Does not reduce alias: Tried to add a lowpass filter before downsampling.

dsp/RecursiveLinearFilter.cpp

@@ -162,3 +162,20 @@ void recursive_linear_filter::HighShelf::SetParams(const recursive_linear_filter
 
   this->_AssignCoefficients(a0, a1, a2, b0, b1, b2);
 }
+
+void recursive_linear_filter::LowPassBiquad::SetParams(const recursive_linear_filter::BiquadParams& params)
+{
+    const double omega0 = params.GetOmega0();
+    const double cosw0 = std::cos(omega0);
+    const double alpha = params.GetAlpha(omega0);
+
+    const double b0 = (1.0 - cosw0) / 2.0;
+    const double b1 = 1.0 - cosw0;
+    const double b2 = (1.0 - cosw0) / 2.0;
+    const double a0 = 1.0 + alpha;
+    const double a1 = -2.0 * cosw0;
+    const double a2 = 1.0 - alpha;
+
+    // Normalize the coefficients by a0 and assign them
+    this->_AssignCoefficients(a0, a1, a2, b0, b1, b2);
+}

dsp/RecursiveLinearFilter.h

@@ -202,4 +202,11 @@ class LowPass : public Base
   }
 };
 
+class LowPassBiquad : public Biquad
+{
+public:
+    LowPassBiquad() : Biquad() {}
+    void SetParams(const BiquadParams& params) override;
+};
+
 }; // namespace recursive_linear_filter

dsp/ResamplingContainer/ResamplingContainer.h

@@ -54,6 +54,8 @@ iPlug 2 includes the following 3rd party libraries (see each license info):
 #include "Dependencies/WDL/ptrlist.h"
 
 #include "Dependencies/LanczosResampler.h"
+#include "AudioDSPTools/dsp/RecursiveLinearFilter.h"
+
 
 namespace dsp
 {
@@ -133,6 +135,14 @@ class ResamplingContainer
       mResampler1 = std::make_unique<LanczosResampler>(mInputSampleRate, mRenderingSampleRate);
       mResampler2 = std::make_unique<LanczosResampler>(mRenderingSampleRate, mInputSampleRate);
 
+
+      // Initialize the LowPassBiquad filter with appropriate parameters
+      double cutoffFrequency = std::min(mInputSampleRate, mRenderingSampleRate) / 2.0 * 0.9; // For example
+      double qualityFactor = 0.707; // A common choice for a Butterworth filter
+      double gainDB = 0; // Typically, no gain change for a low-pass filter
+      recursive_linear_filter::BiquadParams params(mRenderingSampleRate, cutoffFrequency, qualityFactor, gainDB);
+      mLowPassFilter.SetParams(params);
+
       // Zeroes the scratch pointers so that we warm up with silence.
       ClearBuffers();
 
@@ -182,6 +192,9 @@ class ResamplingContainer
       {
         throw std::runtime_error("Got more encapsulated samples than the encapsulated DSP is prepared to handle!");
       }
+      mLowPassFilter.Process(mEncapsulatedOutputPointers.GetList(), 1, populated1);
+
+
       func(mEncapsulatedInputPointers.GetList(), mEncapsulatedOutputPointers.GetList(), (int)populated1);
       // And push the results into the second resampler so that it has what the external context requires.
       mResampler2->PushBlock(mEncapsulatedOutputPointers.GetList(), populated1);
@@ -201,6 +214,8 @@ class ResamplingContainer
   int GetLatency() const { return mLatency; }
 
 private:
+    recursive_linear_filter::LowPassBiquad mLowPassFilter; // Declaration of the LowPassBiquad filter
+
   static inline int LinearInterpolate(T** inputs, T** outputs, int inputLen, double ratio, int maxOutputLen)
   {
     // FIXME check through this!