common code: Fixed de-emphasis by adding high pass filter
This high-pass filter filters out DC and frequencies below 300 Hz. Now de-emphases will not cause low frequencies to saturate audio level range or cause too high levels on low frequencies.
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@ -24,10 +24,14 @@
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#include "emphasis.h"
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#include "debug.h"
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#define PI M_PI
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#define CUT_OFF_E 300.0 /* cut-off frequency for emphasis filters */
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#define CUT_OFF_H 300.0 /* cut-off frequency for high-pass filters */
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int init_emphasis(emphasis_t *state, int samplerate)
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{
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double factor;
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double factor, rc, dt;
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memset(state, 0, sizeof(*state));
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if (samplerate < 24000) {
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@ -35,33 +39,39 @@ int init_emphasis(emphasis_t *state, int samplerate)
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return -1;
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}
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factor = 0.97;
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/* exp (-2 * PI * CUT_OFF * delta_t) */
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factor = exp(-2.0 * PI * CUT_OFF_E / samplerate); /* 1/samplerate == delta_t */
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PDEBUG(DDSP, DEBUG_DEBUG, "Emphasis factor = %.3f\n", factor);
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state->p.factor = factor;
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state->p.amp = samplerate / 6350.0;
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state->d.factor = factor;
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state->d.amp = 1.0 / (samplerate / 6350.0);
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state->d.d_factor = factor;
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state->d.amp = 1.0 / (samplerate / 5550.0);
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rc = 1.0 / (CUT_OFF_H * 2.0 *3.14);
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dt = 1.0 / samplerate;
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state->d.h_factor = rc / (rc + dt);
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PDEBUG(DDSP, DEBUG_DEBUG, "High-Pass factor = %.3f\n", state->d.h_factor);
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return 0;
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}
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void pre_emphasis(emphasis_t *state, int16_t *samples, int num)
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{
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int32_t sample;
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double old_value, new_value, last_value, factor, amp;
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double x, y, x_last, factor, amp;
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int i;
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last_value = state->p.last_value;
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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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old_value = (double)(*samples) / 32768.0;
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x = (double)(*samples) / 32768.0;
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new_value = old_value - factor * last_value;
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y = x - factor * x_last;
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last_value = old_value;
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x_last = x;
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sample = (int)(amp * new_value * 32768.0);
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sample = (int)(amp * y * 32768.0);
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if (sample > 32767)
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sample = 32767;
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else if (sample < -32768)
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@ -69,27 +79,34 @@ void pre_emphasis(emphasis_t *state, int16_t *samples, int num)
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*samples++ = sample;
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}
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state->p.last_value = last_value;
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state->p.x_last = x_last;
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}
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void de_emphasis(emphasis_t *state, int16_t *samples, int num)
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{
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int32_t sample;
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double old_value, new_value, last_value, factor, amp;
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double x, y, z, y_last, z_last, d_factor, h_factor, amp;
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int i;
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last_value = state->d.last_value;
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factor = state->d.factor;
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y_last = state->d.y_last;
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z_last = state->d.z_last;
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d_factor = state->d.d_factor;
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h_factor = state->d.h_factor;
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amp = state->d.amp;
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for (i = 0; i < num; i++) {
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old_value = (double)(*samples) / 32768.0;
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x = (double)(*samples) / 32768.0;
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new_value = old_value + factor * last_value;
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/* de-emphasis */
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y = x + d_factor * y_last;
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last_value = new_value;
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/* high pass */
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z = h_factor * (z_last + y - y_last);
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sample = (int)(amp * new_value * 32768.0);
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y_last = y;
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z_last = z;
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sample = (int)(amp * z * 32768.0);
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if (sample > 32767)
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sample = 32767;
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else if (sample < -32768)
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@ -97,6 +114,7 @@ void de_emphasis(emphasis_t *state, int16_t *samples, int num)
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*samples++ = sample;
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}
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state->d.last_value = last_value;
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state->d.y_last = y_last;
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state->d.z_last = z_last;
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}
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@ -1,12 +1,14 @@
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typedef struct emphasis {
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struct {
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double last_value;
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double x_last;
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double factor;
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double amp;
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} p;
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struct {
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double last_value;
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double factor;
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double y_last;
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double z_last;
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double d_factor;
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double h_factor;
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double amp;
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} d;
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} emphasis_t;
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@ -3,6 +3,7 @@
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#include <math.h>
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#include <string.h>
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#include "../common/emphasis.h"
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#include "../common/debug.h"
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#define level2db(level) (20 * log10(level))
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#define db2level(db) pow(10, (double)db / 20.0)
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@ -10,39 +11,36 @@
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#define SAMPLERATE 48000
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#define DEVIATION 8000.0
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static void check_level(int16_t *samples, const char *desc)
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static double test_freq[] = { 25, 50, 100, 150, 200, 250, 300, 350, 400, 500, 1000, 2000, 4000, 0 };
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static void check_level(int16_t *samples, double freq, const char *desc)
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{
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int i;
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int last = 0, envelope = 0;
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int up = 0;
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int freq;
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for (i = 0; i < SAMPLERATE; i++) {
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if (last < samples[i]) {
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up = 1;
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} else if (last > samples[i]) {
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if (up) {
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envelope = last;
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if (last > envelope)
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envelope = last;
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}
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up = 0;
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}
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if ((i % (SAMPLERATE/40)) == 0) {
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freq = 500 + 500 * (i / (SAMPLERATE / 8));
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printf("%s: f = %d envelop = %.4f\n", desc, freq, level2db((double)envelope / DEVIATION));
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}
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last = samples[i];
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}
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printf("%s: f = %.0f envelop = %.4f\n", desc, freq, level2db((double)envelope / DEVIATION));
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}
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static void gen_samples(int16_t *samples)
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static void gen_samples(int16_t *samples, double freq)
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{
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int i;
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double value;
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int freq;
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for (i = 0; i < SAMPLERATE; i++) {
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freq = 500 + 500 * (i / (SAMPLERATE / 8));
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value = sin(2.0 * M_PI * (double)freq / (double)SAMPLERATE * (double)i);
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value = sin(2.0 * M_PI * freq / (double)SAMPLERATE * (double)i);
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samples[i] = value * DEVIATION;
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}
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}
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@ -51,26 +49,26 @@ int main(void)
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{
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emphasis_t estate;
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int16_t samples[SAMPLERATE];
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int i;
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debuglevel = DEBUG_DEBUG;
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printf("1000 Hz shall be close to 0 dB, that is no significant change in volume.\n\n");
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/* generate sweep 0..4khz */
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gen_samples(samples);
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init_emphasis(&estate, SAMPLERATE);
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// check_level(samples, "unchanged");
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pre_emphasis(&estate, samples, SAMPLERATE);
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check_level(samples, "pre-emphasis");
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for (i = 0; test_freq[i]; i++) {
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gen_samples(samples, test_freq[i]);
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pre_emphasis(&estate, samples, SAMPLERATE);
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check_level(samples, test_freq[i], "pre-emphasis");
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}
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/* generate sweep 0..4khz */
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gen_samples(samples);
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de_emphasis(&estate, samples, SAMPLERATE);
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check_level(samples, "de-emphasis");
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for (i = 0; test_freq[i]; i++) {
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gen_samples(samples, test_freq[i]);
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de_emphasis(&estate, samples, SAMPLERATE);
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check_level(samples, test_freq[i], "de-emphasis");
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}
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return 0;
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}
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