Attempt to model filter leakage (#12)

src/Filter.h

@@ -92,6 +92,10 @@ class Filter
     bool lp = false;
     //@}
 
+    double leakMixer  = 0.001628;  // 6581: 0.002531 / 8580: 0.001628
+    double leakFilter = 0.001733;  // 6581: 0.001688 / 8580: 0.001733
+    double leakV3     = 0.0525;    // 6581: 0.1125   / 8580: 0.0525
+
 private:
     /// Current volume.
     uint8_t vol = 0;
@@ -102,19 +106,13 @@ class Filter
     /// Selects which inputs to route through filter.
     uint8_t filt = 0;
 
+
 private:
     inline int32_t getNormalizedVoice(Voice& v) const
     {
         return fmc.getNormalizedVoice(v.output(), v.envelope()->output());
     }
 
-    // If voice 3 is off we still need to clock the waveform generator
-    inline static int32_t getSilentVoice(Voice& v)
-    {
-        v.wave()->output();
-        return 0;
-    }
-
 protected:
     /**
      * Update filter cutoff frequency.
@@ -140,6 +138,12 @@ class Filter
 
     virtual int32_t solveIntegrators() = 0;
 
+    static inline int32_t signalLeak(int32_t input, double leak)
+    {
+        int32_t leaked = static_cast<int32_t>(leak * (1 << 12));
+        return ((input-32767) * leaked) >> 12;
+    }
+
 public:
     Filter(FilterModelConfig& fmc);
 
@@ -215,16 +219,23 @@ uint16_t Filter::clock(Voice& voice1, Voice& voice2, Voice& voice3)
 {
     const int32_t V1 = getNormalizedVoice(voice1);
     const int32_t V2 = getNormalizedVoice(voice2);
-    // Voice 3 is silenced by voice3off if it is not routed through the filter.
-    const int32_t V3 = (filt3 || !voice3off) ? getNormalizedVoice(voice3) : getSilentVoice(voice3);
+    const int32_t V3 = getNormalizedVoice(voice3);
 
     int32_t Vsum = 0;
     int32_t Vmix = 0;
 
-    (filt1 ? Vsum : Vmix) += V1;
-    (filt2 ? Vsum : Vmix) += V2;
-    (filt3 ? Vsum : Vmix) += V3;
-    (filtE ? Vsum : Vmix) += Ve;
+    Vsum += filt1 ? V1 : signalLeak(V1, leakFilter);
+    Vmix += filt1 ? signalLeak(V1, leakMixer) : V1;
+
+    Vsum += filt2 ? V2 : signalLeak(V2, leakFilter);
+    Vmix += filt2 ? signalLeak(V2, leakMixer) : V2;
+
+    Vsum += filt3 ? V3 : signalLeak(V3, leakFilter);
+    // Voice 3 is silenced by voice3off if it is not routed through the filter.
+    Vmix += filt3 ? signalLeak(V3, leakMixer) : voice3off ? signalLeak(V3, leakV3) : V3;
+
+    Vsum += filtE ? Ve : signalLeak(Ve, leakFilter);
+    Vmix += filtE ? signalLeak(Ve, leakMixer) : Ve;
 
     Vhp = currentSummer[currentResonance[Vbp] + Vlp + Vsum];
 

src/Filter6581.cpp

@@ -35,9 +35,10 @@ int32_t Filter6581::solveIntegrators()
     Vlp = bpIntegrator.solve(Vbp);
 
     int32_t Vfilt = 0;
-    if (lp) Vfilt += Vlp;
-    if (bp) Vfilt += Vbp;
-    if (hp) Vfilt += Vhp;
+
+    Vfilt += lp ? Vlp : signalLeak(Vlp, leakFilter);
+    Vfilt += bp ? Vbp : signalLeak(Vbp, leakFilter);
+    Vfilt += hp ? Vhp : signalLeak(Vhp, leakFilter);
 
     // The filter input resistors are slightly bigger than the voice ones
     // Scale the values accordingly

src/Filter8580.cpp

@@ -33,9 +33,10 @@ int32_t Filter8580::solveIntegrators()
     Vlp = bpIntegrator.solve(Vbp);
 
     int32_t Vfilt = 0;
-    if (lp) Vfilt += Vlp;
-    if (bp) Vfilt += Vbp;
-    if (hp) Vfilt += Vhp;
+
+    Vfilt += lp ? Vlp : signalLeak(Vlp, leakFilter);
+    Vfilt += bp ? Vbp : signalLeak(Vbp, leakFilter);
+    Vfilt += hp ? Vhp : signalLeak(Vhp, leakFilter);
 
     return Vfilt;
 }