NOTE: If I'm understanding this correctly (and I could very well be wrong -- I'm out of my depth here), it might suggest a cool additional feature, not actually part of the Major Havoc quad pokey but could be built onto a quad POKEY for 8-bit machines: a register which allows the audio channel from the 4th POKEY to replace/modify ground on the other channels?
This seems to be even more complicated. So, if someone with some audio electronics skills wants to chime in, here is a ZIP with the Major Havoc schematics (individual pages are in TIFF format). I've pre-digested this document, so to save you the time, here are the good parts:
Page 10A contains the Quad POKEY and the surrounding circuit. Find the large IC "QUAD CUSTOM IO" with the four audio outputs in the lower right of the chip. OUT1-3 are tied together and fed into a circuit, and OUT4 gets its own unique circuit. Then it appears that the output of those two circuits are tied together, and then all of it is processed yet again in a way that creates an AUD1 and AUD2 output that goes to the Atari Audio Regulator II board (as an audio amplifier). This is what mytekcontrols and shupac were talking about.
Page 02A picks up the AUD signals on the Atari Audio Regulator-II board, which is a standardized PCB (but with multiple revs which certain games will require). It creates speaker level outputs. Notice that the SPKRx-RETURN lines are tied to ground, which later a speaker would normally be connected to the SPKRx and SPKRx-RETURN lines.
Page 01B is where we take the SPKR* outputs from the AR-II board and actually feed them into the speakers. The oddity, as described earlier, is that normally, the SPKRx-RETURN lines from the AR-II board are used as audio ground and SPKRx have the audio signal. Instead, here, SPKR1 and SPKR2 are fed to the speakers (with no SPKRx-RETURN lines being used). So that's weird. And as mentioned, the speakers are also oddly wired. One speaker is connected directly to SPKR1 and SPKR2. Then there are two speakers which are connected in series before they are connected to SPKR1 and SPKR2. So this was the oddity on the other side of things that I was talking about.
All of this is so incredibly convoluted. Perhaps someone could help us make sense of what is going on here? Perhaps it was a way to select between sets of speakers? [Specifically, a way to dynamically balance the output of 3 POKEYs between two channels rather than having two fixed audio channels with 2 unique POKEYs on each side?] Or is it as I originally imagined, a way to put a cool twist on the standard POKEY output without burning CPU? Both? Something entirely different? This one is odd, so some expertise would be appreciated.
Possibly worth further examination: how the audio was handled in the official Major Havoc to Tempest conversion PCB kit. The kit made no modification to the cabinet, so it used the more conventional speakers of the Tempest cabinet. (This is how I played Major Havoc. The kit had an authentic Major Havoc PCB, but it included a wiring harness that adapted it to the wiring of a Tempest game's cabinet. It effectively replaced the Tempest arcade PCB.)
Edited by jmccorm, Mon Mar 19, 2018 1:05 PM.