A Music Engine

Overview

One channel plays one note; a sound effect makes it move. Music is the step up: several channels at once, each handed a fresh note on a regular beat. On the NES that beat is already ticking — the NMI fires every frame, 60 times a second on NTSC. Count those frames into rows, and on each row look up a timer value per channel from a note table and write it. Pulse 1 takes the lead, pulse 2 the harmony, the triangle the bass; the noise channel drops in drums. That loop — a tick, a pattern, a note table — is every NES sound driver in miniature.

Code

; =============================================================================
; A MUSIC ENGINE - NES (APU)
; music_tick, called once per NMI, steps a pattern and feeds three voices
; =============================================================================

APUSTATUS  = $4015
PULSE1_VOL = $4000
PULSE1_LO  = $4002
PULSE1_HI  = $4003
PULSE2_VOL = $4004
PULSE2_LO  = $4006
PULSE2_HI  = $4007
TRI_LINEAR = $4008
TRI_LO     = $400A
TRI_HI     = $400B

TEMPO       = 8         ; frames per row (60 / 8 ~ 7.5 rows a second)
PATTERN_LEN = 8

.segment "ZEROPAGE"
mus_row:   .res 1
mus_timer: .res 1

.segment "CODE"

music_init:
    lda #%00000111         ; pulse 1, pulse 2, triangle enabled
    sta APUSTATUS
    lda #%10111111         ; 50% duty, sustained, full volume
    sta PULSE1_VOL
    sta PULSE2_VOL
    lda #%11111111         ; triangle: control set -> plays continuously
    sta TRI_LINEAR
    lda #0
    sta mus_row
    lda #TEMPO
    sta mus_timer
    rts

; --- Call once per NMI; a new row lands only every TEMPO frames ---
music_tick:
    dec mus_timer
    beq @row
    rts                    ; between rows — hold the current notes
@row:
    lda #TEMPO
    sta mus_timer
    ldx mus_row

    ldy pat_p1,x           ; lead: note index for this row (0 = hold)
    beq @p2
    lda note_lo,y
    sta PULSE1_LO
    lda note_hi,y
    sta PULSE1_HI          ; writing $4003 retriggers the note
@p2:
    ldy pat_p2,x           ; harmony
    beq @tri
    lda note_lo,y
    sta PULSE2_LO
    lda note_hi,y
    sta PULSE2_HI
@tri:
    ldy pat_tri,x          ; bass — same table, an octave lower (triangle /32)
    beq @step
    lda note_lo,y
    sta TRI_LO
    lda note_hi,y
    sta TRI_HI
@step:
    inx
    cpx #PATTERN_LEN
    bne @save
    ldx #0                 ; loop the pattern
@save:
    stx mus_row
    rts

.segment "RODATA"
; Pulse timer values (NTSC), 11-bit split lo/hi. Index 0 = rest/hold.
;                 C-4   E-4   G-4
note_lo: .byte 0, $AA,  $52,  $1C
note_hi: .byte 0, $01,  $01,  $01

; The pattern: one note index per channel, per row.
pat_p1:  .byte 1, 2, 3, 2,  1, 2, 3, 2   ; arpeggiated lead
pat_p2:  .byte 3, 3, 1, 1,  3, 3, 1, 1   ; harmony
pat_tri: .byte 1, 0, 1, 0,  1, 0, 1, 0   ; bass on the beat

Trade-offs

Aspect	Cost
CPU	A few bytes of work most frames; a little more on a row change
Memory	The note table, the patterns, two zero-page bytes
Limitation	Five voices total, and an effect must borrow one from the song

When to use: Any in-game music. This is how NES music works — every driver is this shape with more on top.

When to avoid: A single reactive bleep; that’s a sound effect, not a sequencer.

The tick is the tempo

The engine keeps no clock of its own — it rides the frame. Each NMI decrements mus_timer; only when it hits zero does a new row land, and TEMPO sets how many frames that takes. TEMPO 8 gives ≈7.5 rows a second; drop it to 6 and the song speeds up. Real drivers count rows into a song through an order list and run effects — vibrato, slides, arpeggios — on the in-between frames, but the heartbeat is always this: one NMI, one tick.

One table, two octaves

The lead and harmony are pulse channels; the bass is the triangle — yet they share one note table. That’s a gift of the hardware: the triangle divides its timer by 32 where the pulse divides by 16, so the same timer value sounds exactly an octave lower on the triangle. Write $1AA to a pulse channel for C-4 and to the triangle for C-3, no second table needed. The 0 index is the other convention worth keeping — a row of 0 means “leave this channel alone”, so a voice holds its note across rows instead of restriking every beat.

From skeleton to driver

A shipping sound engine adds three things to this frame:

• Instruments — a volume envelope and duty per note, rewritten each row, so a channel can pluck, swell or fade rather than blare one flat tone.
• Drums — the noise channel triggered from its own pattern row; it takes a period index, not a note.
• An order list — a sequence of pattern numbers, so a song reuses a handful of short patterns instead of storing every bar.

Add those and a sound-effect routine that borrows a channel from the music, and you have a driver like FamiTone or the ones behind the NES’s best scores.

Related

Patterns: Sound Effects, A Square Wave, NMI Game Loop

Vault: APU | Nintendo Entertainment System