gibberish-dsp/js/oscillators/fmfeedbackosc.js

Gibberish is designed to be an optimized API for audio synthesis using per-sample techniques.

let g = require( 'genish.js' )

let feedbackOsc = function( frequency, filter, pulsewidth=.5, argumentProps ) {
  if( argumentProps === undefined ) argumentProps = { type: 0 }

  let lastSample = g.history(),
      // determine phase increment and memoize result
      w = g.memo( g.div( frequency, g.gen.samplerate ) ),
      // create scaling factor
      n = g.sub( -.5, w ),
      scaling = g.mul( g.mul( 13, filter ), g.pow( n, 5 ) ),
      // calculate dc offset and normalization factors
      DC = g.sub( .376, g.mul( w, .752 ) ),
      norm = g.sub( 1, g.mul( 2, w ) ),
      // determine phase
      osc1Phase = g.accum( w, 0, { min:-1 }),
      osc1, out

  // create current sample... from the paper:
  // osc = (osc + sin(2*pi*(phase + osc*scaling)))*0.5f;
  osc1 = g.memo( 
    g.mul(
      g.add(
        lastSample.out,
        g.sin(
          g.mul(
            Math.PI * 2,
            g.memo( g.add( osc1Phase, g.mul( lastSample.out, scaling ) ) )
          )
        )
      ),
      .5
    )
  )

  // store sample to use as modulation
  lastSample.in( osc1 )

  // if pwm / square waveform instead of sawtooth...
  if( argumentProps.type === 1 ) { 
    const lastSample2 = g.history() // for osc 2
    const lastSampleMaster = g.history() // for sum of osc1,osc2

    const osc2 = g.mul(
      g.add(
        lastSample2.out,
        g.sin(
          g.mul(
            Math.PI * 2,
            g.memo( g.add( osc1Phase, g.mul( lastSample2.out, scaling ), pulsewidth ) )
          )
        )
      ),
      .5
    )

    lastSample2.in( osc2 )
    out = g.memo( g.sub( lastSample.out, lastSample2.out ) )
    out = g.memo( g.add( g.mul( 2.5, out ), g.mul( -1.5, lastSampleMaster.out ) ) )
    
    lastSampleMaster.in( g.sub( osc1, osc2 ) )

  }else{
     // offset and normalize
    osc1 = g.add( g.mul( 2.5, osc1 ), g.mul( -1.5, lastSample.out ) )
    osc1 = g.add( osc1, DC )
 
    out = osc1
  }

  return g.mul( out, norm )
}

module.exports = feedbackOsc