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common code: Fixed de-emphasis by adding high pass filter

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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
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46
src/test/test_emphasis.c
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@@ -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;
}