Generative Synthesis

  • Idea: Use simple waveforms and filters to generate complex sounds.

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

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

Envelope: Attack, Decay, Sustain, Release

  • ADSR model is standard envelope description

    • Attack: short ramp-up at start of note

    • Decay: short ramp-down just after attack

    • Sustain: constant hold level when decay is complete

    • Release: short ramp-down after key is released

  • Remember, volume is in dB: these ramps should be log-scaled

  • Often linear ramps, sometimes smoothed

  • Used in pretty much every kind of synthesis

Tremolo, Vibrato

  • Both desirable effects: modulate Voltage Controlled Oscillator ("VCO") with Low Frequency Oscillator ("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: Voltage Controlled Filter ("VCF"). Often bandpass, used to get resonance effects etc

  • Filter often controlled by LFO and/or 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 pushpads 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: Saturday, 2 May 2020, 4:33 PM