Loopp/src/dsp/dsp_sidechain_gain.cxx

#include "dsp_sidechain_gain.hxx"

// for pow()
#include <cmath>

SidechainGain::SidechainGain(int rate) :
  /// initial control values
  controlThreshold(0.2),
  controlReduction(1),
  controlReleaseTime(0.5),
  
  /// filter state init
  w(10.0f / (rate * 0.02)),
  a(0.07f),
  b(1.0f / (1.0f - a)),
  g1(0.0f),
  g2(0.0f),
  
  
  peakFrameCounter(0),
  peakCountDuration( rate / 4 ),
  currentTarget(0)
{
}

void SidechainGain::process(unsigned int n_samples, float** inputs, float** outputs)
{
  /// audio inputs
  float* inL  = inputs[0];
  float* inR  = inputs[1];
  float* side = inputs[2];
  float* outL = outputs[0];
  float* outR = outputs[1];
  
  /// control inputs
  float threshold   = controlThreshold;
  float reduction   = controlReduction;
  float releaseTime = controlReleaseTime;
  
  
  /// analyse sidechain input for peak
  float sum = 0.f;
  for( unsigned int i = 0; i < n_samples; i++ )
  {
    if ( *side > 0.000001 )
      sum += *side++;
    else
      sum += -*side++;
  }
  
  currentTarget = 0.f;
  
  /// check for peak level (offset to avoid "always on" peak)
  if ( sum / n_samples > threshold + 0.05 )
  {
    peakFrameCounter = peakCountDuration * releaseTime;
    currentTarget = 1.f - reduction;
  }
  else if ( peakFrameCounter < 0 )
  {
    currentTarget = 1.f;
  }
  else
  {
    currentTarget = 1.f - reduction;
  }
  
  if ( currentTarget < 0.f )
      currentTarget = 0.f;
  
  peakFrameCounter -= n_samples;
  
  for( unsigned int i = 0; i < n_samples; i++ )
  {
    /// smoothing algo is a lowpass, to de-zip the fades
    /// x^^4 approximates linear volume increase for human ears
    g1 += w * ( pow ( currentTarget, 4.f ) - g1 - a * g2 - 1e-20f);
    g2 += w * (b * g1 - g2 + 1e-20f);
    float gain = g2;
    
    *outL++ = *inL++ * gain;
    *outR++ = *inR++ * gain;
  }
  
  /// update output value
  controlSidechainAmp = currentTarget;
}