Added IIR filter (1-order, 3-order) for filtering LP-filtering the mdulation

signal at 5000 Hz (just for testing).

config.h

@@ -56,9 +56,10 @@
 #define REDUCE_LOUDNESS 0
 #define USE_XFADE_DATA 1
 // CHORUS parameters
-#define INTERPOLATION_WINDOW_SIZE 21 // use only odd numbers!!!
-//#define INTERPOLATE_MODE 11
-#define CHORUS_WAVEFORM WAVEFORM_SINE // WAVEFORM_SINE WAVEFORM_TRIANGLE WAVEFORM_SAWTOOTH WAVEFORM_SAWTOOTH_REVERSE
+#define INTERPOLATION_WINDOW_SIZE 7 // use only odd numbers!!!
+#define INTERPOLATE_MODE 11
+#define FILTER_IIR_ORDER_3 1
+#define CHORUS_WAVEFORM WAVEFORM_TRIANGLE // WAVEFORM_SINE WAVEFORM_TRIANGLE WAVEFORM_SAWTOOTH WAVEFORM_SAWTOOTH_REVERSE
 #define CHORUS_DELAY_LENGTH_SAMPLES (15*AUDIO_BLOCK_SAMPLES) // one AUDIO_BLOCK_SAMPLES = 2.902ms; you need doubled length, e.g. delay point is 20ms, so you need up to 40ms delay!
 
 //*************************************************************************************************

effect_modulated_delay.cpp

@@ -26,6 +26,7 @@
 #include "limits.h"
 #include "effect_modulated_delay.h"
 #include "spline.h"
+#include "config.h"
 
 /******************************************************************/
 
@@ -99,7 +100,56 @@ void AudioEffectModulatedDelay::update(void)
 
       // Calculate modulation index as a float, for interpolation later.
       // The index is located around the half of the delay length multiplied by the current amount of the modulator
-      mod_idx = float(*mp) / SHRT_MAX * float(_delay_length >> 1);
+      /**************************************************************************************************************
+        Use an IIR filter on the modulation for avoiding aliasing (http://www-users.cs.york.ac.uk/~fisher/mkfilter/) */
+#if defined(FILTER_IIR_ORDER_1)
+      /* Digital filter designed by mkfilter/mkshape/gencode   A.J. Fisher
+        Command line: /www/usr/fisher/helpers/mkfilter -Bu -Lp -o 1 -a 1.1333499558e-01 0.0000000000e+00 -l
+        Parameters: filtertype  =   Butterworth
+        passtype  =   Lowpass
+        ripple  =
+        order   =   1
+        samplerate  =   44117.64706
+        corner1   =   5000
+        corner2   =
+        adzero  =
+        logmin  =
+      **************************************************************************************************************/
+#define NZEROS 1
+#define NPOLES 1
+#define GAIN   3.688918967e+00
+      static float xv[NZEROS + 1], yv[NPOLES + 1];
+      xv[0] = xv[1];
+      xv[1] = (float(*mp) / SHRT_MAX) / GAIN;
+      yv[0] = yv[1];
+      yv[1] =   (xv[0] + xv[1]) + (0.4578357460 * yv[0]);
+      //new_value = yv[1];
+      mod_idx = yv[1] * float(_delay_length >> 1);
+#elif defined(FILTER_IIR_ORDER_3)
+      /* Digital filter designed by mkfilter/mkshape/gencode   A.J. Fisher
+        Command line: /www/usr/fisher/helpers/mkfilter -Bu -Lp -o 3 -a 1.1333333333e-01 0.0000000000e+00 -l
+        raw alpha1    =   0.1133333333
+        raw alpha2    =   0.1133333333
+        warped alpha1 =   0.1183783855
+        warped alpha2 =   0.1183783855
+        gain at dc    :   mag = 4.028005941e+01   phase =   0.0000000000 pi
+        gain at centre:   mag = 2.848230315e+01   phase =  -0.7500000000 pi
+        gain at hf    :   mag = 0.000000000e+00
+      **************************************************************************************************************/
+#define NZEROS 3
+#define NPOLES 3
+#define GAIN   4.028005941e+01
+      static float xv[NZEROS + 1], yv[NPOLES + 1];
+      xv[0] = xv[1]; xv[1] = xv[2]; xv[2] = xv[3];
+      xv[3] = (float(*mp) / SHRT_MAX) / GAIN;
+      yv[0] = yv[1]; yv[1] = yv[2]; yv[2] = yv[3];
+      yv[3] =   (xv[0] + xv[3]) + 3 * (xv[1] + xv[2])
+                + (  0.2323461955 * yv[0]) + ( -1.0299524229 * yv[1])
+                + (  1.5989967885 * yv[2]);
+      //new_value = yv[3];
+      mod_idx = yv[3] * float(_delay_length >> 1);
+      /************************************************************************/
+#endif
       mod_fraction = modff(mod_idx, &mod_number);
 
 #ifdef INTERPOLATE_MODE