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Work on emphasis filter: Neutral @ 1000 Hz; Use filter before de-emphasis

This commit is contained in:
Andreas Eversberg 2017-01-28 08:02:11 +01:00
parent b3fd53b165
commit 71e556e7ff
5 changed files with 88 additions and 68 deletions
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2
src/cnetz/cnetz.c
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@ -143,7 +143,7 @@
/* uncomment this to do echo debugging (-L) on Speech Channel */
//#define DEBUG_SPK
#define CUT_OFF_EMPHASIS_CNETZ 796 /* 200 uS time constant */
#define CUT_OFF_EMPHASIS_CNETZ 796.0 /* 200 uS time constant */
/* Call reference for calls from mobile station to network
This offset of 0x400000000 is required for MNCC interface. */

4
src/common/debug.c
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@ -126,11 +126,11 @@ const char *debug_db(double level_db)
static char text[128];
int l;
strcpy(text, ": . : . : . : . : . : . : . : . : ");
strcpy(text, ": . : . : . : . : . : . : . : . | . : . : . : . : . : . : . : . :");
if (level_db <= 0.0)
return text;
l = (int)round(level2db(level_db));
if (l > 3)
if (l > 48)
return text;
if (l < -48)
return text;

79
src/common/emphasis.c
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@ -21,37 +21,60 @@
#include <stdint.h>
#include <string.h>
#include <math.h>
#include "filter.h"
#include "emphasis.h"
#include "debug.h"
#define PI M_PI
#define CUT_OFF_H 200.0 /* cut-off frequency for high-pass filters */
#define CUT_OFF_H 300.0 /* cut-off frequency for high-pass filters */
static void gen_sine(double *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(double *samples, int num)
{
int i;
double envelope = 0;
for (i = num/2; i < num; i++) {
if (samples[i] > envelope)
envelope = samples[i];
}
return envelope;
}
int init_emphasis(emphasis_t *state, int samplerate, double cut_off)
{
double factor, rc, dt;
double factor;
double test_samples[samplerate / 10];
memset(state, 0, sizeof(*state));
if (samplerate < 24000) {
PDEBUG(DDSP, DEBUG_ERROR, "Sample rate must be at least 24000 Hz!\n");
return -1;
}
/* exp (-2 * PI * CUT_OFF * delta_t) */
factor = exp(-2.0 * PI * cut_off / samplerate); /* 1/samplerate == delta_t */
factor = exp(-2.0 * PI * cut_off / (double)samplerate); /* 1/samplerate == delta_t */
PDEBUG(DDSP, DEBUG_DEBUG, "Emphasis factor = %.3f\n", factor);
state->p.factor = factor;
state->p.amp = samplerate / 6400.0;
state->d.d_factor = factor;
state->d.amp = 1.0 / (samplerate / 5750.0);
state->p.amp = 1.0;
state->d.factor = factor;
state->d.amp = 1.0;
filter_highpass_init(&state->d.hp, CUT_OFF_H, samplerate, 1);
/* 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]));
/* high-pass filter prevents low frequency noise and dc level
* from being amplified by de-emphasis */
rc = 1.0 / (CUT_OFF_H * 2.0 *3.14);
dt = 1.0 / samplerate;
state->d.h_factor = rc / (rc + dt);
PDEBUG(DDSP, DEBUG_DEBUG, "High-Pass factor = %.3f\n", state->d.h_factor);
return 0;
}
@ -65,13 +88,14 @@ void pre_emphasis(emphasis_t *state, double *samples, int num)
amp = state->p.amp;
for (i = 0; i < num; i++) {
x = *samples / 32768.0;
x = *samples;
/* pre-emphasis */
y = x - factor * x_last;
x_last = x;
*samples++ = (int)(amp * y * 32768.0);
*samples++ = amp * y;
}
state->p.x_last = x_last;
@ -79,31 +103,26 @@ void pre_emphasis(emphasis_t *state, double *samples, int num)
void de_emphasis(emphasis_t *state, double *samples, int num)
{
double x, y, z, y_last, z_last, d_factor, h_factor, amp;
double x, y, y_last, factor, amp;
int i;
filter_process(&state->d.hp, samples, num);
y_last = state->d.y_last;
z_last = state->d.z_last;
d_factor = state->d.d_factor;
h_factor = state->d.h_factor;
factor = state->d.factor;
amp = state->d.amp;
for (i = 0; i < num; i++) {
x = *samples / 32768.0;
x = *samples;
/* de-emphasis */
y = x + d_factor * y_last;
/* high pass */
z = h_factor * (z_last + y - y_last);
y = x + factor * y_last;
y_last = y;
z_last = z;
*samples++ = (int)(amp * z * 32768.0);
*samples++ = amp * y;
}
state->d.y_last = y_last;
state->d.z_last = z_last;
}

7
src/common/emphasis.h
View File

@ -5,15 +5,14 @@ typedef struct emphasis {
double amp;
} p;
struct {
filter_t hp;
double y_last;
double z_last;
double d_factor;
double h_factor;
double factor;
double amp;
} d;
} emphasis_t;
#define CUT_OFF_EMPHASIS_DEFAULT 300.0
#define CUT_OFF_EMPHASIS_DEFAULT 300.0
int init_emphasis(emphasis_t *state, int samplerate, double cut_off);
void pre_emphasis(emphasis_t *state, double *samples, int num);

64
src/test/test_emphasis.c
View File

@ -3,6 +3,7 @@
#include <math.h>
#include <string.h>
#include "../common/sample.h"
#include "../common/filter.h"
#include "../common/emphasis.h"
#include "../common/debug.h"
@ -10,29 +11,17 @@
#define db2level(db) pow(10, (double)db / 20.0)
#define SAMPLERATE 48000
#define DEVIATION 8000.0
static double test_freq[] = { 25, 50, 100, 200, 250, 300, 400, 500, 1000, 2000, 4000, 0 };
static void check_level(sample_t *samples, double freq, const char *desc)
static double get_level(double *samples)
{
int i;
int last = 0, envelope = 0;
int up = 0;
for (i = 0; i < SAMPLERATE; i++) {
if (last < samples[i]) {
up = 1;
} else if (last > samples[i]) {
if (up) {
if (last > envelope)
envelope = last;
}
up = 0;
}
last = samples[i];
double envelope = 0;
for (i = SAMPLERATE/2; i < SAMPLERATE; i++) {
if (samples[i] > envelope)
envelope = samples[i];
}
printf("%s: f = %.0f envelop = %.4f\n", desc, freq, level2db((double)envelope / DEVIATION));
return envelope;
}
static void gen_samples(double *samples, double freq)
@ -41,34 +30,47 @@ static void gen_samples(double *samples, double freq)
double value;
for (i = 0; i < SAMPLERATE; i++) {
value = sin(2.0 * M_PI * freq / (double)SAMPLERATE * (double)i);
samples[i] = value * DEVIATION;
value = cos(2.0 * M_PI * freq / (double)SAMPLERATE * (double)i);
samples[i] = value;
}
}
int main(void)
{
emphasis_t estate;
double cut_off = CUT_OFF_EMPHASIS_DEFAULT;
double samples[SAMPLERATE];
int i;
double level;
double i;
debuglevel = DEBUG_DEBUG;
printf("1000 Hz shall be close to 0 dB, that is no significant change in volume.\n\n");
init_emphasis(&estate, SAMPLERATE, cut_off);
init_emphasis(&estate, SAMPLERATE, CUT_OFF_EMPHASIS_DEFAULT);
printf("testing pre-emphasis filter with cut-off frequency %.1f\n", cut_off);
for (i = 0; test_freq[i]; i++) {
gen_samples(samples, test_freq[i]);
for (i = 31.25; i < 4001; i = i * sqrt(sqrt(2.0))) {
gen_samples(samples, (double)i);
pre_emphasis(&estate, samples, SAMPLERATE);
check_level(samples, test_freq[i], "pre-emphasis");
level = get_level(samples);
printf("%s%.0f Hz: %.1f dB", debug_db(level), i, level2db(level));
if ((int)round(i) == 1000)
printf(" level=%.6f\n", level);
else
printf("\n");
}
/* generate sweep 0..4khz */
for (i = 0; test_freq[i]; i++) {
gen_samples(samples, test_freq[i]);
printf("testing de-emphasis filter with cut-off frequency %.1f\n", cut_off);
for (i = 31.25; i < 4001; i = i * sqrt(sqrt(2.0))) {
gen_samples(samples, (double)i);
de_emphasis(&estate, samples, SAMPLERATE);
check_level(samples, test_freq[i], "de-emphasis");
level = get_level(samples);
printf("%s%.0f Hz: %.1f dB", debug_db(level), i, level2db(level));
if ((int)round(i) == 1000)
printf(" level=%.6f\n", level);
else
printf("\n");
}
return 0;