aptdec/src/filter.c

/*
 * aptdec - A lightweight FOSS (NOAA) APT decoder
 * Copyright (C) 2004-2009 Thierry Leconte (F4DWV) 2019-2022 Xerbo (xerbo@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; 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 "filter.h"

#include <math.h>

#include "util.h"

float convolve(const float *in, const float *taps, size_t len) {
    float sum = 0.0;
    for (size_t i = 0; i < len; i++) {
        sum += in[i] * taps[i];
    }

    return sum;
}

complexf_t hilbert_transform(const float *in, const float *taps, size_t len) {
    float i = 0.0;
    float q = 0.0;

    for (size_t k = 0; k < len; k++) {
        q += in[2 * k] * taps[k];
        i += in[2 * k];
    }

    i = in[len - 1] - (i / len);
#ifdef _MSC_VER
    return _FCbuild(i, q);
#else
    return i + q * I;
#endif
}

float interpolating_convolve(const float *in, const float *taps, size_t len, float offset, float delta) {
    float out = 0.0;
    float n = offset;

    for (size_t i = 0; i < (len - 1) / delta - 1; n += delta, i++) {
        int k = (int)floor(n);
        float alpha = n - k;

        out += in[i] * (taps[k] * (1.0f - alpha) + taps[k + 1] * alpha);
    }
    return out;
}