state variable filter

ofxMaxim/ofxMaxim/libs/maximilian.h

@@ -551,5 +551,73 @@ class maxiDCBlocker {
     }
 };
 
+/*
+ State Variable Filter
+ 
+ algorithm from  http://www.cytomic.com/files/dsp/SvfLinearTrapOptimised.pdf
+ usage:
+ either set the parameters separately as required (to save CPU)
+ 
+ filter.setCutoff(param1);
+ filter.setResonance(param2);
+ 
+ w = filter.play(w, 0.0, 1.0, 0.0, 0.0);
+ 
+ or set everything together at once
+ 
+ w = filter.setCutoff(param1).setResonance(param2).play(w, 0.0, 1.0, 0.0, 0.0);
+ 
+ */
+class maxiSVF {
+public:
+    maxiSVF() : v0z(0), v1(0), v2(0) { setParams(1000, 1);}
+
+    //20 < cutoff < 20000
+    inline maxiSVF& setCutoff(double cutoff) {
+        setParams(cutoff, res);
+        return *this;
+    }
+
+    //from 0 upwards, starts to ring from 2-3ish, cracks a bit around 10
+    inline maxiSVF& setResonance(double q) {
+        setParams(freq, q);
+        return *this;
+    }
+
+    //run the filter, and get a mixture of lowpass, bandpass, highpass and notch outputs
+    inline double play(double w, double lpmix, double bpmix, double hpmix, double notchmix) {
+        double low, band, high, notch;
+        double v1z = v1;
+        double v2z = v2;
+        double v3 = w + v0z - 2.0 * v2z;
+        v1 += g1*v3-g2*v1z;
+        v2 += g3*v3+g4*v1z;
+        v0z = w;
+        low = v2;
+        band = v1;
+        high = w-k*v1-v2;
+        notch = w-k*v1;
+        return (low * lpmix) + (band * bpmix) + (high * hpmix) + (notch * notchmix);
+    }
+
+private:
+    inline void setParams(double _freq, double _res) {
+        freq = _freq;
+        res = _res;
+        g = tan(PI * freq / maxiSettings::sampleRate);
+        damping = 1.0 / res;
+        k = damping;
+        ginv = g / (1.0 + g * (g + k));
+        g1 = ginv;
+        g2 = 2.0 * (g + k) * ginv;
+        g3 = g * ginv;
+        g4 = 2.0 * ginv;
+    }
+
+    double v0z, v1, v2, g, damping, k, ginv, g1, g2, g3 ,g4;
+    double freq, res;
+
+};
+
 
 #endif