Butterworth filter

Author: kunitoki

modules/yup_dsp/designers/yup_FilterDesigner.cpp

@@ -419,10 +419,12 @@ int FilterDesigner<CoeffType>::designButterworth (
         // Lowpass and Highpass filters
         for (int s = 0; s < numStages; ++s)
         {
-            const CoeffType d = static_cast<CoeffType> (2.0) * std::sin (((static_cast<CoeffType> (2 * (s + 1) - 1)) * MathConstants<CoeffType>::pi) / (static_cast<CoeffType> (2 * order)));
+            const CoeffType d = static_cast<CoeffType> (2.0) *
+                std::sin (((static_cast<CoeffType> (2 * (s + 1) - 1)) * MathConstants<CoeffType>::pi) / (static_cast<CoeffType> (2 * order)));
 
-            const CoeffType beta = static_cast<CoeffType> (0.5) * ((static_cast<CoeffType> (1.0) - (d / static_cast<CoeffType> (2.0)) * std::sin (omega)) /
-                                                                  (static_cast<CoeffType> (1.0) + (d / static_cast<CoeffType> (2.0)) * std::sin (omega)));
+            const CoeffType beta = static_cast<CoeffType> (0.5) *
+                ((static_cast<CoeffType> (1.0) - (d / static_cast<CoeffType> (2.0)) * std::sin (omega)) /
+                    (static_cast<CoeffType> (1.0) + (d / static_cast<CoeffType> (2.0)) * std::sin (omega)));
 
             const CoeffType gamma = (static_cast<CoeffType> (0.5) + beta) * std::cos (omega);
 
@@ -512,6 +514,31 @@ int FilterDesigner<CoeffType>::designButterworth (
             coefficients.push_back (coeffs);
         }
     }
+    else if (filterMode.test (FilterMode::allpass))
+    {
+        // Allpass filters - use same structure as lowpass but with different coefficients
+        for (int s = 0; s < numStages; ++s)
+        {
+            const CoeffType d = static_cast<CoeffType> (2.0) * std::sin (((static_cast<CoeffType> (2 * (s + 1) - 1)) * MathConstants<CoeffType>::pi) / (static_cast<CoeffType> (2 * order)));
+
+            const CoeffType beta = static_cast<CoeffType> (0.5) * ((static_cast<CoeffType> (1.0) - (d / static_cast<CoeffType> (2.0)) * std::sin (omega)) /
+                                                                  (static_cast<CoeffType> (1.0) + (d / static_cast<CoeffType> (2.0)) * std::sin (omega)));
+
+            const CoeffType gamma = (static_cast<CoeffType> (0.5) + beta) * std::cos (omega);
+
+            BiquadCoefficients<CoeffType> coeffs;
+            // For allpass: numerator = reversed denominator
+            coeffs.a0 = static_cast<CoeffType> (1.0);
+            coeffs.a1 = static_cast<CoeffType> (-2.0) * gamma;
+            coeffs.a2 = static_cast<CoeffType> (2.0) * beta;
+            coeffs.b0 = static_cast<CoeffType> (2.0) * beta;
+            coeffs.b1 = static_cast<CoeffType> (-2.0) * gamma;
+            coeffs.b2 = static_cast<CoeffType> (1.0);
+
+            coeffs.normalize();
+            coefficients.push_back (coeffs);
+        }
+    }
 
     return static_cast<int> (coefficients.size());
 }

modules/yup_dsp/filters/yup_ButterworthFilter.h

@@ -55,7 +55,7 @@ class ButterworthFilter : public BiquadCascade<SampleType, CoeffType>
 
     //==============================================================================
     /** Maximum supported filter order */
-    static constexpr int maxOrder = 32;
+    static constexpr int maxOrder = 16;
 
 public:
     //==============================================================================
@@ -70,7 +70,7 @@ class ButterworthFilter : public BiquadCascade<SampleType, CoeffType>
     ButterworthFilter (FilterModeType mode, int filterOrder, CoeffType freq)
         : ButterworthFilter()
     {
-        setParameters (mode, filterOrder, freq, static_cast<CoeffType> (0.0), static_cast<CoeffType> (0.0), 44100.0);
+        setParameters (mode, filterOrder, freq, static_cast<CoeffType> (0.0), 44100.0);
     }
 
     //==============================================================================
@@ -91,13 +91,13 @@ class ButterworthFilter : public BiquadCascade<SampleType, CoeffType>
     {
         mode = resolveFilterMode (mode, getSupportedModes());
 
-        jassert (filterOrder >= 1 && filterOrder <= maxOrder);
+        jassert (filterOrder >= 2 && filterOrder <= maxOrder);
         jassert (freq > static_cast<CoeffType> (0.0));
-        if (mode.test (FilterMode::bandpass) || mode.test (FilterMode::bandstop))
-            jassert (freq2 > freq && freq2 > static_cast<CoeffType> (0.0));
 
-        // Ensure order is valid (1 or power of 2)
-        filterOrder = filterOrder == 1 ? filterOrder : jlimit (2, maxOrder, nextPowerOfTwo (filterOrder));
+        if ((mode.test (FilterMode::bandpass) || mode.test (FilterMode::bandstop)) && freq2 < freq)
+            std::swap (freq, freq2);
+
+        filterOrder = jlimit (2, maxOrder, nextEven (filterOrder));
 
         if (filterMode != mode
             || order != filterOrder
@@ -138,7 +138,7 @@ class ButterworthFilter : public BiquadCascade<SampleType, CoeffType>
     */
     void setOrder (int filterOrder) noexcept
     {
-        filterOrder = filterOrder == 1 ? filterOrder : jlimit (2, maxOrder, nextPowerOfTwo (filterOrder));
+        filterOrder = jlimit (2, maxOrder, nextEven (filterOrder));
 
         if (order != filterOrder)
         {