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Filter improvement: LP and HP filters, added test function

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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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37
src/common/filter.c
View File

@@ -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
View File

@@ -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
View File

@@ -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]);