From e533997dbe8dfca7c69a909bbb7fed6dd58d6e6f Mon Sep 17 00:00:00 2001 From: f4exb Date: Tue, 8 Dec 2015 02:00:30 +0100 Subject: [PATCH] BFM demod: apply de-emphasis --- CMakeLists.txt | 2 ++ include/dsp/filterrc.h | 50 +++++++++++++++++++++++++++ plugins/channel/bfm/bfmdemod.cpp | 28 +++++++++++---- plugins/channel/bfm/bfmdemod.h | 5 +++ sdrbase/dsp/filterrc.cpp | 58 ++++++++++++++++++++++++++++++++ sdrbase/dsp/phaselock.cpp | 17 ++-------- 6 files changed, 138 insertions(+), 22 deletions(-) create mode 100644 include/dsp/filterrc.h create mode 100644 sdrbase/dsp/filterrc.cpp diff --git a/CMakeLists.txt b/CMakeLists.txt index 7ed5aacbf..cc40ba3a1 100644 --- a/CMakeLists.txt +++ b/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 diff --git a/include/dsp/filterrc.h b/include/dsp/filterrc.h new file mode 100644 index 000000000..7f1af7c89 --- /dev/null +++ b/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 . // +/////////////////////////////////////////////////////////////////////////////////// + +#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_ */ diff --git a/plugins/channel/bfm/bfmdemod.cpp b/plugins/channel/bfm/bfmdemod.cpp index 2fed4cb94..0541c33ac 100644 --- a/plugins/channel/bfm/bfmdemod.cpp +++ b/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; - m_audioBuffer[m_audioBufferFill].r = sample - sampleStereo; + Real deemph_l, deemph_r; // Pre-emphasis is applied on each channel before multiplexing + 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; diff --git a/plugins/channel/bfm/bfmdemod.h b/plugins/channel/bfm/bfmdemod.h index 114aacd96..9dc2fcba3 100644 --- a/plugins/channel/bfm/bfmdemod.h +++ b/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(); }; diff --git a/sdrbase/dsp/filterrc.cpp b/sdrbase/dsp/filterrc.cpp new file mode 100644 index 000000000..c5a7be190 --- /dev/null +++ b/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 . // +/////////////////////////////////////////////////////////////////////////////////// + +#include +#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; +} + + diff --git a/sdrbase/dsp/phaselock.cpp b/sdrbase/dsp/phaselock.cpp index 4840b9a60..f6464def7 100644 --- a/sdrbase/dsp/phaselock.cpp +++ b/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& samples_in, std::vector& samples_out) { @@ -247,21 +249,6 @@ void PhaseLock::process(const std::vector& samples_in, std::vector& 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)