feat: change to an SVF filter design

core/Filter.cpp

@@ -1,49 +1,90 @@
 #include "Filter.hpp"
 #include <cmath>
 
-Filter::Filter(float initCutOff, float initSampleRate)
-    : cutOff_(initCutOff), feedback_(0.0f), alpha_(0.0f),
-      sampleRate_(initSampleRate), zL_(0.0f), zR_(0.0f) {
-  recompute();
-};
+#ifndef M_PI
+#define M_PI 3.14159265358979323846
+#endif
 
-void Filter::process(const float *in, float *out, uint32_t frames) {
-  filter(in, out, frames, &zL_);
-};
-void Filter::processStereo(const float *inLeft, const float *inRight,
-                           float *outLeft, float *outRight, uint32_t frames) {
-  filter(inLeft, outLeft, frames, &zL_);
-  filter(inRight, outRight, frames, &zR_);
-};
+Filter::Filter(float initCutOff, float initQ, float initSampleRate)
+    : cutOff_(initCutOff), q_(initQ), sampleRate_(initSampleRate),
+      type_(kLowPass), g_(0.0f), lpL_(0.0f), bpL_(0.0f), lpR_(0.0f),
+      bpR_(0.0f) {
+  updateCoeffs();
+}
 
-void Filter::filter(const float *in, float *out, uint32_t frames, float *z) {
-  for (uint32_t i = 0; i < frames; ++i) {
-    float feedbackInput = in[i] + (feedback_ * (*z));
-    *z = *z + alpha_ * (feedbackInput - *z);
-    out[i] = *z;
+Filter::FilterType Filter::getType() const { return type_; }
+
+void Filter::updateCoeffs() {
+  float f = cutOff_ / std::fmax(sampleRate_, 1.0f);
+  if (f > kMaxNormalizedFrequency)
+    f = kMaxNormalizedFrequency;
+  g_ = 2.0f * std::sin(M_PI * f);
+}
+
+float Filter::processFrame(float input, float &lp, float &bp) const {
+
+  float hp = input - (bp / q_) - lp;
+
+  bp = bp + g_ * hp;
+  lp = lp + g_ * bp;
+
+  switch (type_) {
+  case kHighPass:
+    return hp;
+  case kBandPass:
+    return bp;
+  case kNotch:
+    return input - lp;
+  case kLowPass:
+  default:
+    return lp;
   }
 }
 
-void Filter::recompute() {
-  float fc = std::fmax(cutOff_, 1.0f);
-  float rc = 1.0f / (2.0f * M_PI * fc);
-  float val = 1.0f - std::exp(-1.0f / (rc * sampleRate_));
-  alpha_ = std::fmin(val, 0.999f);
-};
+void Filter::process(const float *in, float *out, uint32_t frames) {
+  float lp = lpL_;
+  float bp = bpL_;
 
-void Filter::setCutOff(float newCutOff) {
-  cutOff_ = newCutOff;
-  recompute();
+  for (uint32_t i = 0; i < frames; ++i) {
+    out[i] = processFrame(in[i], lp, bp);
+  }
+
+  lpL_ = lp;
+  bpL_ = bp;
 }
+
+void Filter::processStereo(const float *inL, const float *inR, float *outL,
+                           float *outR, uint32_t frames) {
+  float lpL = lpL_, bpL = bpL_;
+  float lpR = lpR_, bpR = bpR_;
+
+  for (uint32_t i = 0; i < frames; ++i) {
+    outL[i] = processFrame(inL[i], lpL, bpL);
+    outR[i] = processFrame(inR[i], lpR, bpR);
+  }
+
+  lpL_ = lpL;
+  bpL_ = bpL;
+  lpR_ = lpR;
+  bpR_ = bpR;
+}
+
 float Filter::getCutOff() const { return cutOff_; }
-
-void Filter::setFeedback(float newFeedback) {
-  feedback_ = std::fmin(std::fmax(newFeedback, 0.0f), 0.99f);
+void Filter::setCutOff(float freq) {
+  cutOff_ = std::fmax(freq, 1.0f);
+  updateCoeffs();
 }
-float Filter::getFeedback() const { return feedback_; }
 
-void Filter::setSampleRate(float newSampleRate) {
-  sampleRate_ = newSampleRate;
-  recompute();
+float Filter::getQ() const { return q_; }
+void Filter::setQ(float q) {
+  q_ = std::fmax(q, 0.1f);
+  updateCoeffs();
 }
+
+void Filter::setType(FilterType type) { type_ = type; }
+
 float Filter::getSampleRate() const { return sampleRate_; }
+void Filter::setSampleRate(float sr) {
+  sampleRate_ = sr;
+  updateCoeffs();
+}

core/Filter.hpp

@@ -1,33 +1,45 @@
 #ifndef FILTER_HPP_INCLUDED
 #define FILTER_HPP_INCLUDED
-#ifndef M_PI
-#define M_PI 3.14159265358979323846
-#endif
+
 #include <cstdint>
 
+constexpr float kMaxNormalizedFrequency = 0.16667f;
+
 class Filter {
 public:
-  Filter(float initCutOff = 1.0f, float initSampleRate = 48000.0f);
-  void process(const float *in, float *out, uint32_t frames);
-  void processStereo(const float *inLeft, const float *inRight, float *outLeft,
-                     float *outRight, uint32_t frames);
+  Filter(float initCutOff = 1000.0f, float initQ = 0.707f,
+         float initSampleRate = 48000.0f);
+
+  enum FilterType { kLowPass, kHighPass, kBandPass, kNotch, kFilterTypeCount };
+
+  void setCutOff(float freq);
+  void setQ(float q);
+  void setSampleRate(float sr);
+  void setType(FilterType type);
 
-  void recompute();
-  void setCutOff(float newCutOff);
   float getCutOff() const;
-  void setFeedback(float newFeedback);
-  float getFeedback() const;
-  void setSampleRate(float newSampleRate);
+  float getQ() const;
   float getSampleRate() const;
+  FilterType getType() const;
+
+  void process(const float *in, float *out, uint32_t frames);
+  void processStereo(const float *inL, const float *inR, float *outL,
+                     float *outR, uint32_t frames);
 
 private:
-  void filter(const float *in, float *out, uint32_t frames, float *z);
+  void updateCoeffs();
+
+  float processFrame(float input, float &lp, float &bp) const;
+
   float cutOff_;
-  float feedback_;
-  float alpha_;
+  float q_;
   float sampleRate_;
-  float zL_;
-  float zR_;
+  FilterType type_;
+
+  float g_;
+
+  float lpL_, bpL_;
+  float lpR_, bpR_;
 };
 
-#endif // FILTER_HPP_INCLUDED
+#endif