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:

SegaPCM:	incbin	sound\segapcm.bin
		even

now change it to this

SegaPCM:	incbin	sound\segapcm.bin
SegaPCM_end:	even

Then go to Sound_E1, and replace all of this code:

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

With this:

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

Also you can change

move.w	#$14,d0

to get different sample rates. For example replacing 14 with 45 the PCM will play as 8khz.

GitHub disassembly continuation

If you are using the latest version of the GitHub 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:

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:

to this:

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:

or else you'll get an error when building.

|Fix the SEGA Sound]]