Simple 1st-order filter test, as BlackAddr sugested: https://forum.pjrc.com/threads/56532-Modulated-Delay-Chorus-Flanger?highlight=Chorus

effect_modulated_delay.cpp

@@ -65,6 +65,8 @@ 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;
@@ -92,8 +94,12 @@ 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 = *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_fraction = modff(mod_index, &mod_number); // split float of mod_index into integer (= mod_number) and fraction part
 
       // calculate modulation index into circular buffer