SCHG How-to:Fix the SEGA Sound

(Original guide by Puto)

Let's fix a problem that causes a great deal of pain to anyone adding code to Sonic 1: The SEGA sound.

The bug

The bug is when sometimes when you build the ROM, at the very end of the "SEGA" sound is a static noise (probably the bat sound) so it sounds like, "SEGA-TS" . There is also another one, which sounds like "THOUGNGH-THOUGNGH" .So anyway, lets fix this!

Fixing the bug

Go to "segapcm:" and you should see: <asm> SegaPCM: incbin sound\segapcm.bin even </asm> now change it to this <asm> SegaPCM: incbin sound\segapcm.bin SegaPCM_end: even </asm> Then go to Sound_E1, and replace this: <asm>Sound_E1: ; XREF: Sound_ExIndex move.b #$88,($A01FFF).l move.w #0,($A11100).l ; start the Z80 move.w #$11,d1 loc_71FC0: move.w #-1,d0 loc_71FC4: nop dbf d0,loc_71FC4 dbf d1,loc_71FC0 addq.w #4,sp rts</asm>

With this: <asm>Sound_E1: lea (SegaPCM).l,a2 ; Load the SEGA PCM sample into a2. It's important that we use a2 since a0 and a1 are going to be used up ahead when reading the joypad ports move.l #(SegaPCM_End-SegaPCM),d3 ; Load the size of the SEGA PCM sample into d3 move.b #$2A,($A04000).l ; $A04000 = $2A -> Write to DAC channel PlayPCM_Loop: move.b (a2)+,($A04001).l ; Write the PCM data (contained in a2) to $A04001 (YM2612 register D0) move.w #$14,d0 ; Write the pitch ($14 in this case) to d0 dbf d0,* ; Decrement d0; jump to itself if not 0. (for pitch control, avoids playing the sample too fast) sub.l #1,d3 ; Subtract 1 from the PCM sample size beq.s return_PlayPCM ; If d3 = 0, we finished playing the PCM sample, so stop playing, leave this loop, and unfreeze the 68K lea ($FFFFF604).w,a0 ; address where JoyPad states are written lea ($A10003).l,a1 ; address where JoyPad states are read from jsr (Joypad_Read).w ; Read only the first joypad port. It's important that we do NOT do the two ports, we don't have the cycles for that btst #7,($FFFFF604).w ; Check for Start button bne.s return_PlayPCM ; If start is pressed, stop playing, leave this loop, and unfreeze the 68K bra.s PlayPCM_Loop ; Otherwise, continue playing PCM sample return_PlayPCM: addq.w #4,sp rts</asm> Also you can change <asm>move.w #$14,d0</asm> to get different sample rates. For example replacing 14 with 45 the PCM will play as 8khz.

Mercurial disassembly continuation

If you are using the latest version of the Mercurial community disassembly, the Sega PCM will always work and you don't need to apply the above fix.

If, however, you want the ability to stop the Sega sound by pressing start, then you will need to perform the steps on the previous section. You may skip the first part (defining SegaPCM_end) as it is already done; on the remainder, search for PlaySega instead of Sound_E1. At this point, the Sonic 1 z80 sound driver does not allow interrupting the Sega PCM, which is why that is needed. In addition, you will have to go to the ReadJoypads function and change this: <asm>ReadJoypads: ; XREF: VBlank, HBlank lea (v_jpadhold1).w,a0 ; address where joypad states are written lea ($A10003).l,a1 ; first joypad port bsr.s @read ; do the first joypad addq.w #2,a1 ; do the second joypad

@read:</asm> to this: <asm>ReadJoypads: ; XREF: VBlank, HBlank lea (v_jpadhold1).w,a0 ; address where joypad states are written lea ($A10003).l,a1 ; first joypad port bsr.s Joypad_Read ; do the first joypad addq.w #2,a1 ; do the second joypad

Joypad_Read:</asm>

or else you'll get an error when building.