kopia lustrzana https://github.com/f4exb/sdrangel
155 wiersze
5.7 KiB
C++
155 wiersze
5.7 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_RAISEDCOSINE_H
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#define INCLUDE_RAISEDCOSINE_H
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#include <cmath>
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#include <cstdio>
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#include "dsp/dsptypes.h"
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// Raised-cosine low-pass filter for pulse shaping, without intersymbol interference (ISI)
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// https://en.wikipedia.org/wiki/Raised-cosine_filter
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// This could be optimised in to a polyphase filter, as samplesPerSymbol-1 inputs
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// to filter() should be zero, as the data is upsampled to the sample rate
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template <class Type> class RaisedCosine {
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public:
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RaisedCosine() : m_ptr(0) { }
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// beta - roll-off factor
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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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// normaliseUpsampledAmplitude - when true, scale the filter such that an upsampled
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// (by samplesPerSymbol) bipolar sequence (E.g. [1 0 0 -1 0 0..]) has maximum
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// output values close to (1,-1)
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void create(double beta, int symbolSpan, int samplesPerSymbol, bool normaliseUpsampledAmplitude = false)
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{
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int nTaps = symbolSpan * samplesPerSymbol + 1;
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int i, j;
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// check constraints
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if(!(nTaps & 1)) {
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qDebug("Raised cosine 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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// 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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double denominator = 1.0 - std::pow(2.0 * beta * t, 2.0);
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double sinc;
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if (denominator != 0.0)
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{
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if (t == 0)
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sinc = 1.0;
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else
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sinc = sin(M_PI*t)/(M_PI*t);
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m_taps[i] = sinc * (cos(M_PI*beta*t) / denominator) / (double)samplesPerSymbol;
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}
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else
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m_taps[i] = beta * sin(M_PI/(2.0*beta)) / (2.0*samplesPerSymbol);
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}
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// normalize
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if (!normaliseUpsampledAmplitude)
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{
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// normalize energy
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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 += std::pow(m_taps[i], 2.0) * 2;
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sum += std::pow(m_taps[i], 2.0);
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sum = std::sqrt(sum);
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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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else
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{
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// Calculate maximum output of filter, assuming upsampled bipolar input E.g. [1 0 0 -1 0 0..]
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// This doesn't necessarily include the centre tap, so we try each offset
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double maxGain = 0.0;
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for (i = 0; i < samplesPerSymbol; i++)
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{
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double g = 0.0;
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for (j = 0; j < (int)m_taps.size() - 1; j += samplesPerSymbol)
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g += std::fabs(2.0 * m_taps[j]);
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if ((i & 1) == 0)
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g += std::fabs(m_taps[j]);
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if (g > maxGain)
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maxGain = g;
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}
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// Scale up so maximum out is 1
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for(i = 0; i < (int)m_taps.size(); i++)
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m_taps[i] /= maxGain;
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}
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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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// Print taps as a Matlab vector
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void printTaps(const char *name)
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{
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printf("%s = [", name);
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for (int i = 0; i <= m_taps.size() - 1; ++i) {
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printf("%g ", m_taps[i]);
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}
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for (int i = m_taps.size() - 2; i >= 0; --i) {
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printf("%g ", m_taps[i]);
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}
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printf("];\n");
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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_RAISEDCOSINE_H
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