Added function for coeff-calculation.

effect_modulated_delay.cpp

@@ -27,6 +27,8 @@
 #include "effect_modulated_delay.h"
 #include "config.h"
 
+extern config_t configuration;
+
 /******************************************************************/
 
 // Based on;      A u d i o E f f e c t D e l a y
@@ -58,18 +60,37 @@ boolean AudioEffectModulatedDelay::begin(short *delayline, int d_length)
   _delayline = delayline;
   _delay_length = d_length;
 
+  set_modulator_filter_coeffs(1.0, configuration.chorus_frequency / 10, 1.0); // gain, center frerquency
+  modulator_filter_data = {1, &modulator_filter_state, modulator_filter_coeffs};
+
+  return (true);
+}
+
+void AudioEffectModulatedDelay::set_modulator_filter_coeffs(float gain, float fc, float width)
+{
   // modulator filter
   // coefficients calculated with "IOWA Hills IIR Filter Designer 6.5", http://www.iowahills.com/8DownloadPage.html
-  // OmegaC = 0.5, SR = 44117.64706, Fc = 11 kHz, N=2
   // Example: https://web.fhnw.ch/technik/projekte/eit/Fruehling2016/MuelZum/html/parametric_equalizer_example_8c-example.html
-  modulator_filter_coeffs[0] = 0.291938819295525787;  // b0
+
+  float32_t A = sqrt(powf(10, gain / 20.0f));
+  float32_t w0 = 2.0f * PI * fc / ((float32_t)AUDIO_SAMPLE_RATE_EXACT);
+  float32_t cosw0 = cosf(w0);
+  float32_t sinw0 = sinf(w0);
+  float32_t alpha = sinw0 / (2.0f * width);
+  float32_t a0 = 1.0f + alpha / A;
+
+  modulator_filter_coeffs[0] = (1.0f + alpha * A) / a0;  // b0
+  modulator_filter_coeffs[1] = (-2.0f * cosw0)    / a0;  // b1
+  modulator_filter_coeffs[2] = (1.0f - alpha * A) / a0;  // b2
+  modulator_filter_coeffs[3] = -(2.0f * cosw0)    / -a0; // -a1
+  modulator_filter_coeffs[4] = (1.0f - alpha / A) / -a0; // -a2
+
+  // OmegaC = 0.5, SR = 44117.64706, Fc = 11 kHz, N=2
+  /* modulator_filter_coeffs[0] = 0.291938819295525787;  // b0
     modulator_filter_coeffs[1] = 0.582700575839973478;  // b1
     modulator_filter_coeffs[2] = 0.291938819295525787;  // b2
     modulator_filter_coeffs[3] = 0.005242268129729576;  // -a1
-  modulator_filter_coeffs[4] = -0.171820482560754689;  // -a2
-  modulator_filter_data = {1, &modulator_filter_state, modulator_filter_coeffs};
-
-  return (true);
+    modulator_filter_coeffs[4] = -0.171820482560754689;  // -a2 */
 }
 
 void AudioEffectModulatedDelay::update(void)

effect_modulated_delay.h

@@ -49,6 +49,7 @@ class AudioEffectModulatedDelay :
     virtual float offset(float offset_value);
 
   private:
+    void set_modulator_filter_coeffs(float gain, float fc, float width);
     audio_block_t *inputQueueArray[2];
     int16_t *_delayline;  // pointer for the circular buffer
     uint16_t _cb_index;   // current write pointer of the circular buffer