Arpeggio

Overview

When a sound chip can only play one note per channel, how do you play chords? Arpeggios cycle through chord tones faster than the ear can separate them, creating the perception of multiple simultaneous notes. This technique became the signature sound of 8-bit music.

The technique

Basic concept

Instead of C-E-G simultaneously:

Frame 1: C
Frame 2: E
Frame 3: G
Frame 4: C (repeat)

At 50/60 Hz, cycling every frame creates a ~17-20 Hz arpeggio rate.

Speed considerations

Rate	Effect
Slow (every 3-4 frames)	Audible as separate notes
Medium (every 2 frames)	Classic chip sound
Fast (every frame)	Smooth chord perception

Chord types

Common arpeggio patterns:

Chord	Notes	Semitone offset
Major	C-E-G	0-4-7
Minor	C-Eb-G	0-3-7
Diminished	C-Eb-Gb	0-3-6
Major 7th	C-E-G-B	0-4-7-11

Implementation

SID (C64)

Software-driven or using sound driver:

arpeggio_offsets:
    .byte 0, 4, 7        ; major chord

update_arpeggio:
    ldx arp_position
    lda arpeggio_offsets,x
    ; Add to base note
    ; Update frequency register

    inx
    cpx #3
    bcc .no_wrap
    ldx #0
.no_wrap:
    stx arp_position

Tracker notation

FamiTracker/ProTracker effect command:

0xy - Arpeggio
x = first semitone offset
y = second semitone offset

Example: 047 = major chord (0, +4, +7 semitones)

Musical uses

Application	Purpose
Chords	Harmonic support
Lead decoration	Add interest to melody
Bass lines	Implied harmony
Rhythmic drive	Creates momentum

Variations

Direction

Up (C-E-G-C-E-G…)
Down (G-E-C-G-E-C…)
Up-down (C-E-G-E-C-E-G…)

Speed changes

Varying arpeggio speed within a note creates expression.

Combined with effects

Arpeggios layered with:

Volume envelope
Pulse width modulation
Vibrato
Filter sweeps

By platform

SID advantages

Filter can smooth arpeggio
Ring modulation adds harmonics
Three voices allow chord layers

NES approach

Clean, sharp arpeggios
Two pulse channels for thick chords
Triangle for bass arpeggios

AY-3-8910

Bright, cutting arpeggios
Hardware envelope can interact
Distinct character

Composition tips

Don’t overuse (can become tiring)
Match arpeggio speed to tempo
Consider root motion
Use to add energy at key moments

Chords from a single channel