects it, not the other way around.

For technical details, see www.howstuffworks.com/question273. htm, which includes a link to Nintendo's 1989 patent on the technology. The patent explains how the dark-to-light shift prevents you from cheating by pointing the gun at a steadily shining light source, a weakness of earlier light guns. You'll also learn how multiple targets can appear on the screen at the same time and how more advanced systems can indicate on the screen exactly where your shot "hit."

But that's for the gearheads. At the Straight Dope we're more interested in the deeper meaning of it all. Ingenious as the light gun is, magicians have been boggling minds for years with similar cause/effect reversals. Take the "directed choice." You pluck a card at random from a deck, then return it. You alone know the card's identity--let's say it's the ace of spades. The magician then displays four cards so that only you can see the faces. None is the ace. She places the four cards facedown in a square. By sleight of hand, the details of which need not detain us, she replaces one of them with the ace. "Pick two," she says. You do. The magician then discards the other cards (if you picked the ace), or the cards you picked (if not). She points to the two remaining cards and says, "Pick one." If you pick the ace, she discards the other card. If you don't pick the ace, she discards the card you picked. Indicating the one card remaining, the magician says, "Turn it over. Is that your card?" Of course it is. You're amazed and mystified, but as you can see, the trick turns on a simple unsuspected reversal of cause and effect. (Low light, quick fingers, and a couple beers also help.) You thought you were the active agent; in fact you were the patsy. Understand now? Congratulations. You've just passed Marketing 101.

QUESTIONS WE'RE STILL THINKING ABOUT

Dear Cecil:

I am doing a presentation for my college chem class on any gas. I was wondering if you had any unique gases, or ones that would be very informative. Any suggestions would be helpful. --Amanda Buchanan

Dear Cecil:

Cecil, why should I go on living in this unjust, inhumaine, technology dependant world where one marginally sain person can't even delude himself enough to believe that one person can make a differance as nameless, faceless forces seem to conspire against my every hope and dream, leaving me spiritually ravaged and consigned to work at the drive-in window at Wendy's? Also, [are] Cheetos really made with real cheese? --Jeremy, Pittsburgh

Dear Cecil:

I've often wondered why flagellating underwater produces a more horrific odor than an airborne sample from the same batch. --Richard Reneau, assistant director of maintenance, Austin State Hospital 

Cecil replies:

Richard, this is not a question I feel qualified to answer. You should talk to a member of that religious order we discussed a few months ago, the Flatulents.

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