BFM demod: apply de-emphasis

CMakeLists.txt

@@ -79,6 +79,7 @@ set(sdrbase_SOURCES
 	sdrbase/dsp/fftengine.cpp
 	sdrbase/dsp/fftfilt.cxx
 	sdrbase/dsp/fftwindow.cpp
+	sdrbase/dsp/filterrc.cpp
 	sdrbase/dsp/filesink.cpp
 	sdrbase/dsp/interpolator.cpp
 	sdrbase/dsp/inthalfbandfilter.cpp
@@ -155,6 +156,7 @@ set(sdrbase_HEADERS
 	include/dsp/fftfilt.h
 	include/dsp/fftwengine.h
 	include/dsp/fftwindow.h
+	include/dsp/filterrc.h
 	include/dsp/filesink.h
 	include/dsp/gfft.h
 	include/dsp/interpolator.h

include/dsp/filterrc.h

@@ -0,0 +1,50 @@
+///////////////////////////////////////////////////////////////////////////////////
+// Copyright (C) 2015 F4EXB                                                      //
+// written by Edouard Griffiths                                                  //
+//                                                                               //
+// 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 INCLUDE_DSP_FILTERRC_H_
+#define INCLUDE_DSP_FILTERRC_H_
+
+#include "dsp/dsptypes.h"
+
+/** First order low-pass IIR filter for real-valued signals. */
+class LowPassFilterRC
+{
+public:
+
+    /**
+     * Construct 1st order low-pass IIR filter.
+     *
+     * timeconst :: RC time constant in seconds (1 / (2 * PI * cutoff_freq)
+     */
+    LowPassFilterRC(Real timeconst);
+
+    /**
+     * Reconfigure filter with new time constant
+     */
+    void configure(Real timeout);
+
+    /** Process samples. */
+    void process(const Real& sample_in, Real& sample_out);
+
+private:
+    Real m_timeconst;
+    Real m_y1;
+    Real m_a1;
+    Real m_b0;
+};
+
+#endif /* INCLUDE_DSP_FILTERRC_H_ */

plugins/channel/bfm/bfmdemod.cpp

@@ -32,7 +32,9 @@ BFMDemod::BFMDemod(SampleSink* sampleSink) :
 	m_sampleSink(sampleSink),
 	m_audioFifo(4, 250000),
 	m_settingsMutex(QMutex::Recursive),
-	m_pilotPLL(19000/384000, 50/384000, 0.01)
+	m_pilotPLL(19000/384000, 50/384000, 0.01),
+	m_deemphasisFilterX(default_deemphasis * 48000 * 1.0e-6),
+	m_deemphasisFilterY(default_deemphasis * 48000 * 1.0e-6)
 {
 	setObjectName("BFMDemod");
 
@@ -43,6 +45,8 @@ BFMDemod::BFMDemod(SampleSink* sampleSink) :
 	m_config.m_squelch = -60.0;
 	m_config.m_volume = 2.0;
 	m_config.m_audioSampleRate = DSPEngine::instance()->getAudioSampleRate(); // normally 48 kHz
+	m_deemphasisFilterX.configure(default_deemphasis * m_config.m_audioSampleRate * 1.0e-6);
+	m_deemphasisFilterY.configure(default_deemphasis * m_config.m_audioSampleRate * 1.0e-6);
 	m_rfFilter = new fftfilt(-50000.0 / 384000.0, 50000.0 / 384000.0, rfFilterFftLength);
 
 	apply();
@@ -120,7 +124,7 @@ void BFMDemod::feed(const SampleVector::const_iterator& begin, const SampleVecto
 			m_m1Sample = rf[i];
 
 			m_sampleBuffer.push_back(Sample(demod * (1<<15), 0.0));
-			quint16 sampleStereo;
+			Real sampleStereo;
 
 			// Process stereo if stereo mode is selected
 
@@ -135,7 +139,7 @@ void BFMDemod::feed(const SampleVector::const_iterator& begin, const SampleVecto
 
 				if (m_interpolatorStereo.interpolate(&m_interpolatorStereoDistanceRemain, s, &cs))
 				{
-					sampleStereo = (qint16)(cs.real() * 3000 * m_running.m_volume);
+					sampleStereo = cs.real();
 				}
 			}
 
@@ -143,15 +147,19 @@ void BFMDemod::feed(const SampleVector::const_iterator& begin, const SampleVecto
 
 			if (m_interpolator.interpolate(&m_interpolatorDistanceRemain, e, &ci))
 			{
-				quint16 sample = (qint16)(ci.real() * 3000 * m_running.m_volume);
-
 				if (m_running.m_audioStereo)
 				{
-					m_audioBuffer[m_audioBufferFill].l = sample + sampleStereo;
+					Real deemph_l, deemph_r; // Pre-emphasis is applied on each channel before multiplexing
-					m_audioBuffer[m_audioBufferFill].r = sample - sampleStereo;
+					m_deemphasisFilterX.process(ci.real() + sampleStereo, deemph_l);
+					m_deemphasisFilterY.process(ci.real() - sampleStereo, deemph_r);
+					m_audioBuffer[m_audioBufferFill].l = (qint16)(deemph_l * 3000 * m_running.m_volume);
+					m_audioBuffer[m_audioBufferFill].r = (qint16)(deemph_r * 3000 * m_running.m_volume);
 				}
 				else
 				{
+					Real deemph;
+					m_deemphasisFilterX.process(ci.real() + sampleStereo, deemph);
+					quint16 sample = (qint16)(deemph * 3000 * m_running.m_volume);
 					m_audioBuffer[m_audioBufferFill].l = sample;
 					m_audioBuffer[m_audioBufferFill].r = sample;
 				}
@@ -319,6 +327,12 @@ void BFMDemod::apply()
 		m_squelchLevel *= m_squelchLevel;
 	}
 
+	if (m_config.m_audioSampleRate != m_running.m_audioSampleRate)
+	{
+		m_deemphasisFilterX.configure(default_deemphasis * m_config.m_audioSampleRate * 1.0e-6);
+		m_deemphasisFilterY.configure(default_deemphasis * m_config.m_audioSampleRate * 1.0e-6);
+	}
+
 	m_running.m_inputSampleRate = m_config.m_inputSampleRate;
 	m_running.m_inputFrequencyOffset = m_config.m_inputFrequencyOffset;
 	m_running.m_rfBandwidth = m_config.m_rfBandwidth;

plugins/channel/bfm/bfmdemod.h

@@ -27,6 +27,7 @@
 #include "dsp/movingaverage.h"
 #include "dsp/fftfilt.h"
 #include "dsp/phaselock.h"
+#include "dsp/filterrc.h"
 #include "audio/audiofifo.h"
 #include "util/message.h"
 
@@ -147,6 +148,10 @@ private:
 	StereoPhaseLock m_pilotPLL;
 	Real m_pilotPLLSamples[2];
 
+	LowPassFilterRC m_deemphasisFilterX;
+	LowPassFilterRC m_deemphasisFilterY;
+	static const Real default_deemphasis = 50.0; // 50 us
+
 	void apply();
 };
 

sdrbase/dsp/filterrc.cpp

@@ -0,0 +1,58 @@
+///////////////////////////////////////////////////////////////////////////////////
+// Copyright (C) 2015 F4EXB                                                      //
+// written by Edouard Griffiths                                                  //
+//                                                                               //
+// 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 <QDebug>
+#include "dsp/filterrc.h"
+
+// Construct 1st order low-pass IIR filter.
+LowPassFilterRC::LowPassFilterRC(Real timeconst) :
+    m_timeconst(timeconst),
+	m_y1(0)
+{
+	m_a1 = - exp(-1/m_timeconst);
+	m_b0 = 1 + m_a1;
+}
+
+// Reconfigure
+void LowPassFilterRC::configure(Real timeconst)
+{
+	m_timeconst = timeconst;
+	m_y1 = 0;
+	m_a1 = - exp(-1/m_timeconst);
+	m_b0 = 1 + m_a1;
+
+	qDebug() << "LowPassFilterRC::configure: t: " << m_timeconst
+			<< " a1: " << m_a1
+			<< " b0: " << m_b0;
+}
+
+// Process samples.
+void LowPassFilterRC::process(const Real& sample_in, Real& sample_out)
+{
+    /*
+     * Continuous domain:
+     *   H(s) = 1 / (1 - s * timeconst)
+     *
+     * Discrete domain:
+     *   H(z) = (1 - exp(-1/timeconst)) / (1 - exp(-1/timeconst) / z)
+     */
+
+	m_y1 = (sample_in * m_b0) - (m_y1 * m_a1);
+	sample_out = m_y1;
+}
+
+

sdrbase/dsp/phaselock.cpp

@@ -81,6 +81,7 @@ PhaseLock::PhaseLock(Real freq, Real bandwidth, Real minsignal)
     m_sample_cnt    = 0;
 }
 
+
 void PhaseLock::configure(Real freq, Real bandwidth, Real minsignal)
 {
 	qDebug("PhaseLock::configure: freq: %f bandwidth: %f minsignal: %f", freq, bandwidth, minsignal);
@@ -142,6 +143,7 @@ void PhaseLock::configure(Real freq, Real bandwidth, Real minsignal)
     m_sample_cnt    = 0;
 }
 
+
 // Process samples. Bufferized version
 void PhaseLock::process(const std::vector<Real>& samples_in, std::vector<Real>& samples_out)
 {
@@ -247,21 +249,6 @@ void PhaseLock::process(const std::vector<Real>& samples_in, std::vector<Real>&
     m_sample_cnt += n;
 }
 
-/*
-void PhaseLock::process(const Real& sample_in, Real& sample_out)
-{
-	m_phase += m_freq;
-
-	if (m_phase > 2.0 * M_PI) {
-		m_phase -= 2.0 * M_PI;
-	}
-
-	Real psin = sin(m_phase);
-	Real pcos = cos(m_phase);
-
-	sample_out = 2 * psin * pcos;
-}*/
-
 
 // Process samples. Multiple output
 void PhaseLock::process(const Real& sample_in, Real *samples_out)