-Added SidechainGain class

src/dsp/dsp_sidechain_gain.cxx

@@ -0,0 +1,83 @@
+
+#include "dsp_sidechain_gain.hxx"
+
+// for pow()
+#include <cmath>
+
+SidechainGain::SidechainGain(int rate) :
+  /// 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[0];
+  
+  /// 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;
+}

src/dsp/dsp_sidechain_gain.hxx

@@ -0,0 +1,49 @@
+
+#ifndef OPENAV_DSP_SIDECHAIN_GAIN_H
+#define OPENAV_DSP_SIDECHAIN_GAIN_H
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+class SidechainGain
+{
+  public:
+    SidechainGain(int rate);
+    ~SidechainGain(){}
+    
+    void threshold(float t){controlThreshold = t;}
+    void reduction(float r){controlReduction = r;}
+    void time     (float t){controlReleaseTime = t;}
+    
+    void process(unsigned int n_samples, float** inputs, float** outputs);
+  
+  private:
+    /// audio buffers
+    float* audioInputL;
+    float* audioInputR;
+    float* audioSidechain;
+    float* audioOutputL;
+    float* audioOutputR;
+    
+    /// control signals
+    float controlThreshold;
+    float controlReduction;
+    float controlReleaseTime;
+    float controlSidechainAmp;
+    
+    /// filter state
+    float w, a, b, g1, g2;
+    
+    /// last peak history
+    bool nowIsAPeak;
+    long peakFrameCounter;
+    
+    /// nframes available for countdown
+    long peakCountDuration;
+    
+    /// control output
+    float currentTarget;
+};
+
+#endif // OPENAV_DSP_SIDECHAIN_GAIN_H

src/jack.hxx

@@ -20,6 +20,7 @@
 
 #include "dsp/dsp_reverb.hxx"
 #include "dsp/dsp_dbmeter.hxx"
+#include "dsp/dsp_sidechain_gain.hxx"
 
 using namespace std;
 

wscript

@@ -39,6 +39,8 @@ def build(bld):
              'src/controller/apc.cxx',
              'src/controller/gui.cxx',
              
+             'src/dsp/dsp_sidechain_gain.cxx',
+             
              'src/eventhandlergui.cxx',
              'src/eventhandlerdsp.cxx']