Filter for modulator implemented again (just testing).

effect_modulated_delay.cpp

@@ -58,6 +58,16 @@ boolean AudioEffectModulatedDelay::begin(short *delayline, int d_length)
   _delayline = delayline;
   _delay_length = d_length;
 
+  // modulator filter
+  // coefficients calculated with "IOWA Hills IIR Filter Designer 6.5"
+  // 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);
 }
 
@@ -65,8 +75,6 @@ void AudioEffectModulatedDelay::update(void)
 {
   audio_block_t *block;
   audio_block_t *modulation;
-  float alpha = 0.9f; // 90% new value, 10% feedback
-  static float filteredOutput;
 
   if (_delayline == NULL)
     return;
@@ -85,6 +93,7 @@ void AudioEffectModulatedDelay::update(void)
 
     bp = block->data;
     arm_q15_to_float(modulation->data, modulation_f32, AUDIO_BLOCK_SAMPLES);
+    arm_biquad_cascade_df1_f32(&modulator_filter_data, modulation_f32, modulation_f32, AUDIO_BLOCK_SAMPLES);
     mp = modulation_f32;
 
     for (uint16_t i = 0; i < AUDIO_BLOCK_SAMPLES; i++)
@@ -94,12 +103,8 @@ void AudioEffectModulatedDelay::update(void)
         _cb_index = 0;
       _delayline[_cb_index] = *bp;
 
-      // Simple 1st order IIR, mix the new value with the old value (for modulation to get a band-limited signal)
-      filteredOutput = (*mp * alpha) + filteredOutput * (1 - alpha);
-
       // Calculate the modulation-index as a floating point number for interpolation
-      //mod_index = *mp * (1 - MODULATION_MAX_FACTOR) * _delay_length; // "(1 - MODULATION_MAX_FACTOR) * _delay_length" means: maximum bytes of modulation allowed by given delay length
-      mod_index = filteredOutput * (1 - MODULATION_MAX_FACTOR) * _delay_length; // "(1 - MODULATION_MAX_FACTOR) * _delay_length" means: maximum bytes of modulation allowed by given delay length
+      mod_index = *mp * (1 - MODULATION_MAX_FACTOR) * _delay_length; // "(1 - MODULATION_MAX_FACTOR) * _delay_length" means: maximum bytes of modulation allowed by given delay length
       mod_fraction = modff(mod_index, &mod_number); // split float of mod_index into integer (= mod_number) and fraction part
 
       // calculate modulation index into circular buffer

effect_modulated_delay.h

@@ -55,6 +55,9 @@ class AudioEffectModulatedDelay :
     uint16_t _delay_offset; // number of samples for the read offset of the modulation inside the circular buffer
     uint16_t _delay_length; // calculated number of samples of the delay
     int16_t cb_mod_index; // current read pointer with modulation for the circular buffer
+    arm_biquad_casd_df1_inst_f32 modulator_filter_data;
+    float32_t modulator_filter_state;
+    float32_t modulator_filter_coeffs[5];
 };
 
 #endif