Mathematically correct key tracking

include/synth/dsp/filter.h

@@ -7,11 +7,14 @@
 
 #include "util.h"
 
+#define FILTER_K_BASE 6.907755278982137
+
+inline float noteToK(float note) {
+    return ((note - 69) / 12.0) / (FILTER_K_BASE * M_LOG2E);
+}
+
 class SVF12 {
 public:
-    float frequency;
-    float Q;
-
     typedef struct {
         float lp;   // low-pass
         float bp;   // band-pass
@@ -66,9 +69,9 @@ private:
 public:
     void assign(Settings const * settings);
     
-    inline float tick(float in, float freqAdd, float freqScale) {
-        float freq = clamp(freqScale * settings->freq + freqAdd, 0, 1);
-        float kK = tan(M_PI_4 * clamp(0.001 * exp(freq * 6.907755278982137), 0, 1));
+    inline float tick(float in, float freqAdd) {
+        float freq = clamp(settings->freq + freqAdd, 0, 1);
+        float kK = tan(M_PI_4 * clamp(0.001 * exp(freq * FILTER_K_BASE), 0, 1));
         float kQ = M_SQRT1_2 + settings->res;
         
         SVF12::Output outA = fltA.tick(in, kK, kQ);

include/synth/voice.h

@@ -48,13 +48,13 @@ public:
     inline bool isIdle() { return adsrAmp.isIdle(); }
 
     inline float tick(float pitchMod) {
-        const float keyTrackMul = 1 + settings->keyTrack * noteToFreq(note + pitchMod) / 440.0;
         const float osc1PhaseStep = noteToFreq(note + pitchMod - settings->oscDetune) * PIx2 / SAMPLE_RATE;
         const float osc2PhaseStep = noteToFreq(note + pitchMod + settings->oscDetune + settings->osc2Pitch) * PIx2 / SAMPLE_RATE;
         float out = 0;
         out += (1 - settings->oscMix) * osc1.tick(osc1PhaseStep);
         out +=      settings->oscMix  * osc2.tick(osc2PhaseStep);
-        out = filter.tick(out, settings->modEnvFltGain * adsrMod.tick(), keyTrackMul);
+        const float keyTrackVal = settings->keyTrack * noteToK(note);
+        out = filter.tick(out, settings->modEnvFltGain * adsrMod.tick() + keyTrackVal);
         out *= adsrAmp.tick();
         return out;
     }