processor 6502

;;;
;;; Apple ][ Delta Modulation Sample Playback
;;;
;;; This is the same technique used in the NES
;;; for DMC samples, but adapted for the Apple ][.
;;; Basically: Every 1 bit increases the signal
;;; by 1 step, every 0 bit decreases by 1 step.
;;; The signal is clipped on the top and bottom.
;;;
;;; The NES has a 6-bit DAC, but we only have a
;;; PWM-encoded signal. We have 13 different
;;; routines to generate pulses ranging from 4 to
;;; 14 microseconds long, with a 29-30 microsecond
;;; interval (when we reach 50% duty cycle we just
;;; invert the signal)
;;;
;;; Every 8 bits we have to fetch a new byte
;;; and increment the pointer. To avoid a high-
;;; frequency tone, we generate another pulse
;;; (the same width as the last) in this macro.
;;;
;;; also see http://michaeljmahon.com/RTSynth.html
;;; and https://github.com/oliverschmidt/Play-BTc
;;;

; uninitialized zero-page variables
	seg.u ZEROPAGE	
	org $0
PWMBUF	.ds 2
PWMLEN  .ds 1
TRASH	.ds 1

; speaker toggle
SPKR    equ $c030

; max pulse width = PULWID*2+4 cycles
PULWID = 6

; sleep {1} cycles
; must be multiple of 2
 MAC SLEEP2
.CYCLES     SET {1}
                IF .CYCLES == 1
                    ECHO "MACRO ERROR: 'SLEEP': Duration must be > 1"
                    ERR
                ENDIF
                IF .CYCLES >= 12
                    jsr HandyRTS
.CYCLES             SET .CYCLES - 12
                ENDIF
                REPEAT .CYCLES / 2
                    nop
                REPEND
            ENDM
 ENDM

; pulse macro
; flip speaker, wait N*2 cycles
; flip again, wait (MAX-N)*2 cycles
; total = 8 + PULWID*2 cycles
 MAC PULSE
 sta SPKR
 SLEEP2 {1}*2
 sta SPKR
 SLEEP2 (PULWID-{1})*2
 ENDM

; shift next bit into carry flag
; load next byte and increment
; pointer if needed
; {1} : index of previous routine
; {2} : index of current routine
; we also interleave a speaker pulse in this routine
; using macros (these are the IF/ENDIF blocks)
 MAC NEXTPULSE
 lsr		; next bit -> carry flag
 dex		; bit count == 0?
 bne .noinc	; skip inc/reload cycle
                ; total pulse = 15 + PULWID*2 cycles
   IF {2} > 0
     sta SPKR
   ELSE
     SLEEP2 8   ; no SPKR toggle, take up 8 cycles
   ENDIF
   IF {2} == 1
     sta SPKR
   ENDIF
 
; increment pointer lo byte
 iny		; 2-1 = 1
 
   IF {2} == 2
     sta SPKR
   ENDIF
 
 bne .noinchi	; 4
; increment pointer hi byte
 inc PWMBUF+1
 dec PWMLEN
 bne .noinchi
 rts		; end of sound data
.noinchi

   IF {2} == 3
     sta SPKR
   ENDIF
 
; reset X counter
 ldx #8		; 6
 
   IF {2} == 4 || {2} == 5
     sta SPKR
   ENDIF
 
; fetch new byte
 lda (PWMBUF),y ; 11
 
  ; a few NOPs to match the normal pulse width
  ; (inx, lsr, branch)
   IF {2} == 6
     sta SPKR
     nop
   ENDIF
   IF {2} >= 7
     nop
     sta SPKR
   ENDIF
  ; make up cycles we missed
   SLEEP2 PULWID*2-6
 
.noinc
; carry flag still valid from lsr
 bcc LVLS{1}
 ENDM

;;; start of code
	seg CODE
	org $803	; starting address

Start
        lda #>(SAMPLE_END-SAMPLES)+1
        sta PWMLEN
	lda #<SAMPLES
        sta PWMBUF
        lda #>SAMPLES
        sta PWMBUF+1
        jsr Play
        jmp Start
; handy RTS for wasting 6+6 cycles
HandyRTS
	rts

; each pulse takes PULWID*2+18(-1) cycles
; every 8 bits, the last pulse is repeated
; 1022727 / 29.5 / (9/8) = 30817 Hz
; Y = PWMBUF lo address
; PWMBUF+0 = 0
Play
	ldy PWMBUF
        lda #0
        sta PWMBUF
	ldx #8
        lda (PWMBUF),y
; each label emits a pulse at a duty cycle
; from 0 (silent) to 7
; if bit is 1, falls through to next highest level
; otherwise, jumps back to previous level
LVLS0
	SLEEP2 PULWID*2+8 ; sleep, don't click
        NEXTPULSE 0,0
LVLS1
	PULSE 0
        NEXTPULSE 0,1
LVLS2
	PULSE 1
        NEXTPULSE 1,2
LVLS3
	PULSE 2
        NEXTPULSE 2,3
LVLS4
	PULSE 3
        NEXTPULSE 3,4
LVLS5
	PULSE 4
        NEXTPULSE 4,5
LVLS6
LVLS6INV equ *+3 ; skip STA SPKR to invert signal
	PULSE 5
        NEXTPULSE 5,6
        bcs LVLS7INV ; 1 = invert signal
LVLS7
LVLS7INV equ *+3 ; skip STA SPKR to invert signal
	PULSE 4
        NEXTPULSE 6INV,5 ; 0 = invert signal
LVLS8
	PULSE 3
        NEXTPULSE 7,4
LVLS9
	PULSE 2
        NEXTPULSE 8,3
LVLS10
	PULSE 1
        NEXTPULSE 9,2
LVLS11
	PULSE 0
        NEXTPULSE 10,1
LVLS12
	SLEEP2 PULWID*2+8 ; sleep, don't click
        NEXTPULSE 11,0
        bcs LVLS12	; if 1, stay at highest level

; delta-encoded samples
SAMPLES
 .incbin "springchicken.dat12.bin" ; "Spring Chicken" by BryanTeoh
SAMPLE_END