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Filter improvement: LP and HP filters, added test function 

The -3 dB level at cut-off frequency is now maintained for multiple
iterations.
This commit is contained in:
Andreas Eversberg 2017-01-15 11:08:42 +01:00
parent 799b96c801
commit ac0da3f76e
10 changed files with 186 additions and 32 deletions
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2
.gitignore vendored
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@ -9,7 +9,6 @@ config.sub
configure
depcomp
install-sh
libcolorize.pc
libtool
ltmain.sh
missing
@ -28,6 +27,7 @@ src/cnetz/cnetz
src/nmt/nmt
src/amps/amps
sim/cnetz_sim
src/test/test_filter
src/test/test_compandor
src/test/test_emphasis
src/test/test_dms

4
src/cnetz/scrambler.c
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@ -48,7 +48,7 @@ void scrambler_init(void)
void scrambler_setup(scrambler_t *scrambler, int samplerate)
{
filter_lowpass_init(&scrambler->lp, CARRIER_HZ - FILTER_BELOW, samplerate);
filter_lowpass_init(&scrambler->lp, CARRIER_HZ - FILTER_BELOW, samplerate, FILTER_TURNS);
scrambler->carrier_phaseshift256 = 256.0 / ((double)samplerate / CARRIER_HZ);
}
@ -77,7 +77,7 @@ void scrambler(scrambler_t *scrambler, int16_t *samples, int length)
scrambler->carrier_phase256 = phase;
/* cut off carrier frequency and modulation above carrier frequency */
filter_lowpass_process(&scrambler->lp, spl, length, FILTER_TURNS);
filter_process(&scrambler->lp, spl, length);
for (i = 0; i < length; i++) {
/* store result */

6
src/cnetz/scrambler.h
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@ -1,9 +1,9 @@
#include "../common/filter.h"
typedef struct scrambler {
double carrier_phaseshift256; /* carrier phase shift per sample */
double carrier_phase256; /* current phase of carrier frequency */
filter_lowpass_t lp; /* filter to remove carrier frequency */
double carrier_phaseshift256; /* carrier phase shift per sample */
double carrier_phase256; /* current phase of carrier frequency */
filter_t lp; /* filter to remove carrier frequency */
} scrambler_t;
void scrambler_init(void);

37
src/common/filter.c
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@ -26,14 +26,18 @@
#define PI M_PI
//#define CASCADE
void filter_lowpass_init(filter_lowpass_t *bq, double frequency, int samplerate)
void filter_lowpass_init(filter_t *bq, double frequency, int samplerate, int iterations)
{
double Fc, Q, K, norm;
if (iterations > 64) {
fprintf(stderr, "%s failed: too many iterations, please fix!\n", __func__);
abort();
}
memset(bq, 0, sizeof(*bq));
Q = sqrt(0.5); /* 0.7071... */
bq->iter = iterations;
Q = pow(sqrt(0.5), 1.0 / (double)iterations); /* 0.7071 @ 1 iteration */
Fc = frequency / (double)samplerate;
K = tan(PI * Fc);
norm = 1 / (1 + K / Q + K * K);
@ -44,18 +48,31 @@ void filter_lowpass_init(filter_lowpass_t *bq, double frequency, int samplerate)
bq->b2 = (1 - K / Q + K * K) * norm;
}
void filter_lowpass_process(filter_lowpass_t *bq, double *samples, int length, int iterations)
void filter_highpass_init(filter_t *bq, double frequency, int samplerate, int iterations)
{
double Fc, Q, K, norm;
memset(bq, 0, sizeof(*bq));
bq->iter = iterations;
Q = pow(sqrt(0.5), 1.0 / (double)iterations); /* 0.7071 @ 1 iteration */
Fc = frequency / (double)samplerate;
K = tan(PI * Fc);
norm = 1 / (1 + K / Q + K * K);
bq->a0 = 1 * norm;
bq->a1 = -2 * bq->a0;
bq->a2 = bq->a0;
bq->b1 = 2 * (K * K - 1) * norm;
bq->b2 = (1 - K / Q + K * K) * norm;
}
void filter_process(filter_t *bq, double *samples, int length)
{
double a0, a1, a2, b1, b2;
double *z1, *z2;
double in, out;
int iterations = bq->iter;
int i, j;
if (iterations > 10) {
fprintf(stderr, "%s failed: too many iterations, please fix!\n", __func__);
abort();
}
/* get states */
a0 = bq->a0;
a1 = bq->a1;

12
src/common/filter.h
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@ -1,12 +1,14 @@
#ifndef _FILTER_H
#define _FILTER_H
typedef struct filter_lowpass {
typedef struct filter {
int iter;
double a0, a1, a2, b1, b2;
double z1[10], z2[10];
} filter_lowpass_t;
double z1[64], z2[64];
} filter_t;
void filter_lowpass_init(filter_lowpass_t *bq, double frequency, int samplerate);
void filter_lowpass_process(filter_lowpass_t *bq, double *samples, int length, int iterations);
void filter_lowpass_init(filter_t *bq, double frequency, int samplerate, int iterations);
void filter_highpass_init(filter_t *bq, double frequency, int samplerate, int iterations);
void filter_process(filter_t *bq, double *samples, int length);
#endif /* _FILTER_H */

8
src/common/samplerate.c
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@ -37,8 +37,8 @@ int init_samplerate(samplerate_t *state, double samplerate)
memset(state, 0, sizeof(*state));
state->factor = samplerate / 8000.0;
filter_lowpass_init(&state->up.lp, 4000.0, samplerate);
filter_lowpass_init(&state->down.lp, 4000.0, samplerate);
filter_lowpass_init(&state->up.lp, 4000.0, samplerate, 1);
filter_lowpass_init(&state->down.lp, 4000.0, samplerate, 1);
return 0;
}
@ -56,7 +56,7 @@ int samplerate_downsample(samplerate_t *state, int16_t *input, int input_num, in
spl[i] = *input++ / 32768.0;
/* filter down */
filter_lowpass_process(&state->down.lp, spl, input_num, 1);
filter_process(&state->down.lp, spl, input_num);
/* resample filtered result */
in_index = state->down.in_index;
@ -125,7 +125,7 @@ int samplerate_upsample(samplerate_t *state, int16_t *input, int input_num, int1
state->up.in_index = in_index;
/* filter up */
filter_lowpass_process(&state->up.lp, spl, output_num, 1);
filter_process(&state->up.lp, spl, output_num);
/* convert double to samples */
for (i = 0; i < output_num; i++) {

4
src/common/samplerate.h
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@ -3,11 +3,11 @@
typedef struct samplerate {
double factor;
struct {
filter_lowpass_t lp;
filter_t lp;
double in_index;
} down;
struct {
filter_lowpass_t lp;
filter_t lp;
double in_index;
} up;
} samplerate_t;

10
src/common/sdr.c
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@ -39,7 +39,7 @@ typedef struct sdr_chan {
double rx_rot; /* rotation step per sample to shift rx frequency (used to shift) */
double rx_phase; /* current rotation phase (used to shift) */
double rx_last_phase; /* last phase of FM (used to demodulate) */
filter_lowpass_t rx_lp[2]; /* filters received IQ signal */
filter_t rx_lp[2]; /* filters received IQ signal */
} sdr_chan_t;
typedef struct sdr {
@ -123,8 +123,8 @@ void *sdr_open(const char __attribute__((__unused__)) *audiodev, double *tx_freq
PDEBUG(DSDR, DEBUG_INFO, "Frequency #%d: TX = %.6f MHz, RX = %.6f MHz\n", c, tx_frequency[c] / 1e6, rx_frequency[c] / 1e6);
sdr->chan[c].tx_frequency = tx_frequency[c];
sdr->chan[c].rx_frequency = rx_frequency[c];
filter_lowpass_init(&sdr->chan[c].rx_lp[0], bandwidth, samplerate);
filter_lowpass_init(&sdr->chan[c].rx_lp[1], bandwidth, samplerate);
filter_lowpass_init(&sdr->chan[c].rx_lp[0], bandwidth, samplerate, 1);
filter_lowpass_init(&sdr->chan[c].rx_lp[1], bandwidth, samplerate, 1);
}
if (sdr->paging_channel) {
PDEBUG(DSDR, DEBUG_INFO, "Paging Frequency: TX = %.6f MHz\n", paging_frequency / 1e6);
@ -378,8 +378,8 @@ int sdr_read(void *inst, int16_t **samples, int num, int channels)
Q[s] = i * sin(phase) + q * cos(phase);
}
sdr->chan[c].rx_phase = phase;
filter_lowpass_process(&sdr->chan[c].rx_lp[0], I, count, 1);
filter_lowpass_process(&sdr->chan[c].rx_lp[1], Q, count, 1);
filter_process(&sdr->chan[c].rx_lp[0], I, count);
filter_process(&sdr->chan[c].rx_lp[1], Q, count);
last_phase = sdr->chan[c].rx_last_phase;
for (s = 0; s < count; s++) {
phase = atan2(Q[s], I[s]);

10
src/test/Makefile.am
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@ -2,11 +2,21 @@ AUTOMAKE_OPTIONS = subdir-objects
AM_CPPFLAGS = -Wall -g $(all_includes)
noinst_PROGRAMS = \
test_filter \
test_compandor \
test_emphasis \
test_dms \
test_sms
test_filter_SOURCES = test_filter.c dummy.c
test_filter_LDADD = \
$(COMMON_LA) \
$(top_builddir)/src/common/libcommon.a \
$(ALSA_LIBS) \
$(UHD_LIBS) \
-lm
test_compandor_SOURCES = test_compandor.c
test_compandor_LDADD = \

125
src/test/test_filter.c Normal file
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@ -0,0 +1,125 @@
#include <stdio.h>
#include <stdint.h>
#include <math.h>
#include <string.h>
#include "../common/filter.h"
#include "../common/debug.h"
#define level2db(level) (20 * log10(level))
#define db2level(db) pow(10, (double)db / 20.0)
#define SAMPLERATE 48000
static double get_level(double *samples)
{
#if 0
int i;
double last = 0, envelope = 0;
int up = 0;
for (i = SAMPLERATE/2; 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];
}
#else
int i;
double envelope = 0;
for (i = SAMPLERATE/2; i < SAMPLERATE; i++) {
if (samples[i] > envelope)
envelope = samples[i];
}
#endif
return envelope;
}
static void gen_samples(double *samples, double freq)
{
int i;
double value;
for (i = 0; i < SAMPLERATE; i++) {
value = cos(2.0 * M_PI * freq / (double)SAMPLERATE * (double)i);
samples[i] = value;
}
}
int main(void)
{
filter_t filter_low;
filter_t filter_high;
double samples[SAMPLERATE];
double level;
int iter = 2;
int i;
debuglevel = DEBUG_DEBUG;
printf("testing low-pass filter with %d iterations\n", iter);
filter_lowpass_init(&filter_low, 1000.0, SAMPLERATE, iter);
for (i = 0; i < 4001; i += 100) {
gen_samples(samples, (double)i);
filter_process(&filter_low, samples, SAMPLERATE);
level = get_level(samples);
printf("%4d Hz: %.1f dB", i, level2db(level));
if (i == 1000)
printf(" cutoff\n");
else if (i == 2000)
printf(" double frequency\n");
else if (i == 4000)
printf(" quad frequency\n");
else
printf("\n");
}
printf("testing high-pass filter with %d iterations\n", iter);
filter_highpass_init(&filter_high, 2000.0, SAMPLERATE, iter);
for (i = 0; i < 4001; i += 100) {
gen_samples(samples, (double)i);
filter_process(&filter_high, samples, SAMPLERATE);
level = get_level(samples);
printf("%4d Hz: %.1f dB", i, level2db(level));
if (i == 2000)
printf(" cutoff\n");
else if (i == 1000)
printf(" half frequency\n");
else if (i == 500)
printf(" quarter frequency\n");
else
printf("\n");
}
printf("testing band-pass filter with %d iterations\n", iter);
filter_lowpass_init(&filter_low, 2000.0, SAMPLERATE, iter);
filter_highpass_init(&filter_high, 1000.0, SAMPLERATE, iter);
for (i = 0; i < 4001; i += 100) {
gen_samples(samples, (double)i);
filter_process(&filter_low, samples, SAMPLERATE);
filter_process(&filter_high, samples, SAMPLERATE);
level = get_level(samples);
printf("%4d Hz: %.1f dB", i, level2db(level));
if (i == 1000)
printf(" cutoff high\n");
else if (i == 2000)
printf(" cutoff low\n");
else
printf("\n");
}
return 0;
}