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sdrangel/sdrbase/dsp/firfilter.cpp
2020-11-04 23:32:46 +01:00

71 lines
2.4 KiB
C++

///////////////////////////////////////////////////////////////////////////////////
// Copyright (C) 2020 kasper93 //
// written by Kacper Michajłow and 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 //
// (at your option) any later version. //
// //
// 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 "firfilter.h"
namespace FirFilterGenerators
{
void generateLowPassFilter(int nTaps, double sampleRate, double cutoff, std::vector<Real> &taps)
{
if (!(nTaps & 1))
{
qDebug("Filter has to have an odd number of taps");
nTaps++;
}
double Wc = (2.0 * M_PI * cutoff) / sampleRate;
int halfTaps = nTaps / 2 + 1;
taps.resize(halfTaps);
for (int i = 0; i < halfTaps; ++i)
{
if (i == halfTaps - 1)
{
taps[i] = Wc / M_PI;
}
else
{
int n = i - (nTaps - 1) / 2;
taps[i] = sin(n * Wc) / (n * M_PI);
}
}
// Blackman window
for (int i = 0; i < halfTaps; i++)
{
int n = i - (nTaps - 1) / 2;
taps[i] *= 0.42 + 0.5 * cos((2.0 * M_PI * n) / nTaps) + 0.08 * cos((4.0 * M_PI * n) / nTaps);
}
Real sum = 0;
size_t i;
for (i = 0; i < taps.size() - 1; ++i) {
sum += taps[i] * 2.0;
}
sum += taps[i];
for (i = 0; i < taps.size(); ++i) {
taps[i] /= sum;
}
}
}