Re: [stella] stick/paddle combos

Subject: Re: [stella] stick/paddle combos
From: Eckhard_Stolberg@xxxxxxxxxxxxxxxxxxxxx (Eckhard Stolberg)
Date: Tue, 30 Sep 1997 16:52:12 +0200
>	I was wondering how difficult it is to implement paddles since
>Glenn in an earlier message said they're sometimes difficult to get things
>working with.  However there's a number of Paddle games which implement
>all four pots (or there's a number of 4 player paddle games with really,
>really, really good programmers behind them)

Paddles are just potentiometers. To read their position you have to
discharge them and then increase a counter until the paddle port signals
that the pot has recharged again. Since this takes some time, you usually
discharge the pot at the beginning of a TV frame and then read the port
and increase the counter in every scanline. Or if you can live with lower
resolution, read the port every couple of scanlines. Reading all four
ports in a scanline takes a lot of processing power, that you need to
update the graphics registers for that line. So reading paddles and
displaying good graphics is quite a challenge.

>	How much of a challenge are the Driving controllers?  Technically
>they're just joysticks with the left & right contacts ripped out.  But
>turning it one direction involves more than a simple "up up up up up..."

Bob Colbert uses driving controlers in Stell-A-Scetch. You can check his
source code, if you are interested in his routine for reading DCs.

In the DC the up and down pins get moved around in a way that they connect
to open or closed circuits, so that they can produce all four possible
constellations. So, e.g. turning left would produce a sequence of
00 01 10 11 and turning right would be 11 10 01 00, or something similar.

To read a DC you would have to read the joystick port and check if the
current up/down status is before or after the last checked one in this 

>	And, how about the light gun?  Thing I'm leading up to is creating
>a "Lucky & Wild" rip.  Think Night Driver, but this time you get to shoot
>those cars. >=^)  Enduro, but you get to shoot those cars.  Pole Position,
>but... you get the idea.  (Or use the model of Gran Prix if perspective
>causes a problem.)

I don't have a lightgun, but I think it is the same as a paddle with a
photo sensor instead of a potentiometer. You would read it the same way
and it would take longer to recharge depending on how much light reaches 
the sensor. Since the amount of light, that reaches the sensor, depends
on the room your TV is in, you can only distinguish between black 
background and bright objects. If you have more than one object on the
screen, you would need to turn them off one per frame and check if the
value changes. This also help against lightbulp cheating. :-)

Ciao, Eckhard Stolberg

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