This is the circuit that generates the "hit" sound. It uses the vertical ball counter. Some circuitry has been added on the left to generate hits periodically.

When a hit happens, it clears a D Flip Flop which turns back on again when the vertical ball counter reaches 240 (when the high 4-bits are all 1). A hit can only happen when the vertical ball counter is 252 or greater. So, the hit sound only lasts about 1/60th of a second. When this flip flop is cleared, bit 4 of the counter is used to generate the hit sound. That makes the hit sound about 490 Hz.

The bit used to generate the hit sound is not quite a square wave, though. The vertical ball counter only counts from 7 to 255, and more importantly, it stops counting during the vertical blanking interval. So the hit sound will be different depending on where the hit happens. If it happens in the middle of the screen, then there will be a short silence in the middle, so it seems like two separate sounds. (The counter will keep counting until we hit the bottom of the screen, stop briefly, and then start again when we hit the top of the screen.) If it happens at the top or bottom of the screen, then the vertical blanking interval will happen at the beginning or ending of the sound, so the sound will seem like a single sound.

Wait for the Audio Out to fill up, then click "Play Audio".

Next: Top/Bottom Bounce Sound.