Fix Song Restoration Bugs in Sonic 1's Sound Driver

(Original guide by MarkeyJester)

When collecting an extra life, the engine stores away the currently playing music track, and then plays the extra life jingle, once the extra life jingle has finished playing, the previous track is restored at very low volume, and then fades into normal volume, the DAC (drum samples) do not have fading in capabilities for this engine, so it does not resume until the music has finished fading into full volume.

Explaining the issue

If you pause the game while the music track is fading back in, and then unpause, the DAC channel does not resume, it remains mute, it seems that the pause routine not only sets the keys off, but it also turns off the left and right speaker panning for each channel (because of the release rate), and unpausing causes the system to reset all of the channel's panning/AMS/FMS values, of course, if the music is going through a fade in, then the DAC channel (a.k.a. FM 6) must remain mute until the music has fully faded, hence, the panning/AMS/FMS value for FM 6 does not get updated while DAC is running, until of course; either a new track is played, the game is paused and unpaused again after fading, or the DAC channel changes speakers during its script.

Fixing the issue

We need to go to "loc_726D6:"

<asm> bclr #2,$40(a6) clr.b $24(a6) rts</asm> The first instruction clears the fade out mute bit, which was set by the "E4" flag (fade out into previous track), the second instruction clears the fading out flag (telling the system that fading out is complete), the "key on" values are resumed by the system already through natural course, all that's left is to reset the L/R/AMS/FMS of FM6 on the YM2612, like so:

<asm>loc_726D6: bclr #2,$40(a6) clr.b $24(a6) tst.b $40(a6) ; is the DAC channel running? bpl.s Resume_NoDAC ; if not, branch moveq #$FFFFFFB6,d0 ; prepare FM channel 3/6 L/R/AMS/FMS address move.b $4A(a6),d1 ; load DAC channel's L/R/AMS/FMS value jsr sub_72764(pc) ; write to FM 6

Resume_NoDAC: rts</asm> Now the DAC channel will resume correctly after fade in.