Synthesis

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

Harmonics

• 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"

Envelope

• 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

Filtering

• 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

Noise

• 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

Effects

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

• Reverb and delay effects are particularly common

Controllers

• 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