An implementation of Analog cellular networks like A-Netz, B-Netz, C-Netz, NMT, AMPS, TACS, JTACS, Radiocom 2000, IMTS, MPT1327, Eurosignal and more http://osmocom-analog.eversberg.eu/
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osmocom-analog/src/libgoertzel/goertzel.c

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/* Goertzel functions
*
* (C) 2016 by Andreas Eversberg <jolly@eversberg.eu>
* All Rights Reserved
*
* 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 3 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 <http://www.gnu.org/licenses/>.
*/
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <math.h>
#include "../libsample/sample.h"
#include "../libdebug/debug.h"
#include "goertzel.h"
/*
* audio level calculation
*/
/* Return average value (rectified value)
* The input must not have any dc offset!
* For a perfect rectangualr wave, the result would equal the peak level.
* For a sine wave the result would be factor (2 / PI) below peak level.
*/
double audio_level(sample_t *samples, int length)
{
double level, sk;
int n;
/* level calculation */
level = 0;
for (n = 0; n < length; n++) {
sk = samples[n];
if (sk < 0)
level -= (double)sk;
if (sk > 0)
level += (double)sk;
}
level = level / (double)length;
return level;
}
void audio_goertzel_init(goertzel_t *goertzel, double freq, int samplerate)
{
memset(goertzel, 0, sizeof(*goertzel));
goertzel->coeff = 2.0 * cos(2.0 * M_PI * freq / (double)samplerate);
}
/*
* goertzel filter
*/
/* filter frequencies and return their levels
*
* samples: pointer to sample buffer
* length: length of buffer
* offset: for ring buffer, start here and wrap around to 0 when length has been hit
* coeff: array of coefficients (coeff << 15)
* result: array of result levels (average value of the sine, that is 1 / (PI/2) of the sine's peak)
* k: number of frequencies to check
*/
void audio_goertzel(goertzel_t *goertzel, sample_t *samples, int length, int offset, double *result, int k)
{
double sk, sk1, sk2;
double cos2pik;
int i, n;
/* we do goertzel */
for (i = 0; i < k; i++) {
sk = 0;
sk1 = 0;
sk2 = 0;
cos2pik = goertzel[i].coeff;
/* note: after 'length' cycles, offset is restored to its initial value */
for (n = 0; n < length; n++) {
sk = (cos2pik * sk1) - sk2 + samples[offset++];
sk2 = sk1;
sk1 = sk;
if (offset == length)
offset = 0;
}
/* compute level of signal */
result[i] = sqrt(
(sk * sk) -
(cos2pik * sk * sk2) +
(sk2 * sk2)
) / (double)length * 2.0 * 0.63662; /* 1 / (PI/2) */
}
}