Audio UDP/RTP: added a lowpass filter for decimation

sdrbase/CMakeLists.txt

@@ -7,6 +7,7 @@ set(sdrbase_SOURCES
     audio/audiocompressor.cpp
     audio/audiodevicemanager.cpp
     audio/audiofifo.cpp
+    audio/audiofilter.cpp
     audio/audiooutput.cpp
     audio/audioinput.cpp
     audio/audionetsink.cpp
@@ -99,6 +100,7 @@ set(sdrbase_HEADERS
 	audio/audiocompressor.h
     audio/audiodevicemanager.h
     audio/audiofifo.h
+    audio/audiofilter.h
     audio/audiooutput.h
     audio/audioinput.h
     audio/audionetsink.h

sdrbase/audio/audiofilter.cpp

@@ -0,0 +1,48 @@
+///////////////////////////////////////////////////////////////////////////////////
+// Copyright (C) 2019 Edouard Griffiths, F4EXB                                   //
+//                                                                               //
+// This program is free software; you can redistribute it and/or modify          //
+// it under the terms of the GNU General Public License as published by          //
+// the Free Software Foundation as version 3 of the License, or                  //
+//                                                                               //
+// This program is distributed in the hope that it will be useful,               //
+// but WITHOUT ANY WARRANTY; without even the implied warranty of                //
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                  //
+// GNU General Public License V3 for more details.                               //
+//                                                                               //
+// You should have received a copy of the GNU General Public License             //
+// along with this program. If not, see <http://www.gnu.org/licenses/>.          //
+///////////////////////////////////////////////////////////////////////////////////
+
+#include "audiofilter.h"
+
+const float AudioFilter::m_lpa[3] = {1.0,           1.392667E+00, -5.474446E-01};
+const float AudioFilter::m_lpb[3] = {3.869430E-02,  7.738860E-02,  3.869430E-02};
+// f(-3dB) = 300 Hz @ 8000 Hz SR (w = 0.075):
+const float AudioFilter::m_hpa[3] = {1.000000e+00,  1.667871e+00, -7.156964e-01};
+const float AudioFilter::m_hpb[3] = {8.459039e-01, -1.691760e+00,  8.459039e-01};
+
+AudioFilter::AudioFilter() :
+        m_filterLP(m_lpa, m_lpb),
+        m_filterHP(m_hpa, m_hpb),
+        m_useHP(false)
+{
+}
+
+AudioFilter::~AudioFilter()
+{}
+
+float AudioFilter::run(const float& sample)
+{
+    return m_useHP ? m_filterLP.run(m_filterHP.run(sample)) : m_filterLP.run(sample);
+}
+
+float AudioFilter::runHP(const float& sample)
+{
+    return m_filterHP.run(sample);
+}
+
+float AudioFilter::runLP(const float& sample)
+{
+    return m_filterLP.run(sample);
+}

sdrbase/audio/audiofilter.h

@@ -0,0 +1,62 @@
+///////////////////////////////////////////////////////////////////////////////////
+// Copyright (C) 2019 Edouard Griffiths, F4EXB                                   //
+//                                                                               //
+// This program is free software; you can redistribute it and/or modify          //
+// it under the terms of the GNU General Public License as published by          //
+// the Free Software Foundation as version 3 of the License, or                  //
+//                                                                               //
+// This program is distributed in the hope that it will be useful,               //
+// but WITHOUT ANY WARRANTY; without even the implied warranty of                //
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                  //
+// GNU General Public License V3 for more details.                               //
+//                                                                               //
+// You should have received a copy of the GNU General Public License             //
+// along with this program. If not, see <http://www.gnu.org/licenses/>.          //
+///////////////////////////////////////////////////////////////////////////////////
+
+#ifndef _SDRBASE_AUDIO_AUDIOFILTER_H_
+#define _SDRBASE_AUDIO_AUDIOFILTER_H_
+
+#include "export.h"
+#include "dsp/iirfilter.h"
+
+/**
+ * This is a 2 pole lowpass Chebyshev (recursive) filter at fc=0.075 using coefficients found in table 20-1 of
+ * http://www.analog.com/media/en/technical-documentation/dsp-book/dsp_book_Ch20.pdf
+ *
+ * At the interpolated sampling frequency of 48 kHz the -3 dB corner is at 48 * .075 = 3.6 kHz which is perfect for voice
+ *
+ * a0= 3.869430E-02
+ * a1= 7.738860E-02 b1= 1.392667E+00
+ * a2= 3.869430E-02 b2= -5.474446E-01
+ *
+ * given x[n] is the new input sample and y[n] the returned output sample:
+ *
+ * y[n] = a0*x[n] + a1*x[n] + a2*x[n] + b1*y[n-1] + b2*y[n-2]
+ *
+ * This one works directly with floats
+ *
+ */
+
+class SDRBASE_API AudioFilter {
+public:
+    AudioFilter();
+    ~AudioFilter();
+
+    void useHP(bool useHP) { m_useHP = useHP; }
+    bool usesHP() const { return m_useHP; }
+    float run(const float& sample);
+    float runHP(const float& sample);
+    float runLP(const float& sample);
+
+private:
+    IIRFilter<float, 2> m_filterLP;
+    IIRFilter<float, 2> m_filterHP;
+    bool m_useHP;
+    static const float m_lpa[3];
+    static const float m_lpb[3];
+    static const float m_hpa[3];
+    static const float m_hpb[3];
+};
+
+#endif // _SDRBASE_AUDIO_AUDIOFILTER_H_

sdrbase/sdrbase.pro

@@ -56,6 +56,7 @@ MINGW32 || MINGW64 || MSVC {
 SOURCES += audio/audiodevicemanager.cpp\
         audio/audiocompressor.cpp\
         audio/audiofifo.cpp\
+        audio/audiofilter.cpp\
         audio/audiooutput.cpp\
         audio/audioinput.cpp\
         audio/audionetsink.cpp\
@@ -134,6 +135,7 @@ SOURCES += audio/audiodevicemanager.cpp\
 HEADERS  += audio/audiodevicemanager.h\
         audio/audiocompressor.h\
         audio/audiofifo.h\
+        audio/audiofilter.h\
         audio/audiooutput.h\
         audio/audioinput.h\
         audio/audionetsink.h\