DanBoris' Tech Blog

Interesting that the length of the "hit" sound is generated based on the frame timing, but the score sound uses a 555. I wonder if there's a list somewhere showing what different chips cost in 1971-1972.

 

Also, I couldn't find where VPOS256 and VPOS32 are generated? Are those taps off the ball's timer chain?

Interesting that the length of the "hit" sound is generated based on the frame timing, but the score sound uses a 555. I wonder if there's a list somewhere showing what different chips cost in 1971-1972.

 

Also, I couldn't find where VPOS256 and VPOS32 are generated? Are those taps off the ball's timer chain?

 

Yes, VPOSxxx are taps off of the vertical ball counter. The 555 in necessary for the score sound since it's a much longer delay the could easily be generated with an existing counter signal. By using the counter in this circuit they saved themselves a chip and a couple discrete components (at least 2 caps and a resistor). The miss sound, which I haven't talked about yet, also uses one of the VPOS counters.

 

I do wonder why they used the vertical position counter instead of the main vertical sync counter. The only reason I can think of is that they were already driving to many chips off the vertical sync counter.

 

Dan

The 555 in necessary for the score sound since it's a much longer delay the could easily be generated with an existing counter signal. By using the counter in this circuit they saved themselves a chip and a couple discrete components (at least 2 caps and a resistor). The miss sound, which I haven't talked about yet, also uses one of the VPOS counters.

 

I can't find part 2 of "Ball horizontal control". Did you maybe forget to "publish" it?

 

I wonder whether it would have been practical to have the ball travel "invisibly" following a miss until it reached the net position and then "serve" from there? When the ball is blanked, the XOR of the upper bits of the ball's counter and the hsync counter might have made an interesting source of audio.

 

I do wonder why they used the vertical position counter instead of the main vertical sync counter. The only reason I can think of is that they were already driving to many chips off the vertical sync counter.

 

I would guess they used VPOS256 instead of vertical sync to ensure that the hit sound would be the same length whether the hit occurred at the top or bottom of the screen. The use of VPOS16 instead of the display scan line counter would ensure a consistent "start" and "end" to the sound, though the "missing" counts every vertical blank would still create annoying distortion which would vary depending upon the ball's position.

You are right, I missed part 2 of the Ball Horizontal control.