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// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
/****************************************************************************
*
* Copyright (C) 2012 PX4 Development Team. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in
* the documentation and/or other materials provided with the
* distribution.
* 3. Neither the name PX4 nor the names of its contributors may be
* used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
* OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
* ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*
****************************************************************************/
/// @file LowPassFilter.cpp
/// @brief A class to implement a second order low pass filter
/// Author: Leonard Hall <LeonardTHall@gmail.com>
#include "LowPassFilter2p.hpp"
#include "math.h"
namespace math
{
void LowPassFilter2p::set_cutoff_frequency(float sample_freq, float cutoff_freq)
{
_cutoff_freq = cutoff_freq;
if (_cutoff_freq <= 0.0f) {
// no filtering
return;
}
float fr = sample_freq/_cutoff_freq;
float ohm = tanf(M_PI_F/fr);
float c = 1.0f+2.0f*cosf(M_PI_F/4.0f)*ohm + ohm*ohm;
_b0 = ohm*ohm/c;
_b1 = 2.0f*_b0;
_b2 = _b0;
_a1 = 2.0f*(ohm*ohm-1.0f)/c;
_a2 = (1.0f-2.0f*cosf(M_PI_F/4.0f)*ohm+ohm*ohm)/c;
}
float LowPassFilter2p::apply(float sample)
{
if (_cutoff_freq <= 0.0f) {
// no filtering
return sample;
}
// do the filtering
float delay_element_0 = sample - _delay_element_1 * _a1 - _delay_element_2 * _a2;
if (isnan(delay_element_0) || isinf(delay_element_0)) {
// don't allow bad values to propogate via the filter
delay_element_0 = sample;
}
float output = delay_element_0 * _b0 + _delay_element_1 * _b1 + _delay_element_2 * _b2;
_delay_element_2 = _delay_element_1;
_delay_element_1 = delay_element_0;
// return the value. Should be no need to check limits
return output;
}
} // namespace math
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