145 lines
3.8 KiB
C
145 lines
3.8 KiB
C
/* Pre-Emphasis and De-Emphasis implementation
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*
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* (C) 2016 by Andreas Eversberg <jolly@eversberg.eu>
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* All Rights Reserved
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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#include <stdio.h>
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#include <stdint.h>
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#include <string.h>
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#include <math.h>
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#include "../libsample/sample.h"
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#include "../libfilter/iir_filter.h"
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#include "emphasis.h"
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#define PI M_PI
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static void gen_sine(sample_t *samples, int num, int samplerate, double freq)
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{
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int i;
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for (i = 0; i < num; i++)
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samples[i] = cos(2.0 * M_PI * freq / (double)samplerate * (double)i);
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}
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static double get_level(sample_t *samples, int num)
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{
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int i;
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double envelope = 0;
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for (i = num/2; i < num; i++) {
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if (samples[i] > envelope)
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envelope = samples[i];
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}
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return envelope;
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}
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/* calculate cut off from time constant in uS */
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double timeconstant2cutoff(double time_constant_us)
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{
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return 1.0 / (2.0 * PI * time_constant_us / 1e6);
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}
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int init_emphasis(emphasis_t *state, int samplerate, double cut_off, double cut_off_h, double cut_off_l)
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{
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double factor;
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sample_t test_samples[samplerate / 10];
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memset(state, 0, sizeof(*state));
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/* exp (-2 * PI * CUT_OFF * delta_t) */
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factor = exp(-2.0 * PI * cut_off / (double)samplerate); /* 1/samplerate == delta_t */
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// printf("Emphasis factor = %.3f\n", factor);
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state->p.factor = factor;
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state->p.amp = 1.0;
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state->d.factor = factor;
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state->d.amp = 1.0;
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/* do not de-emphasis below CUT_OFF_H */
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iir_highpass_init(&state->d.hp, cut_off_h, samplerate, 1);
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/* do not pre-emphasis above CUT_OFF_L
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* Mobile network specifications want -18 dB per octave.
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* With two iterations we have 24 dB, - 6 dB (from emphasis). */
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iir_lowpass_init(&state->p.lp, cut_off_l, samplerate, 2);
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/* calibrate amplification to be neutral at 1000 Hz */
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gen_sine(test_samples, sizeof(test_samples) / sizeof(test_samples[0]), samplerate, 1000.0);
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pre_emphasis(state, test_samples, sizeof(test_samples) / sizeof(test_samples[0]));
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state->p.amp = 1.0 / get_level(test_samples, sizeof(test_samples) / sizeof(test_samples[0]));
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gen_sine(test_samples, sizeof(test_samples) / sizeof(test_samples[0]), samplerate, 1000.0);
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de_emphasis(state, test_samples, sizeof(test_samples) / sizeof(test_samples[0]));
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state->d.amp = 1.0 / get_level(test_samples, sizeof(test_samples) / sizeof(test_samples[0]));
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return 0;
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}
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void pre_emphasis(emphasis_t *state, sample_t *samples, int num)
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{
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double x, y, x_last, factor, amp;
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int i;
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iir_process(&state->p.lp, samples, num);
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x_last = state->p.x_last;
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factor = state->p.factor;
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amp = state->p.amp;
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for (i = 0; i < num; i++) {
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x = *samples;
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/* pre-emphasis */
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y = x - factor * x_last;
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x_last = x;
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*samples++ = amp * y;
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}
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state->p.x_last = x_last;
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}
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void de_emphasis(emphasis_t *state, sample_t *samples, int num)
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{
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double x, y, y_last, factor, amp;
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int i;
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y_last = state->d.y_last;
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factor = state->d.factor;
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amp = state->d.amp;
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for (i = 0; i < num; i++) {
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x = *samples;
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/* de-emphasis */
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y = x + factor * y_last;
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y_last = y;
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*samples++ = amp * y;
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}
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state->d.y_last = y_last;
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}
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/* high pass filter to remove DC and low frequencies */
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void dc_filter(emphasis_t *state, sample_t *samples, int num)
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{
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iir_process(&state->d.hp, samples, num);
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}
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