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/libemphasis/emphasis.c

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/* Pre-Emphasis and De-Emphasis implementation
*
* (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 <stdio.h>
#include <stdint.h>
#include <string.h>
#include <math.h>
#include "../libsample/sample.h"
#include "../libfilter/iir_filter.h"
#include "emphasis.h"
#define PI M_PI
static void gen_sine(sample_t *samples, int num, int samplerate, double freq)
{
int i;
for (i = 0; i < num; i++)
samples[i] = cos(2.0 * M_PI * freq / (double)samplerate * (double)i);
}
static double get_level(sample_t *samples, int num)
{
int i;
double envelope = 0;
for (i = num/2; i < num; i++) {
if (samples[i] > envelope)
envelope = samples[i];
}
return envelope;
}
/* calculate cut off from time constant in uS */
double timeconstant2cutoff(double time_constant_us)
{
return 1.0 / (2.0 * PI * time_constant_us / 1e6);
}
int init_emphasis(emphasis_t *state, int samplerate, double cut_off, double cut_off_h, double cut_off_l)
{
double factor;
sample_t test_samples[samplerate / 10];
memset(state, 0, sizeof(*state));
/* exp (-2 * PI * CUT_OFF * delta_t) */
factor = exp(-2.0 * PI * cut_off / (double)samplerate); /* 1/samplerate == delta_t */
// printf("Emphasis factor = %.3f\n", factor);
state->p.factor = factor;
state->p.amp = 1.0;
state->d.factor = factor;
state->d.amp = 1.0;
/* do not de-emphasis below CUT_OFF_H */
iir_highpass_init(&state->d.hp, cut_off_h, samplerate, 1);
/* do not pre-emphasis above CUT_OFF_L
* Mobile network specifications want -18 dB per octave.
* With two iterations we have 24 dB, - 6 dB (from emphasis). */
iir_lowpass_init(&state->p.lp, cut_off_l, samplerate, 2);
/* calibrate amplification to be neutral at 1000 Hz */
gen_sine(test_samples, sizeof(test_samples) / sizeof(test_samples[0]), samplerate, 1000.0);
pre_emphasis(state, test_samples, sizeof(test_samples) / sizeof(test_samples[0]));
state->p.amp = 1.0 / get_level(test_samples, sizeof(test_samples) / sizeof(test_samples[0]));
gen_sine(test_samples, sizeof(test_samples) / sizeof(test_samples[0]), samplerate, 1000.0);
de_emphasis(state, test_samples, sizeof(test_samples) / sizeof(test_samples[0]));
state->d.amp = 1.0 / get_level(test_samples, sizeof(test_samples) / sizeof(test_samples[0]));
return 0;
}
void pre_emphasis(emphasis_t *state, sample_t *samples, int num)
{
double x, y, x_last, factor, amp;
int i;
iir_process(&state->p.lp, samples, num);
x_last = state->p.x_last;
factor = state->p.factor;
amp = state->p.amp;
for (i = 0; i < num; i++) {
x = *samples;
/* pre-emphasis */
y = x - factor * x_last;
x_last = x;
*samples++ = amp * y;
}
state->p.x_last = x_last;
}
void de_emphasis(emphasis_t *state, sample_t *samples, int num)
{
double x, y, y_last, factor, amp;
int i;
y_last = state->d.y_last;
factor = state->d.factor;
amp = state->d.amp;
for (i = 0; i < num; i++) {
x = *samples;
/* de-emphasis */
y = x + factor * y_last;
y_last = y;
*samples++ = amp * y;
}
state->d.y_last = y_last;
}
/* high pass filter to remove DC and low frequencies */
void dc_filter(emphasis_t *state, sample_t *samples, int num)
{
iir_process(&state->d.hp, samples, num);
}