55 lines
1.9 KiB
C++
55 lines
1.9 KiB
C++
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// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
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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, either version 3 of the License, or
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(at your option) any later version.
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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 for more details.
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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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/// @file LowPassFilter.cpp
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/// @brief A class to implement a second order low pass filter
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/// Author: Leonard Hall <LeonardTHall@gmail.com>
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#include <inttypes.h>
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#include <AP_Math.h>
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#include "LowPassFilter2p.h"
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void LowPassFilter2p::set_cutoff_frequency(float sample_freq, float cutoff_freq)
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{
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_cutoff_freq = cutoff_freq;
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float fr = sample_freq/_cutoff_freq;
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float ohm = tanf(PI/fr);
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float c = 1.0f+2.0f*cosf(PI/4.0f)*ohm + ohm*ohm;
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_b0 = ohm*ohm/c;
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_b1 = 2.0f*_b0;
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_b2 = _b0;
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_a1 = 2.0f*(ohm*ohm-1.0f)/c;
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_a2 = (1.0f-2.0f*cosf(PI/4.0f)*ohm+ohm*ohm)/c;
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}
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float LowPassFilter2p::apply(float sample)
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{
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// do the filtering
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float delay_element_0 = sample - _delay_element_1 * _a1 - _delay_element_2 * _a2;
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if (isnan(delay_element_0) || isinf(delay_element_0)) {
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// don't allow bad values to propogate via the filter
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delay_element_0 = sample;
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}
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float output = delay_element_0 * _b0 + _delay_element_1 * _b1 + _delay_element_2 * _b2;
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_delay_element_2 = _delay_element_1;
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_delay_element_1 = delay_element_0;
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// return the value. Should be no need to check limits
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return output;
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}
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