Remove high frequency artifacts from Lanczos resampling.

Run the TIA signal through a high pass with 10Hz cutoff.

src/common/audio/HighPass.cxx

@@ -0,0 +1,41 @@
+//============================================================================
+//
+//   SSSS    tt          lll  lll
+//  SS  SS   tt           ll   ll
+//  SS     tttttt  eeee   ll   ll   aaaa
+//   SSSS    tt   ee  ee  ll   ll      aa
+//      SS   tt   eeeeee  ll   ll   aaaaa  --  "An Atari 2600 VCS Emulator"
+//  SS  SS   tt   ee      ll   ll  aa  aa
+//   SSSS     ttt  eeeee llll llll  aaaaa
+//
+// Copyright (c) 1995-2018 by Bradford W. Mott, Stephen Anthony
+// and the Stella Team
+//
+// See the file "License.txt" for information on usage and redistribution of
+// this file, and for a DISCLAIMER OF ALL WARRANTIES.
+//============================================================================
+
+#include <cmath>
+#ifndef M_PI
+  #define M_PI 3.14159265358979323846f
+#endif
+
+#include "HighPass.hxx"
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+HighPass::HighPass(float cutOffFrequency, float frequency)
+  : myLastValueIn(0),
+    myLastValueOut(0),
+    myAlpha(1.f / (1.f + 2.f*M_PI*cutOffFrequency/frequency))
+{}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+float HighPass::apply(float valueIn)
+{
+  float valueOut = myAlpha * (myLastValueOut + valueIn - myLastValueIn);
+
+  myLastValueIn = valueIn;
+  myLastValueOut = valueOut;
+
+  return valueOut;
+}

src/common/audio/HighPass.hxx

@@ -0,0 +1,42 @@
+//============================================================================
+//
+//   SSSS    tt          lll  lll
+//  SS  SS   tt           ll   ll
+//  SS     tttttt  eeee   ll   ll   aaaa
+//   SSSS    tt   ee  ee  ll   ll      aa
+//      SS   tt   eeeeee  ll   ll   aaaaa  --  "An Atari 2600 VCS Emulator"
+//  SS  SS   tt   ee      ll   ll  aa  aa
+//   SSSS     ttt  eeeee llll llll  aaaaa
+//
+// Copyright (c) 1995-2018 by Bradford W. Mott, Stephen Anthony
+// and the Stella Team
+//
+// See the file "License.txt" for information on usage and redistribution of
+// this file, and for a DISCLAIMER OF ALL WARRANTIES.
+//============================================================================
+
+#ifndef HIGH_PASS_HXX
+#define HIGH_PASS_HXX
+
+class HighPass
+{
+  public:
+
+    HighPass(float cutOffFrequency, float frequency);
+
+    float apply(float value);
+
+  private:
+
+    float myLastValueIn;
+
+    float myLastValueOut;
+
+    float myAlpha;
+
+  private:
+
+    HighPass() = delete;
+};
+
+#endif // HIGH_PASS_HXX

src/common/audio/LanczosResampler.cxx

@@ -25,6 +25,7 @@
 namespace {
 
   constexpr float CLIPPING_FACTOR = 0.75;
+  constexpr float HIGH_PASS_CUT_OFF = 10;
 
   uInt32 reducedDenominator(uInt32 n, uInt32 d)
   {
@@ -78,6 +79,9 @@ LanczosResampler::LanczosResampler(
   myCurrentFragment(nullptr),
   myFragmentIndex(0),
   myIsUnderrun(true),
+  myHighPassL(HIGH_PASS_CUT_OFF, formatFrom.sampleRate),
+  myHighPassR(HIGH_PASS_CUT_OFF, formatFrom.sampleRate),
+  myHighPass(HIGH_PASS_CUT_OFF, formatFrom.sampleRate),
   myTimeIndex(0)
 {
   myPrecomputedKernels = make_unique<float[]>(myPrecomputedKernelCount * myKernelSize);
@@ -184,11 +188,11 @@ inline void LanczosResampler::shiftSamples(uInt32 samplesToShift)
 {
   while (samplesToShift-- > 0) {
     if (myFormatFrom.stereo) {
-      myBufferL->shift(myCurrentFragment[2*myFragmentIndex] / static_cast<float>(0x7fff));
-      myBufferR->shift(myCurrentFragment[2*myFragmentIndex + 1] / static_cast<float>(0x7fff));
+      myBufferL->shift(myHighPassL.apply(myCurrentFragment[2*myFragmentIndex] / static_cast<float>(0x7fff)));
+      myBufferR->shift(myHighPassR.apply(myCurrentFragment[2*myFragmentIndex + 1] / static_cast<float>(0x7fff)));
     }
     else
-      myBuffer->shift(myCurrentFragment[myFragmentIndex] / static_cast<float>(0x7fff));
+      myBuffer->shift(myHighPass.apply(myCurrentFragment[myFragmentIndex] / static_cast<float>(0x7fff)));
 
     myFragmentIndex++;
 

src/common/audio/LanczosResampler.hxx

@@ -21,6 +21,7 @@
 #include "bspf.hxx"
 #include "Resampler.hxx"
 #include "ConvolutionBuffer.hxx"
+#include "HighPass.hxx"
 
 class LanczosResampler : public Resampler
 {
@@ -59,6 +60,10 @@ class LanczosResampler : public Resampler
     uInt32 myFragmentIndex;
     bool myIsUnderrun;
 
+    HighPass myHighPassL;
+    HighPass myHighPassR;
+    HighPass myHighPass;
+
     uInt32 myTimeIndex;
 };
 

src/common/audio/module.mk

@@ -3,7 +3,8 @@ MODULE := src/common/audio
 MODULE_OBJS := \
 	src/common/audio/SimpleResampler.o \
 	src/common/audio/ConvolutionBuffer.o \
-	src/common/audio/LanczosResampler.o
+	src/common/audio/LanczosResampler.o \
+	src/common/audio/HighPass.o
 
 MODULE_DIRS += \
 	src/emucore/tia