Parallel resonance

src/yaw-vowel/filter.cpp

@@ -2,23 +2,58 @@
 
 void Resonator::clear()
 {
-    xp = xpp = yp = ypp = 0;
+  xp = xpp = yp = ypp = 0;
 }
 
-void Resonator::process(float* output, const float* const input, uint32_t frames)
+inline float Resonator::process(float x)
 {
-  for(uint32_t i = 0; i < frames; ++i){
-    float x = static_cast<float>(input[i] * scale);
-    float y = x - xpp - app * ypp + ap * yp;
-    output[i] = y;
-    xpp = xp;
-    xp = x;
-    ypp = yp;
-    yp = y;
-  }
+  // Apply biquad filter.
+  float y = scale * x - xpp - app * ypp + ap * yp;
+  // Update biquad memory.
+  xpp = xp;
+  xp = x;
+  ypp = yp;
+  yp = y;
+  return y;
 }
 
 void Resonator::set(double _app, double _ap, double _scale)
 {
-    app = _app; ap = _ap; scale = _scale;
+  app = _app;
+  ap = _ap;
+  scale = _scale;
+}
+
+Filter::Filter(){};
+
+void Filter::process(float **outputs, const float **inputs, uint32_t frames)
+{
+  for (uint chn = 0; chn < 2; ++chn)
+  {
+    for (auto &r : res[chn])
+    {
+      for (uint32_t j = 0; j < frames; ++j)
+      {
+        outputs[chn][j] += r.process(inputs[chn][j]);
+      }
+    }
+  }
+}
+
+// Compute filter coefficients from
+// frequency as a proportion of the sample rate
+// REMEMBER: THAT'S DOUBLE THE Nyquist RATE, dummy
+// and resonance in [0, 1]
+void Filter::set(double x, double y, double p)
+{
+  double r = p * 0.049 + 0.95; // Filter unstable at r = 1.
+  double app = r * r;
+  double xap = (1.0 + r * r) * cos(M_PI * x);
+  double yap = (1.0 + r * r) * cos(M_PI * y);
+  double scale = sqrt(0.5 - 0.5 * r * r);
+  for (auto &channelFilter : res)
+  {
+    channelFilter[0].set(app, xap, scale);
+    channelFilter[1].set(app, yap, scale);
+  }
 }

src/yaw-vowel/filter.h

@@ -8,7 +8,7 @@ public:
     Resonator():
     xp(0), xpp(0), yp(0), ypp(0), app(0), ap(0), scale(1){};
 
-    void process(float *output, const float *const input, uint32_t frames);
+    inline float process(float x);
     void clear();
     void set(double _app, double _ap, double _scale);
 
@@ -25,46 +25,10 @@ private:
 class Filter
 {
 public:
-    Filter(){};
-
-    void process(float **outputs, const float **inputs, uint32_t frames)
-    {
-        for (uint chn = 0; chn < 2; ++chn)
-        {
-            for (uint32_t j = 0; j < frames; ++j)
-            {
-                outputs[chn][j] = inputs[chn][j];
-            }
-            
-            for (auto& r : res[chn])
-            {
-                r.process(outputs[chn], outputs[chn], frames);
-            }
-        }
-
-
-    }
-
-    //Compute filter coefficients from
-    //frequency as a proportion of the sample rate
-    //REMEMBER: THAT'S DOUBLE THE Nyquist RATE, dummy
-    //and resonance in [0, 1]
-    void set(double x, double y, double p)
-    {
-        double r = p * 0.99;
-        double app = r * r;
-        double xap = (1.0 + r * r) * cos(M_PI * x);
-        double yap = (1.0 + r * r) * cos(M_PI * y);
-        double scale = sqrt(0.5 - 0.5 * r * r);
-        for (auto& channelFilter : res)
-        {
-            channelFilter[0].set(app, xap, scale);
-            channelFilter[1].set(app, xap, scale);
-            channelFilter[2].set(app, yap, scale);
-            channelFilter[3].set(app, yap, scale);
-        }
-    }
+    Filter();
+    void process(float **outputs, const float **inputs, uint32_t frames);
+    void set(double x, double y, double p);
 
 private:
-    std::array<std::array<Resonator, 4>, 2> res;
+    std::array<std::array<Resonator, 2>, 2> res;
 };