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122 lines
4.4 KiB
C++
122 lines
4.4 KiB
C++
///////////////////////////////////////////////////////////////////////////////////
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// Copyright (C) 2015 Edouard Griffiths, F4EXB //
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// Copyright (C) 2020 Jon Beniston, M7RCE //
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// //
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// This program is free software; you can redistribute it and/or modify //
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// it under the terms of the GNU General Public License as published by //
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// the Free Software Foundation as version 3 of the License, or //
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// (at your option) any later version. //
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// //
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// This program is distributed in the hope that it will be useful, //
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// but WITHOUT ANY WARRANTY; without even the implied warranty of //
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the //
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// GNU General Public License V3 for more details. //
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// //
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// You should have received a copy of the GNU General Public License //
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// along with this program. If not, see <http://www.gnu.org/licenses/>. //
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///////////////////////////////////////////////////////////////////////////////////
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#ifndef INCLUDE_GAUSSIAN_H
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#define INCLUDE_GAUSSIAN_H
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#include <math.h>
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#include "dsp/dsptypes.h"
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// Standard values for bt
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#define GAUSSIAN_BT_BLUETOOTH 0.5
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#define GAUSSIAN_BT_GSM 0.3
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#define GAUSSIAN_BT_CCSDS 0.25
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#define GAUSSIAN_BT_802_15_4 0.5
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#define GAUSSIAN_BT_AIS 0.5
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// Gaussian low-pass filter for pulse shaping
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// https://onlinelibrary.wiley.com/doi/pdf/10.1002/9780470041956.app2
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// Unlike raisedcosine.h, this should be feed NRZ values rather than impulse stream, as described here:
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// https://www.mathworks.com/matlabcentral/answers/107231-why-does-the-pulse-shape-generated-by-gaussdesign-differ-from-that-used-in-the-comm-gmskmodulator-ob
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template <class Type> class Gaussian {
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public:
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Gaussian() : m_ptr(0) { }
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// bt - 3dB bandwidth symbol time product
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// symbolSpan - number of symbols over which the filter is spread
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// samplesPerSymbol - number of samples per symbol
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void create(double bt, int symbolSpan, int samplesPerSymbol)
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{
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int nTaps = symbolSpan * samplesPerSymbol + 1;
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int i;
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// check constraints
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if(!(nTaps & 1)) {
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qDebug("Gaussian filter has to have an odd number of taps");
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nTaps++;
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}
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// make room
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m_samples.resize(nTaps);
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for(int i = 0; i < nTaps; i++)
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m_samples[i] = 0;
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m_ptr = 0;
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m_taps.resize(nTaps / 2 + 1);
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// See eq B.2 - this is alpha over Ts
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double alpha_t = std::sqrt(std::log(2.0) / 2.0) / (bt);
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double sqrt_pi_alpha_t = std::sqrt(M_PI) / alpha_t;
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// calculate filter taps
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for(i = 0; i < nTaps / 2 + 1; i++)
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{
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double t = (i - (nTaps / 2)) / (double)samplesPerSymbol;
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// See eq B.5
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m_taps[i] = sqrt_pi_alpha_t * std::exp(-std::pow(t * M_PI / alpha_t, 2.0));
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}
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// normalize
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double sum = 0;
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for(i = 0; i < (int)m_taps.size() - 1; i++)
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sum += m_taps[i] * 2;
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sum += m_taps[i];
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for(i = 0; i < (int)m_taps.size(); i++)
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m_taps[i] /= sum;
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}
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Type filter(Type sample)
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{
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Type acc = 0;
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unsigned int n_samples = m_samples.size();
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unsigned int n_taps = m_taps.size() - 1;
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unsigned int a = m_ptr;
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unsigned int b = a == n_samples - 1 ? 0 : a + 1;
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m_samples[m_ptr] = sample;
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for (unsigned int i = 0; i < n_taps; ++i)
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{
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acc += (m_samples[a] + m_samples[b]) * m_taps[i];
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a = (a == 0) ? n_samples - 1 : a - 1;
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b = (b == n_samples - 1) ? 0 : b + 1;
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}
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acc += m_samples[a] * m_taps[n_taps];
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m_ptr = (m_ptr == n_samples - 1) ? 0 : m_ptr + 1;
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return acc;
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}
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/*
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void printTaps()
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{
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for (int i = 0; i < m_taps.size(); i++)
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printf("%.4f ", m_taps[i]);
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printf("\n");
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}
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*/
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private:
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std::vector<Real> m_taps;
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std::vector<Type> m_samples;
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unsigned int m_ptr;
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};
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#endif // INCLUDE_GAUSSIAN_H
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