updated DcBlockingFilter, add doc reference, add get/setTimeConstant()

Author: Martin Klang

LibSource/DcBlockingFilter.h

@@ -5,23 +5,35 @@
 #include "SignalProcessor.h"
 
 /**
- * DC Blocking IIR filter:
- * Leaky differentiator.
+ * DC Blocking IIR filter, aka Leaky differentiator.
+ * Ref: https://www.dsprelated.com/freebooks/filters/DC_Blocker.html
  */
 class DcBlockingFilter : public SignalProcessor {
 private:
-  const float lambda;
   float x1, y1;
+  float R;
 public:
-  DcBlockingFilter(float lambda = 0.995): lambda(lambda), x1(0), y1(0) {}
+  DcBlockingFilter(float R = 0.995): R(R), x1(0), y1(0) {}
 
+  /**
+   * Get adaptation time constant in samples.
+   */
+  float getTimeConstant(){
+    return 1/(1-R); // approximate
+  }
+  /**
+   * Set adaptation time constant in samples.
+   */
+  void setTimeConstant(float tc){
+    R = (tc - 1) / tc;
+  }
   void reset(){
     x1 = y1 = 0;
   }
 
   /* process a single sample and return the result */
   float process(float x){
-    y1 = x - x1 + lambda*y1;
+    y1 = x - x1 + R*y1;
     x1 = x;
     return y1;
   }
@@ -31,7 +43,7 @@ class DcBlockingFilter : public SignalProcessor {
     float y = y1;
     while(size--){
       x = *input++;
-      y = x - x1 + lambda*y;
+      y = x - x1 + R*y;
       x1 = x;
       *output++ = y;
     }
@@ -52,8 +64,8 @@ class DcBlockingFilter : public SignalProcessor {
     process(in, out, in.getSize());
   }
 
-  static DcBlockingFilter* create(float lambda=0.995){
-    return new DcBlockingFilter(lambda);
+  static DcBlockingFilter* create(float R=0.995){
+    return new DcBlockingFilter(R);
   }
 
   static void destroy(DcBlockingFilter* obj){
@@ -65,14 +77,29 @@ class StereoDcBlockingFilter : public MultiSignalProcessor {
 private:
   DcBlockingFilter left, right;
 public:
-  StereoDcBlockingFilter(float lambda = 0.995): left(lambda), right(lambda) {}
+  StereoDcBlockingFilter(float R = 0.995): left(R), right(R) {}
+
+  /**
+   * Get adaptation time constant in samples.
+   */
+  float getTimeConstant(){
+    return left.getTimeConstant();
+  }
+  /**
+   * Set adaptation time constant in samples.
+   */
+  void setTimeConstant(float tc){
+    left.setTimeConstant(tc);
+    right.setTimeConstant(tc);
+  }
+
   void process(AudioBuffer& input, AudioBuffer& output){
     left.process(input.getSamples(LEFT_CHANNEL), output.getSamples(LEFT_CHANNEL));
     right.process(input.getSamples(RIGHT_CHANNEL), output.getSamples(RIGHT_CHANNEL));
   }
 
-  static StereoDcBlockingFilter* create(float lambda){
-    return new StereoDcBlockingFilter(lambda);
+  static StereoDcBlockingFilter* create(float R=0.995){
+    return new StereoDcBlockingFilter(R);
   }
 
   static void destroy(StereoDcBlockingFilter* obj){