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While reading about CRT on Wikipedia, an information (CRT Allows the use of light guns/pens; https://en.wikipedia.org/wiki/Cathode_ray_tube#Advantages_and_disadvantages) suddenly triggered a a vast lots of memories.

Around early 1990's when I was a toddler, in a local "mela" or fair each year, there was a stall for video games .

They had several big boxes (maybe CPUs), several tiny to large sized, portable monochrome televisions with a mesh of cables. They provided any 1 of 2 kinds of input devices, 1. a game-control with arrow-keys & fire-key etc; as-well, 2. a toy Gun-shaped device.

Game in CRT with a gun-like input device

(The above diagram drawn completely based-upon memory.)

In the game, to destroy the enemy (a rocket, or an octopus, or a ghost etc), the tip of the toy-gun were almost-touched on that object on screen, and on pressing the trigger, the selected rocket were destroyed.

It was, specifically surprising to the kids, because that was a pre-touchscreen era. The television set, acted quite like a touchscreen.

Now I want to know, how that systems worked?

Which-one is acting as the sensor (of the particular place where the rocket was fired)? the toy-gun or the CRT? What is the mechanism to identify a particular spatial-position (of target-object such as rocket) out of many-other objects (other rockets)?

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    \$\begingroup\$ Did you try reading the wiki article for Light Gun? en.wikipedia.org/wiki/Light_gun Complete technical breakdown of various implementations, including the CRT versions. \$\endgroup\$ – Passerby Jun 30 '16 at 5:35
  • \$\begingroup\$ If the work of CRT display is just to show the signal, and the whole function of sensing that signal is upon the toy-gun; then it should work on LCD or LED type displays also. Isn't? \$\endgroup\$ – Always Confused Jun 30 '16 at 7:34
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    \$\begingroup\$ There was a nice article about this on hackaday <hackaday.com/2015/11/16/resurrecting-duckhunt/#more-177788> \$\endgroup\$ – JakkeFire Jun 30 '16 at 9:13
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    \$\begingroup\$ Too broad? but satisfactory answers came . Neither of these answers are too broad, nor distracting. And the possible answer set is already made narrow by last three sentences (last three question marks) of the question. \$\endgroup\$ – Always Confused Jun 30 '16 at 12:58
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    \$\begingroup\$ I agree that this should not be closed as too broad. The latency of displays is controversial with gamers, and is due to buffering the signal to process it in modern systems. \$\endgroup\$ – pjc50 Jun 30 '16 at 14:06
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The CRT TV picture was generated by deflecting an electron beam fired from the back of the tube in a raster scan as shown in Figure 1. Two sawtooth oscillators control this: one for the vertical and one for the horizontal - the horizontal one running much faster than the vertical. For PAL used in Europe there are 625 lines (horizontal) refreshed at 25 frames/second (vertical). (It's a little more complex as the lines are interlaced to reduce flicker. The vertical scan runs at double the frequency alternately drawing the odd and even lines.)

enter image description here

Figure 1. CRT TV raster scan. Source: Wikipedia Analog Television.

The electron beam excites the phosphor on the inside of the tube making a sudden step change in the brightness of the dot. This then decays until the next refresh. The persistence of the dot was a balance between keeping bright between refresh and "motion smear" if it was too slow.

The gun contains the required optics to view a small spot on the screen and a fast light sensor. Some analog logic is required to keep track of the vertical and horizontal dot position relative to the position of the on-screen "target" and match this with the step change in light level by the gun sensor. If the two coincide at the moment the trigger is pressed a "hit" is registered.

Note that this system could only work with bright targets as there would be no dot impulse on black or very dark areas of the screen.


I think @Passerby's answer is more accurate and obviously a much simpler method of working without the timing issues of my answer in the case of the gun. I predate video games. He obviously had a mis-spent youth - although it has paid back on this occasion.

enter image description here

Figure 2. The cutting edge - once upon a time. This would be easier to date if we could see how wide his trousers flares were.

The method I have described is how the light pen worked - to the best of my recollection.

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  • \$\begingroup\$ Thanks a good-many valuable informations you have provided. \$\endgroup\$ – Always Confused Jun 30 '16 at 7:13
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    \$\begingroup\$ See my update regarding the light pen. \$\endgroup\$ – Transistor Jun 30 '16 at 9:08
  • \$\begingroup\$ @ transistor Please do-not worry, only 1 answer could be selected as accepted (green tick-mark). No-one answer came is wrong. And yours one carries the signature of hardwork. But @passerby's answer was so simple, that not accepting that answer would be an injustice to that user. I'm sorry. \$\endgroup\$ – Always Confused Jun 30 '16 at 9:19
  • \$\begingroup\$ Also, I can put only 1 vote to a question, and not more. For such a subjective and "fuzzy" task like rate a broad question; a +1 also cannot express the truth. Rather, a 5-star or 10-star rating would be quite better. \$\endgroup\$ – Always Confused Jun 30 '16 at 10:21
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The quintessential example of a light gun is Duck Hunt with the NES Zapper.

When the trigger was pressed, the Nintendo would trigger a special frame at CRT refresh rates. The screen would draw a white square at the target, too fast for human vision to see, but slow enough that a brightness sensor (A simple Phototransistor) with a very narrow field of vision in the Zapper could tell the difference between Black (miss) and White (hit). Multiple targets have their own target frame (or slightly overlapping).

enter image description here

The Zapper is the sensor in both meaning of the word. It senses the trigger press, and it senses the screen brightness.

It didn't work on newer TV technology because the Zapper was designed to work with the precision refresh and scan rates of CRTs. Newer technology threw scanning away and don't have precision refresh rates. Newer light gun technology was created to adjust to newer TV technology. But at this point, check the wiki article for more information.

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  • \$\begingroup\$ if so, the sensing-task is solely upon the gun. Isnt'? thein it could be used for both CRT and LCD or LED display? \$\endgroup\$ – Always Confused Jun 30 '16 at 7:21
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    \$\begingroup\$ @AlwaysConfused one frame for one duck, then the next for the second. The sensor result would be timed with the frame that is up, so it knows which was hit. As for Plasma or LCD or LED or projectors, no, because the original Zapper uses the precision timing of CRT refresh and scan rates to work. New technologies have replaced it. For the Wii, Duck Hunt uses the IR point tracking and accelerometer/gyroscope to figure out where its pointing, completely independent of the TV. \$\endgroup\$ – Passerby Jun 30 '16 at 7:34
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    \$\begingroup\$ LCDs don't blink, like a CRT does... \$\endgroup\$ – Passerby Jun 30 '16 at 7:39
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    \$\begingroup\$ I remember playing this game as a kid, and I also remember some sort of discoloured square appearing around the duck each time you shot it. I think I mistook it for some sort of visual effect, graphics weren't all that awesome back then :) \$\endgroup\$ – Lundin Jun 30 '16 at 12:52
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    \$\begingroup\$ @Passerby are you sure it needs two frames for two ducks? With a CRT you could use the scanning effect (rows of pixels being drawn sequentially) to determine which duck is aimed at via precision timing as long as they're at different heights. (The top target box would light up before the bottom box.) \$\endgroup\$ – Dan Neely Jun 30 '16 at 13:30
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The television set, acted quite like a touchscreen.

No it didn't because it would not work with your finger, it only works with the "gun".

The operation is quite simple, pictures on CRT screens are drawn by moving a "dot" which lights up, across the screen. Human eyes are too slow to notice this so we see only the complete picture.

The gun consists of a pipe and a fast light sensor. When you point it at a spot on the screen the sensor will give a signal when the "dot" passes by and light shines into the pipe and onto the sensor. Then the system "knows" where the gun is pointing as it got a signal from the gun when the "dot" was at a certain position.

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  • \$\begingroup\$ Which-one is acting as the input-device (of the fire-event on a particular-rocket at particular place)? the toygun or the CRT? \$\endgroup\$ – Always Confused Jun 30 '16 at 6:30
  • \$\begingroup\$ Think about it, I explained how the location where you point the gun is found. Now a button needs to be pressed to "fire". Where's that button ? \$\endgroup\$ – Bimpelrekkie Jun 30 '16 at 7:15
  • \$\begingroup\$ Where's the fire button? the trigger of the gun-shaped tool. \$\endgroup\$ – Always Confused Jun 30 '16 at 7:26

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