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80 lines
3.4 KiB
C++
80 lines
3.4 KiB
C++
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///////////////////////////////////////////////////////////////////////////////////
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// Copyright (C) 2005,2007,2012 Free Software Foundation, Inc.
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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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#include <cmath>
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#include <QDebug>
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#include "dsp/fmpreemphasis.h"
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FMPreemphasis::FMPreemphasis(int sampleRate, Real tau, Real highFreq)
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{
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configure(sampleRate, tau, highFreq);
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}
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void FMPreemphasis::configure(int sampleRate, Real tau, Real highFreq)
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{
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// Based on: https://github.com/gnuradio/gnuradio/blob/master/gr-analog/python/analog/fm_emph.py
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// Compare to freq response in https://www.mathworks.com/help/comm/ref/comm.fmbroadcastmodulator-system-object.html
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// High frequency corner at which to flatten the gain
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double fh = std::min((double)highFreq, 0.925 * sampleRate/2.0);
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// Digital corner frequencies
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double w_cl = 1.0 / tau;
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double w_ch = 2.0 * M_PI * fh;
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// Prewarped analog corner frequencies
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double w_cla = 2.0 * sampleRate * std::tan(w_cl / (2.0 * sampleRate));
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double w_cha = 2.0 * sampleRate * std::tan(w_ch / (2.0 * sampleRate));
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// Resulting digital pole, zero, and gain term from the bilinear
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// transformation of H(s) = (s + w_cla) / (s + w_cha) to
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// H(z) = b0 (1 - z1 z^-1)/(1 - p1 z^-1)
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double kl = -w_cla / (2.0 * sampleRate);
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double kh = -w_cha / (2.0 * sampleRate);
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double z1 = (1.0 + kl) / (1.0 - kl);
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double p1 = (1.0 + kh) / (1.0 - kh);
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double b0 = (1.0 - kl) / (1.0 - kh);
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// Adjust with a gain, g, so 0 dB gain at DC
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double g = std::abs(1.0 - p1) / (b0 * std::abs(1.0 - z1));
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// Caclulate IIR taps
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m_b0 = (Real)(g * b0 * 1.0);
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m_b1 = (Real)(g * b0 * -z1);
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m_a1 = (Real)-p1;
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// Zero delay line so we get reproducible results
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m_z = 0;
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qDebug() << "FMPreemphasis::configure: tau: " << tau
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<< " sampleRate: " << sampleRate
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<< " b0: " << m_b0
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<< " b1: " << m_b1
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<< " a1: " << m_a1;
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}
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Real FMPreemphasis::filter(const Real sampleIn)
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{
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Real sampleOut;
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// See Transposed Direct form 2 - https://en.wikipedia.org/wiki/Digital_biquad_filter
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sampleOut = sampleIn * m_b0 + m_z;
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m_z = sampleIn * m_b1 + sampleOut * -m_a1;
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return sampleOut;
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}
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