aptdec/src/filter.c

/* 
 *  This file is part of Aptdec.
 *  Copyright (c) 2004-2009 Thierry Leconte (F4DWV), Xerbo (xerbo@protonmail.com) 2019-2020
 *
 *  Aptdec 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, either version 2 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 for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
 *
 */

#include <math.h>

// Finite impulse response
float fir(float *buff, const float *coeff, const int len) {
	double r;

	r = 0.0;
	for (int i = 0; i < len; i++) {
		r += buff[i] * coeff[i];
	}
	return (float)r;
}

/* IQ finite impulse response
 * Turn samples into a single IQ sample
 */
void iqfir(float *buff, const float *coeff, const int len, double *I, double *Q) {
	double i = 0.0, q = 0.0;

	for (int k = 0; k < len; k++) {
		q += buff[2*k] * coeff[k];
		i += buff[2*k];
	}

	i = buff[len-1] - (i / len);
	*I = i, *Q = q;
}

/* Gaussian finite impulse responce compensation
 * https://www.recordingblogs.com/wiki/gaussian-window
 */
float rsfir(double *buff, const float *coeff, const int len, const double offset, const double delta) {
	double out;

	out = 0.0;
	double n = offset;
	for (int i = 0; i < (len-1)/delta-1; n += delta, i++) {
		int k;
		double alpha;

		k = (int)floor(n);
		alpha = n - k;
		out += buff[i] * (coeff[k] * (1.0 - alpha) + coeff[k + 1] * alpha);
	}
	return (float)out;
}