Additive Synthesis

Additive and Subtractive Synthesis

  • Idea: Instead of using a wavetable, use "principled" sound generators

  • A pipe organ is an additive synthesizer; the electric organ builds on that idea

  • Remember, any periodic sound can be represented as the sum of sinusoids

  • But that isn't too practical, so tricks are used


  • Recall that a distorted sinusoid has "harmonics": multiples of the fundamental frequency

    • Square wave has strong odd harmonics

    • Triangle, various other waves have odd harmonics, sometimes even harmonics

  • Organ plan: Make power-of-two pipe lengths for each note ("ranks") with adjustable volume

  • But start with distorted sinusoids so even more harmonic content

  • Can add non-octave multiples for even more interesting sounds

  • Electric organ "oscillators"; synthesizer "VCO"


  • Organ only manages quickish attack to sustain level, quickish release

  • Probably want an ADSR envelope (at least) so that sounds can have attack or decay

  • May want each note to have multiple envelopes to control other things

Tremolo, Vibrato

  • Both desirable effects: modulate VCO with "LFO"

    • Tremolo: modulate amplitude

    • Vibrato: modulate frequency


  • Sounds have different character depending on harmonic content

  • May want global filter (lowpass, bandpass, fancy) to control overall sound shape

  • May want per-note filters that track the note: "VCF". Typically bandpass, used to get resonance effects etc

  • Filter may be modded with LFO, ADSR


  • Adding filtered noise is good

  • Several kinds and scales would be nice: white noise, scalable max frequency

  • Use very slow noise as a control to modulate pitch etc


  • Almost any of the effects we discussed are nice additions to a synth

  • Reverb and delay effects are particularly common


  • Big strength of additive synthesis: be able to turn the knobs (you have knobs, right?) in real-time to modulate the sound

  • This is why most MIDI keyboards have a pitch wheel and mod wheel: ideally the pitch wheel can be used for other things

  • MIDI controller boards are really common, with lots of knobs and other fancy things

Modeling Natural Sources

  • Weird sounds are great, but it's also great to imitate orchestral instruments etc

  • String "pads" are fairly easy to achieve

  • Wind instruments are a bit sketch: use filtered "breath" noise and resonant filters

  • Brass instruments are meh: they have fairly fancy changes in frequency over time. They are basically subtractive instruments

  • Plucked strings are pretty garbage, really: hard to do well

Last modified: Tuesday, 7 May 2019, 1:06 AM