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.
This commit is contained in:
Andreas Eversberg 2016-05-11 18:42:22 +02:00
parent 3a54f46b81
commit 42eefe90bf
3 changed files with 64 additions and 46 deletions

View File

@ -24,10 +24,14 @@
#include "emphasis.h"
#include "debug.h"
#define PI M_PI
#define CUT_OFF_E 300.0 /* cut-off frequency for emphasis filters */
#define CUT_OFF_H 300.0 /* cut-off frequency for high-pass filters */
int init_emphasis(emphasis_t *state, int samplerate)
{
double factor;
double factor, rc, dt;
memset(state, 0, sizeof(*state));
if (samplerate < 24000) {
@ -35,33 +39,39 @@ int init_emphasis(emphasis_t *state, int samplerate)
return -1;
}
factor = 0.97;
/* exp (-2 * PI * CUT_OFF * delta_t) */
factor = exp(-2.0 * PI * CUT_OFF_E / samplerate); /* 1/samplerate == delta_t */
PDEBUG(DDSP, DEBUG_DEBUG, "Emphasis factor = %.3f\n", factor);
state->p.factor = factor;
state->p.amp = samplerate / 6350.0;
state->d.factor = factor;
state->d.amp = 1.0 / (samplerate / 6350.0);
state->d.d_factor = factor;
state->d.amp = 1.0 / (samplerate / 5550.0);
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;
}
void pre_emphasis(emphasis_t *state, int16_t *samples, int num)
{
int32_t sample;
double old_value, new_value, last_value, factor, amp;
double x, y, x_last, factor, amp;
int i;
last_value = state->p.last_value;
x_last = state->p.x_last;
factor = state->p.factor;
amp = state->p.amp;
for (i = 0; i < num; i++) {
old_value = (double)(*samples) / 32768.0;
x = (double)(*samples) / 32768.0;
new_value = old_value - factor * last_value;
y = x - factor * x_last;
last_value = old_value;
x_last = x;
sample = (int)(amp * new_value * 32768.0);
sample = (int)(amp * y * 32768.0);
if (sample > 32767)
sample = 32767;
else if (sample < -32768)
@ -69,27 +79,34 @@ void pre_emphasis(emphasis_t *state, int16_t *samples, int num)
*samples++ = sample;
}
state->p.last_value = last_value;
state->p.x_last = x_last;
}
void de_emphasis(emphasis_t *state, int16_t *samples, int num)
{
int32_t sample;
double old_value, new_value, last_value, factor, amp;
double x, y, z, y_last, z_last, d_factor, h_factor, amp;
int i;
last_value = state->d.last_value;
factor = state->d.factor;
y_last = state->d.y_last;
z_last = state->d.z_last;
d_factor = state->d.d_factor;
h_factor = state->d.h_factor;
amp = state->d.amp;
for (i = 0; i < num; i++) {
old_value = (double)(*samples) / 32768.0;
x = (double)(*samples) / 32768.0;
new_value = old_value + factor * last_value;
/* de-emphasis */
y = x + d_factor * y_last;
last_value = new_value;
/* high pass */
z = h_factor * (z_last + y - y_last);
sample = (int)(amp * new_value * 32768.0);
y_last = y;
z_last = z;
sample = (int)(amp * z * 32768.0);
if (sample > 32767)
sample = 32767;
else if (sample < -32768)
@ -97,6 +114,7 @@ void de_emphasis(emphasis_t *state, int16_t *samples, int num)
*samples++ = sample;
}
state->d.last_value = last_value;
state->d.y_last = y_last;
state->d.z_last = z_last;
}

View File

@ -1,12 +1,14 @@
typedef struct emphasis {
struct {
double last_value;
double x_last;
double factor;
double amp;
} p;
struct {
double last_value;
double factor;
double y_last;
double z_last;
double d_factor;
double h_factor;
double amp;
} d;
} emphasis_t;

View File

@ -3,6 +3,7 @@
#include <math.h>
#include <string.h>
#include "../common/emphasis.h"
#include "../common/debug.h"
#define level2db(level) (20 * log10(level))
#define db2level(db) pow(10, (double)db / 20.0)
@ -10,39 +11,36 @@
#define SAMPLERATE 48000
#define DEVIATION 8000.0
static void check_level(int16_t *samples, const char *desc)
static double test_freq[] = { 25, 50, 100, 150, 200, 250, 300, 350, 400, 500, 1000, 2000, 4000, 0 };
static void check_level(int16_t *samples, double freq, const char *desc)
{
int i;
int last = 0, envelope = 0;
int up = 0;
int freq;
for (i = 0; i < SAMPLERATE; i++) {
if (last < samples[i]) {
up = 1;
} else if (last > samples[i]) {
if (up) {
envelope = last;
if (last > envelope)
envelope = last;
}
up = 0;
}
if ((i % (SAMPLERATE/40)) == 0) {
freq = 500 + 500 * (i / (SAMPLERATE / 8));
printf("%s: f = %d envelop = %.4f\n", desc, freq, level2db((double)envelope / DEVIATION));
}
last = samples[i];
}
printf("%s: f = %.0f envelop = %.4f\n", desc, freq, level2db((double)envelope / DEVIATION));
}
static void gen_samples(int16_t *samples)
static void gen_samples(int16_t *samples, double freq)
{
int i;
double value;
int freq;
for (i = 0; i < SAMPLERATE; i++) {
freq = 500 + 500 * (i / (SAMPLERATE / 8));
value = sin(2.0 * M_PI * (double)freq / (double)SAMPLERATE * (double)i);
value = sin(2.0 * M_PI * freq / (double)SAMPLERATE * (double)i);
samples[i] = value * DEVIATION;
}
}
@ -51,26 +49,26 @@ int main(void)
{
emphasis_t estate;
int16_t samples[SAMPLERATE];
int i;
debuglevel = DEBUG_DEBUG;
printf("1000 Hz shall be close to 0 dB, that is no significant change in volume.\n\n");
/* generate sweep 0..4khz */
gen_samples(samples);
init_emphasis(&estate, SAMPLERATE);
// check_level(samples, "unchanged");
pre_emphasis(&estate, samples, SAMPLERATE);
check_level(samples, "pre-emphasis");
for (i = 0; test_freq[i]; i++) {
gen_samples(samples, test_freq[i]);
pre_emphasis(&estate, samples, SAMPLERATE);
check_level(samples, test_freq[i], "pre-emphasis");
}
/* generate sweep 0..4khz */
gen_samples(samples);
de_emphasis(&estate, samples, SAMPLERATE);
check_level(samples, "de-emphasis");
for (i = 0; test_freq[i]; i++) {
gen_samples(samples, test_freq[i]);
de_emphasis(&estate, samples, SAMPLERATE);
check_level(samples, test_freq[i], "de-emphasis");
}
return 0;
}