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/*
***************************************************************************
*
* Author: Teunis van Beelen
*
* Copyright (C) 2007 - 2025 Teunis van Beelen
*
* Email: teuniz@protonmail.com
*
***************************************************************************
*
* 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, version 3 of the License.
*
* 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 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 "filter.h"
filtset_t * create_filter(int is_LPF, double cutoff_frequency, double sample_frequency)
{
filtset_t *settings;
settings = (filtset_t *)calloc(1, sizeof(filtset_t));
if(settings==NULL) return NULL;
settings->is_LPF = is_LPF;
settings->cutoff_frequency = cutoff_frequency;
settings->sample_frequency = sample_frequency;
settings->factor = get_filter_factor(cutoff_frequency, sample_frequency);
settings->old_input = 0.0;
settings->old_output = 0.0;
settings->first_sample = 1;
return settings;
}
filtset_t * create_filter_copy(filtset_t *src)
{
filtset_t *settings;
settings = (filtset_t *)calloc(1, sizeof(filtset_t));
if(settings==NULL)
{
return NULL;
}
memcpy(settings, src, sizeof(filtset_t));
return settings;
}
void reset_filter(double offset, filtset_t *settings)
{
settings->first_sample = 0;
settings->old_input = offset;
if(settings->is_LPF)
{
settings->old_output = offset;
}
else
{
settings->old_output = 0.0;
}
}
double first_order_filter(double new_input, filtset_t *settings)
{
double new_output;
if(settings->first_sample)
{
settings->first_sample = 0;
settings->old_input = new_input;
if(settings->is_LPF)
{
settings->old_output = new_input;
return new_input;
}
else
{
settings->old_output = 0.0;
return 0;
}
}
if(settings->is_LPF)
{
new_output = settings->old_output + ((settings->old_input - settings->old_output) * settings->factor);
}
else
{
new_output = settings->old_output * (1.0 - settings->factor);
new_output += new_input - settings->old_input;
}
settings->old_input = new_input;
settings->old_output = new_output;
return new_output;
}
double get_filter_factor(double cutoff_frequency, double sample_frequency)
{
double t;
t = (cutoff_frequency * 2.0 * M_PI) / sample_frequency;
t = exp(-t);
t = 1.0 - t; // now t will be in the range 0.001 (at high sample-frequency and low cut-off)
// to 0.999 (at low sample-frequency and high cut-off)
return t;
}
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