So, in theory, if the position disc was missing teeth, would the guy on the screen jump to the up position when the calibration disc hit its position? I am just wondering how it would actually calibrate if the position disc is off for whatever reason.
Yes, the skater will align to "up" when the calibration signal is triggered.
- IIRC, you can confirm this in MAME v0.142 or newer versions where the "Machine Configuration" menu option of controller type includes "real". Not sure what other settings to adjust to get the snap alignment, but there was a related thread about it
here.
The position disc works just like a spinner.
- It outputs
relative position changes. i.e. two steps clockwise, 5 steps counter-clockwise
- When the game board boots up, it doesn't know what direction the joystick is pointing
until the calibration disc triggers.
The calibration disc is mounted so the tooth is aligned with the calibration optos when the handle is in the 12 o'clock position.
- This provides an
absolute position. i.e. When the calibration optos trigger, the joystick is passing through the 12 o'clock position. Now that it knows where "up" is, any changes from the position wheel are relative to that "up".
Or does it only calibrate on a specific screen?
It calibrates throughout gameplay.
Every time the joystick handle passes through the 12 o'clock position, the slots on the calibration disc pass through the calibration optos, triggering a calibration signal that tells the game PCB to turn the skater to face 12 o'clock.
I would think, once it knows which way it up, it would not need to calibrate again as it would be programmed for that encoder wheel.
Remember that the encoder wheel outputs
relative position changes, not an absolute position.
- If the spinner alone could hold calibration, there would be no need for the calibration disc.
I suspect that part of the problem also has to do with how the game logic applies the directional angle and speed (a vector) to the grid-like layout of the playfield.
- Add in the possibility of jitter or backlash on the position encoder wheel and it makes sense that Atari would add the calibration disc.
Scott