Testing with fixed values.

effect_modulated_delay.cpp

@@ -167,19 +167,43 @@ void AudioEffectModulatedDelay::setDelay(float milliseconds)
 void AudioEffectModulatedDelay::calcModFilterCoeff(float32_t cFrq, float32_t gain, float32_t width)
 {
   /* Calculate intermediate values */
-  float32_t A = sqrt(pow(10, gain / 20.0f));
-  float32_t w0 = 2.0f * PI * cFrq / ((float32_t)AUDIO_SAMPLE_RATE_EXACT);
-  float32_t cosw0 = cos(w0);
-  float32_t sinw0 = sin(w0);
-  float32_t alpha = sinw0 / (2.0f * width);
+  //  float32_t A = sqrt(pow(10, gain / 20.0f));
+  //  float32_t w0 = 2.0f * PI * cFrq / ((float32_t)AUDIO_SAMPLE_RATE_EXACT);
+  //  float32_t cosw0 = cos(w0);
+  //  float32_t sinw0 = sin(w0);
+  //  float32_t alpha = sinw0 / (2.0f * width);
 
   /* Calculate coefficients */
-  float32_t b0 =  1.0f + alpha * A;
-  float32_t b1 = -2.0f * cosw0;
-  float32_t b2 =  1.0f - alpha * A;
-  float32_t a0 =  1.0f + alpha / A;
-  float32_t a1 = -2.0f * cosw0;
-  float32_t a2 =  1.0f - alpha / A;
+  //  float32_t b0 =  1.0f + alpha * A;
+  //  float32_t b1 = -2.0f * cosw0;
+  //  float32_t b2 =  1.0f - alpha * A;
+  //  float32_t a0 =  1.0f + alpha / A;
+  //  float32_t a1 = -2.0f * cosw0;
+  //  float32_t a2 =  1.0f - alpha / A;
+
+/* https://stackoverflow.com/questions/20924868/calculate-coefficients-of-2nd-order-butterworth-low-pass-filter/20932062
+    ff=cutoff_frq/sample_rate=AUDIO_SAMPLE_RATE_EXACT/1000
+    const double ita =1.0/ tan(M_PI*ff);
+    const double q=sqrt(2.0);
+    b0 = 1.0 / (1.0 + q*ita + ita*ita);
+    b1= 2*b0;
+    b2= b0;
+    a1 = 2.0 * (ita*ita - 1.0) * b0;
+    a2 = -(1.0 - q*ita + ita*ita) * b0;
+
+    ff=1000/44117.64706=0.02266666666
+    ita=804.60898525
+    q=1.414213
+    
+ */
+  // 1kHz 2nd order Butterworth lowpass filter coefficients
+  // calculated with Iowa IIR FIlter Designer 6.5
+  float32_t b0 = 0.124589380980617656;
+  float32_t b1 = 0.124589380980617656;
+  float32_t b2 = 0.0;
+  float32_t a0 = 1.000000000000000000;
+  float32_t a1 = -0.750821238038764660;
+  float32_t a2 = 0.0;
 
   /* Normalize so a0 = 1 */
   filter_coeffs[0] = b0 / a0;