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The 1-Bit Forum → Other Platforms → 1-bit sound on BBC Micro / Acorn Electron
The noise channel probably won't be useful since it starts on a low cycle.
However https://www.smspower.org/Development/SN76489 claims that
If the register value is zero or one then the output is a constant value of +1. This is often used for sample playback on the SN76489.
so we won't get any more volume range than we already have anyway. Might still be worth a try to see if 0 gives a better range than 1.
Unfortunately I don’t hear any difference. However, I’ve now ported the code to the Acorn Electron which only has an on/off state for sound (no volume control) and I can hear a recognisable tune - it has a slower 1MHz processor so needs some adjustment. Based on what I’m hearing there may still be some issues with the pattern parsing. I’ll look through a disassembly of XXL’s Atari Tritone code to see if there’s anything obviously different.
Posting an update as I finally managed to get the Tritone engine working for the 6502. Drums aren’t implemented, but sounds pretty good.
;Tritone beeper music engine
;Original Z80 code by Shiru 03'2011
;6502 engine port by utz 03'2024
;BBC Micro port by Negative Charge 01'2025
; Constants
OSBYTE = $FFF4
OSWRCH = $FFEE
OSNEWL = $FFE7
SHEILABASE = $FE00
SYSVIA_DDRA = SHEILABASE + $43
SYSVIA_ORAS = SHEILABASE + $4F
SYSVIA_REGB = SHEILABASE + $40
DISPLAY_START = $7c00
; Zero Page
ORG $60
GUARD $90
.vars_start
.ch0_acc_lo SKIP 1
.ch0_acc_hi SKIP 1
.ch1_acc_lo SKIP 1
.ch1_acc_hi SKIP 1
.ch2_acc_lo SKIP 1
.ch2_acc_hi SKIP 1
.ch0_div_lo SKIP 1
.ch0_div_hi SKIP 1
.ch1_div_lo SKIP 1
.ch1_div_hi SKIP 1
.ch2_div_lo SKIP 1
.ch2_div_hi SKIP 1
.ch0_duty SKIP 1
.ch1_duty SKIP 1
.ch2_duty SKIP 1
.temp_output SKIP 1
.loop_ptr SKIP 2
.pattern_ptr SKIP 2
.order_ptr SKIP 2
.drum SKIP 1
.row_length SKIP 2
.vars_end
ORG &1900
GUARD DISPLAY_START
.start
MACRO sound_write_slow
sta SYSVIA_ORAS
lda #%00000000
sta SYSVIA_REGB
nop:nop:nop
lda #%00001000
sta SYSVIA_REGB
ENDMACRO
MACRO RESET_SOUND_CHIP
lda #%11111111
sound_write_slow
lda #%11011111
sound_write_slow
lda #%10111111
sound_write_slow
lda #%10011111
sound_write_slow
ENDMACRO
.init
lda #%11111111
sta SYSVIA_DDRA
sei
RESET_SOUND_CHIP
lda #%10000001
sound_write_slow
lda #%00000000
sound_write_slow
lda #%10100001
sound_write_slow
lda #%00000000
sound_write_slow
lda #%11000001
sound_write_slow
lda #%00000000
sound_write_slow
lda #%00000000
sta SYSVIA_REGB
lda #LO(music_data)
ldx #HI(music_data)
.play
pha
txa
pha
; Clear ZP
lda #0
ldx #vars_end-vars_start
.clear_loop
sta vars_start,x
dex
bpl clear_loop
pla
sta pattern_ptr+1
pla
sta pattern_ptr+0
lda pattern_ptr+0
clc
adc #2
sta loop_ptr+0
lda pattern_ptr+1
adc #0
sta loop_ptr+1
.play_loop
lda pattern_ptr+0
sta order_ptr+0
lda pattern_ptr+1
sta order_ptr+1
ldy #0
lda (pattern_ptr),y
tax
iny
lda (pattern_ptr),y
bne no_loop
cpx #0
bne no_loop
.return_loop
lda loop_ptr+0
sta pattern_ptr+0
lda loop_ptr+1
sta pattern_ptr+1
jmp play_loop
.no_loop
; Set pattern_ptr to pattern data
sta pattern_ptr+1
stx pattern_ptr+0
; Read speed from pattern
ldy #0
lda (pattern_ptr),y
sta row_length+0
iny
lda (pattern_ptr),y
sta row_length+1
lda pattern_ptr+0
adc #1
sta pattern_ptr+0
bcc row
inc pattern_ptr+1
jmp row
.pattern_end
lda order_ptr+0
clc
adc #2
sta pattern_ptr+0
lda order_ptr+1
adc #0
sta pattern_ptr+1
jmp play_loop
.row
ldy #0
lda (pattern_ptr),y
iny
cmp #255
beq pattern_end
cmp #128
bcs ch0
cmp #2
bcc ch0
sta drum
lda (pattern_ptr),y
iny
.ch0
cmp #1
beq skip_ch0
cmp #2
bcs ch0_note
sta ch0_duty
sta ch0_div_lo
sta ch0_div_hi
jmp skip_ch0
.ch0_note
tax
and #$0f
sta ch0_div_hi
txa
and #$f0
sta ch0_duty
lda (pattern_ptr),y
iny
sta ch0_div_lo
.skip_ch0
lda (pattern_ptr),y
iny
cmp #1
beq skip_ch1
cmp #2
bcs ch1_note
sta ch1_duty
sta ch1_div_lo
sta ch1_div_hi
jmp skip_ch1
.ch1_note
tax
and #$0f
sta ch1_div_hi
txa
and #$f0
sta ch1_duty
lda (pattern_ptr),y
iny
sta ch1_div_lo
.skip_ch1
lda (pattern_ptr),y
iny
cmp #1
beq skip_ch2
cmp #2
bcs ch2_note
sta ch2_duty
sta ch2_div_lo
sta ch2_div_hi
jmp skip_ch2
.ch2_note
tax
and #$0f
sta ch2_div_hi
txa
and #$f0
sta ch2_duty
lda (pattern_ptr),y
iny
sta ch2_div_lo
.skip_ch2
tya
clc
adc pattern_ptr+0
sta pattern_ptr+0
bcc setup_play_loop
inc pattern_ptr+1
.setup_play_loop
lda row_length+0
ora row_length+1
beq row_finished
ldx row_length+0
ldy row_length+1
.play_note
; CH0
lda ch0_acc_lo ; 3
adc ch0_div_lo ; 3
sta ch0_acc_lo ; 3
lda ch0_acc_hi ; 3
adc ch0_div_hi ; 3
sta ch0_acc_hi ; 3
cmp ch0_duty ; 3 - carry set if acc_hi >= duty
lda #$00 ; 2
sbc #$00 ; 2 - A = $00 if carry, $FF if !carry
sta temp_output ; 3
; CH1
lda ch1_acc_lo ; 3
adc ch1_div_lo ; 3
sta ch1_acc_lo ; 3
lda ch1_acc_hi ; 3
adc ch1_div_hi ; 3
sta ch1_acc_hi ; 3
cmp ch1_duty ; 3
lda #$00 ; 2
sbc #$00 ; 2
ora temp_output ; 3 - mix CH0 | CH1
sta temp_output ; 3
; CH2
lda ch2_acc_lo ; 3
adc ch2_div_lo ; 3
sta ch2_acc_lo ; 3
lda ch2_acc_hi ; 3
adc ch2_div_hi ; 3
sta ch2_acc_hi ; 3
cmp ch2_duty ; 3
lda #$00 ; 2
sbc #$00 ; 2
ora temp_output ; 3 - final mix (A = CH0 | CH1 | CH2)
bne play_sound ; 2/3
lda #$9F ; 2 - silent
bne play_output ; 3 (always branches)
.play_sound
lda #$90 ; 2 - sound
.play_output
sta SYSVIA_ORAS ; 4
dex ; 2
bne play_note ; 3
dey ; 2
bne play_note ; 3
.row_finished
jmp row
INCLUDE "tracks\lb_zx_tritone.6502"
.end
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