Pulse Width Modulation

Overview

A square wave at 50% duty cycle sounds hollow and thin. Change that ratio—make the high portion wider or narrower—and the harmonic content shifts dramatically. The SID chip could sweep through duty cycles in real-time, creating dynamic, living sounds from simple waveforms.

Understanding duty cycle

Duty cycle	Sound character
12.5%	Thin, nasal, clarinet-like
25%	Hollow, woody
50%	Pure square, hollow
75%	Same as 25% (inverted)

The duty cycle determines which harmonics are present:

50%: Only odd harmonics (1, 3, 5, 7…)
Other ratios: Mix of odd and even

SID implementation

Register layout

Register	Function
$D402/03	Voice 1 pulse width (12-bit)
$D409/0A	Voice 2 pulse width
$D410/11	Voice 3 pulse width

Setting pulse width

    lda #$00
    sta $d402        ; Low byte
    lda #$08         ; 50% duty cycle ($800 = 2048)
    sta $d403        ; High byte

Sweeping (PWM)

pulse_sweep:
    lda pulse_lo
    clc
    adc #$10         ; Increment
    sta pulse_lo
    sta $d402
    lda pulse_hi
    adc #$00
    and #$0f         ; Keep in range
    sta pulse_hi
    sta $d403
    rts

Musical applications

Brass sounds

Fast PWM sweep creates brassy attack:

Attack:  Sweep 12% → 50% quickly
Sustain: Hold at 50% or slow sweep

String pads

Slow, subtle PWM creates warmth:

Continuous slow sweep: 25% ↔ 50% over 2-4 seconds

Lead sounds

Moderate PWM adds movement and interest to melodic lines.

NES pulse channels

The NES has fixed duty cycle options:

Setting	Duty cycle
0	12.5%
1	25%
2	50%
3	75%

No continuous sweep—must switch between presets.

Tracker notation

In SID trackers:

Pulse width command: Pxy
x = direction (0=down, 1=up)
y = speed

Combining with other effects

PWM layers beautifully with:

Combination	Result
PWM + Filter sweep	Rich, evolving pads
PWM + Vibrato	Organic lead sounds
PWM + Arpeggio	Shimmering chords

The “SID sound”

PWM is central to the distinctive SID character:

Rob Hubbard’s brass stabs
Martin Galway’s warm pads
Tim Follin’s aggressive leads

Shaping square waves