SID Note Trigger

Overview

Trigger a note by setting frequency, waveform, and ADSR envelope, then toggling the gate bit. The gate-on starts the attack phase; gate-off triggers release. This is the core technique for all SID sound — music and effects both build on this foundation.

Code

; =============================================================================
; SID NOTE TRIGGER
; Play a single note on voice 1
; Taught: Game 6 (SID Symphony), Unit 1
; CPU: ~50 cycles | Memory: ~60 bytes
; =============================================================================

SID_BASE    = $D400

; Voice 1 registers
SID_V1_FREQ_LO  = SID_BASE + $00
SID_V1_FREQ_HI  = SID_BASE + $01
SID_V1_PW_LO    = SID_BASE + $02
SID_V1_PW_HI    = SID_BASE + $03
SID_V1_CTRL     = SID_BASE + $04
SID_V1_AD       = SID_BASE + $05
SID_V1_SR       = SID_BASE + $06

SID_VOLUME      = SID_BASE + $18

; Control register bits
GATE_ON     = %00000001
GATE_OFF    = %00000000
WAVE_PULSE  = %01000000
WAVE_SAW    = %00100000
WAVE_TRI    = %00010000
WAVE_NOISE  = %10000000

; Initialise SID for playing notes
init_sid:
    lda #$0F            ; Max volume
    sta SID_VOLUME

    lda #$08            ; Pulse width 50%
    sta SID_V1_PW_HI
    lda #$00
    sta SID_V1_PW_LO

    lda #$00            ; Attack=0, Decay=0 (instant)
    sta SID_V1_AD
    lda #$F9            ; Sustain=15, Release=9
    sta SID_V1_SR
    rts

; Play a note
; Input: A = frequency high byte, X = frequency low byte
play_note:
    stx SID_V1_FREQ_LO
    sta SID_V1_FREQ_HI
    lda #WAVE_PULSE | GATE_ON
    sta SID_V1_CTRL     ; Gate on - note starts
    rts

; Release the note (start release phase)
release_note:
    lda #WAVE_PULSE | GATE_OFF
    sta SID_V1_CTRL     ; Gate off - release begins
    rts

Usage:

    jsr init_sid

    ; Play middle C (C4 = 261.63 Hz on PAL = $1167)
    lda #$11            ; Frequency high
    ldx #$67            ; Frequency low
    jsr play_note

    ; ... wait for note duration ...

    jsr release_note    ; Let note fade out

Trade-offs

Aspect	Cost
CPU	~50 cycles to trigger
Memory	~60 bytes
Limitation	Single voice; no music sequencing

When to use: Sound effects, learning SID basics, simple audio feedback.

When to avoid: Full music playback (use a music driver instead).

SID rate tables

The 4-bit Attack, Decay, and Release values map to durations defined in the SID datasheet. Release rates are slower than attack rates by roughly 3× for the same value:

Value	Attack	Decay / Release
$0	2 ms	6 ms
$1	8 ms	24 ms
$2	16 ms	48 ms
$3	24 ms	72 ms
$4	38 ms	114 ms
$5	56 ms	168 ms
$6	68 ms	204 ms
$7	80 ms	240 ms
$8	100 ms	300 ms
$9	250 ms	750 ms
$A	500 ms	1.5 s
$B	800 ms	2.4 s
$C	1 s	3 s
$D	3 s	9 s
$E	5 s	15 s
$F	8 s	24 s

So Release=9 used in init_sid above gives a ~750 ms tail — long enough to hear the note fade after release_note.

Frequency formula

For any note, the 16-bit SID frequency value is:

FREQ = Hz × 16777216 / clock

Where clock is 985,248 Hz (PAL) or 1,022,727 Hz (NTSC). So C4 (261.63 Hz) on PAL is 261.63 × 16777216 / 985248 ≈ 4455 = $1167.

Related

Vault: The SID Chip | Commodore 64

Evolution: Note Trigger → Envelope Shapes → Music Driver