Multi-Frequency Mixer

A multi-frequency mixer sums several simultaneous tones into one signal. Three of audio's most important phenomena — musical chords, beats, and complex timbres — live in this combination layer. Four independent oscillators is enough to cover triads, 7th chords, two-tone DTMF, beat-frequency demos, and the first four partials of a harmonic series.

Chords and intervals

A chord is two or more pitches sounding together. Western music's primary triads (major and minor) come from stacking specific musical intervals: a major triad is root + major 3rd + perfect 5th; a minor triad swaps the major 3rd for a minor 3rd. The preset list above contains the canonical chord shapes in the key of C so you can hear each one back-to-back. Switch the waveform from sine to sawtooth to hear how the same notes acquire timbre — more harmonics, brighter sound.

Beat frequencies

Two close-but-not-equal tones produce beats: a slow amplitude modulation at the difference frequency. Sum 440 Hz and 442 Hz and the combined waveform "breathes" at 2 Hz — you can see this clearly on the waveform plot. Beats are the basis of how piano tuners tune in unison (slowing the beat until it stops) and how psychoacoustic phenomena like binaural beats are constructed.

DTMF telephone tones

A telephone keypad encodes each digit as the sum of two specific sine tones — one from a "row" frequency (697 / 770 / 852 / 941 Hz) and one from a "column" (1209 / 1336 / 1477 / 1633 Hz). When you press "5", the phone sends 770 + 1336 Hz simultaneously; the central office decodes by FFT. Apply the DTMF presets to hear and visualise these classic two-tone signals.

Harmonic series

The harmonic series at a fundamental f₀ consists of integer multiples: f₀, 2f₀, 3f₀, 4f₀, etc. Acoustic instruments produce close approximations of this series (with each harmonic at a slightly different amplitude). The "Harmonic series @ 110 Hz" preset puts the first 4 partials in your ears at equal amplitude — the synthetic timbre that results is the building block of all additive synthesis.

Reading the plots

• Time-domain waveform — the analyser's last 4096 samples of the combined output. For two close frequencies you'll see the slow beat envelope; for a major triad you'll see a complex periodic pattern that repeats once per LCM of the periods.
• Spectrum — log-frequency bars showing the FFT peak per bar. Each unmuted oscillator's fundamental shows as a tall bar; if you use a non-sine waveform you'll also see that oscillator's harmonics at integer multiples.