by what mechanism does the nintendo zapper work?  it seems as though the zapper
sends a signal to the  television, but that is absurd.  how does the nintendo
know where you are pointing the zapper?

8 responses total.

What *is* a Nintendo zapper?  A fake gun?

I've noticed that when you pull the trigger, the targets on the screen
flash.  I assume the optics in the gun pick up this flash.  If the Nintendo
sees a pulse at the right time, it knows you were pointing at the target. 
If you look down the barrel of a Nintendo 'zapper', you'll see it's got a
set of lenses focused on a phototransistor.

yeah that is the most reasonable assumption.  that s what i thought too.  but
here s why i no longer think it:

in the game duck hunt, if you shoot at the empty field a dot appears in line
with your gun.  if the zapper is  detecting signal from the tv, then how does
it distinguish between some pixel of blue sky on the left side,  and any other
blue pixel on the screen?  it knows not only whether you hit the target or not,
but also exactly  where your bullet did end up on the tv.

if every pixel on the tv screen has some kind of id tag, so the zapper
distinguishes exactly what part it is  reading, i would say this is a very very
sensitive phototransistor indeed!  

in a new system, playstation i think, you start by calibrating the gun.  you
don t pull the trigger, you just  point at a blank screen.  there is a dot on
the screen where your gun is pointing, and it follows your gun  around as you
wave it in front of the screen.  as far as i can tell, there are no
distinguishable features on  the screen which the gun could detect and use to
get it s bearings.

your explanation, mr brodbeck, sounds very reasonable, and i want to believe
it.  i mean that s how i always  thought it worked.  but i can t explain to
myself how this $30 piece of plastic childrens toy can 'look' at the  tv and
determine exactly where it s pointing.

Well, maybe it works like the old Commodore light pen did.

The Commodore light pen had a phototransistor in the tip.  When you held it
up to the screen, the phototransistor generated a pulse every time the TV's
electron beam scanned by and illuminated the particular set of pixels it was
on top of, once every 1/60th of a second.  By measuring the time between the
start of each video field and when the pulse occured, the computer could
tell where on the screen the light pen was.

Doing this from across the room would require better optics, but I'm sure
it's possible.

Then all the TV has to do is illuminate that point on the next sweep, and you
will think it had happened simultaneously. 

Re #2:  That's one way to do it, and if the targets are flashing it
probably is how it's done.  Do the targets flash even if the gadget
is pointed at the wall?  Seems almost airtight.

Another way is to watch the flicker of the phosphor as the beam writes
the picture on the tube.  If the resolution of the optics is good
enough to pick out an area about one line high on the screen, and the
detector is sensitive and fast enough to see the flicker as the beam
flashes by, you can pick out the point of aim down to the scan line
and pixel.  This is how light pens work.  However, the need to flash
parts of the display make that scenario seem rather unlikely for the
Nintendo.

(Postscript after reading 3-5:  Yeah, they're probably right.)

Yeah.  I'm still convinced the Nintendo gun uses the 'flash the targets'
method.  (They flash even if you point the gun at a wall.)  The Playstation
gun may use the 'detect the scan line' method; it's probably more
sophisticated.

heh they explained it in an issue of nintendo power
when you press the trigger, the screen goes white, and targets go black (or
other way around cant remeber) and the gone detects which its pointed at and
registers it as a hit or miss. try aiming it at a black peice of paper and
firing (if black = hit)

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