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Zapper works on small-LCD screen
http://forums.nesdev.com/viewtopic.php?f=9&t=16650
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Author:  krzysiobal [ Wed Nov 01, 2017 10:44 am ]
Post subject:  Zapper works on small-LCD screen

Wtf?
https://www.youtube.com/watch?v=ENWrb73ZSE8

Author:  lidnariq [ Wed Nov 01, 2017 11:28 am ]
Post subject:  Re: Zapper works on small-LCD screen

Sure?

Unlike other light pen/guns, code that uses the Zapper doesn't strictly convert time to an X,Y coordinate. And the 16kHz resonator in the Zapper is astoundingly nonspecific; any sharp transition from dark to bright can get it to trigger. So all that's necessary is that the latency from the video input to what's on screen be low enough for the game to see a dark-to-light transition during the same frame.

Given that it's one of those LCDs that appear to only support composite, I'm not too surprised? If there's no frame buffer, there's probably only one scanline of latency. More latency would cost more money.

I'd be curious to see what tepples's Zapruder test ROM does.

Author:  krzysiobal [ Wed Nov 01, 2017 12:35 pm ]
Post subject:  Re: Zapper works on small-LCD screen

I could not get any rational results in Y or XY mode in LCD display.
https://www.youtube.com/watch?v=mF8UYuSVh64

In big 100 HZ tube TV i could get good Y results (but it was delayed almost by half of screen) but X result were random.

Also, when I plug zapper in port 1, I can no longer push B to start 1 gun test! (ROM bug?)

Author:  tepples [ Wed Nov 01, 2017 12:53 pm ]
Post subject:  Re: Zapper works on small-LCD screen

X results are very noisy on an authentic CRT as well, so don't get too worried about those.

Author:  lidnariq [ Wed Nov 01, 2017 12:55 pm ]
Post subject:  Re: Zapper works on small-LCD screen

Interesting, thanks for sharing.

krzysiobal wrote:
Also, when I plug zapper in port 1, I can no longer push B to start 1 gun test! (ROM bug?)
Tepples probably (reasonably) assumed that one couldn't plug a zapper and a standard controller both into port 1 at the same time...

Author:  rainwarrior [ Wed Nov 01, 2017 1:38 pm ]
Post subject:  Re: Zapper works on small-LCD screen

Wondering also if the type of backlight in the TV is important here too?

E.g. if it's a PWM LED that might help with high momentary brightness and/or a convenient modulation frequency?

Author:  mikejmoffitt [ Wed Nov 01, 2017 2:06 pm ]
Post subject:  Re: Zapper works on small-LCD screen

A lot of these tiny simple LCDs don't have a buffer and basically decode NTSC and throw it at the LCD immediately.

Author:  krzysiobal [ Thu Nov 02, 2017 6:52 pm ]
Post subject:  Re: Zapper works on small-LCD screen

But the small LCD, where most Zapper games work is the one on which Teppes' test program DOES NOT work properly even in Y direction.

On big 100 HZ CRT TV where it works in Y (but is delayed), no single Zapper game works.

I previously tested about 8 of my different zappers and they hardly work, but this one (that I got recently) works pretty good.

I examined its interior and there is nothing special, but the lens is placed very close to the diode and it's shape is:
Image

While for other zappers it is:
Image

Author:  lidnariq [ Thu Nov 02, 2017 7:26 pm ]
Post subject:  Re: Zapper works on small-LCD screen

I think that makes sense? As long as there's still a light/dark alternation (such as *any* CRT), regardless of the phase, tepples's demonstration will draw something: in fact, tepples's code is one of the few things that will work with multi-frame lag.

In contrast, a game has to get the result with less than 20ms latency, and possibly much less depending on how long it polls the photodiode after the light patches are drawn to the screen.

Author:  tepples [ Thu Nov 02, 2017 8:21 pm ]
Post subject:  Re: Zapper works on small-LCD screen

If you can't get a result out of the Zap Ruder tech demo, you won't be able to get one out of Operation Wolf either. What these have in common is that they read the Y coordinate by waiting from the top of the frame until when light is first received. This works well for a CRT but might not work so well for a continuously illuminated LCD.

Author:  tokumaru [ Thu Nov 02, 2017 11:10 pm ]
Post subject:  Re: Zapper works on small-LCD screen

krzysiobal wrote:
But the small LCD, where most Zapper games work is the one on which Teppes' test program DOES NOT work properly even in Y direction.

Most games don't measure coordinates at all, they just check whether the zapper is seeing black or white.

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