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SDR: Use filter to remove harmonics caused by downsampling/upsampling

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This drastically increases the signal-noise-ratio and removes co-channel
interferences. It gains CPU usage, but only on SDR threads, not on the
main thread.
This commit is contained in:
Andreas Eversberg
2017-09-24 15:59:24 +02:00
parent ff5faa8697
commit f469879ed0
3 changed files with 63 additions and 1 deletions
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29
src/common/sdr.c
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@@ -47,6 +47,10 @@ enum paging_signal;
/* enable to debug buffer handling */
//#define DEBUG_BUFFER
/* enable to test without oversampling filter */
//#define DISABLE_FILTER
typedef struct sdr_thread {
int use;
volatile int running, exit; /* flags to control exit of threads */
@@ -56,6 +60,7 @@ typedef struct sdr_thread {
volatile int in, out; /* in and out pointers (atomic, so no locking required) */
int max_fill; /* measure maximum buffer fill */
double max_fill_timer; /* timer to display/reset maximum fill */
iir_filter_t lp[2]; /* filter for upsample/downsample IQ data */
} sdr_thread_t;
typedef struct sdr_chan {
@@ -148,6 +153,10 @@ void *sdr_open(const char __attribute__((__unused__)) *audiodev, double *tx_freq
return NULL;
}
sdr->thread_read.in = sdr->thread_read.out = 0;
if (oversample > 1) {
iir_lowpass_init(&sdr->thread_read.lp[0], samplerate / 2.0, sdr_config->samplerate, 2);
iir_lowpass_init(&sdr->thread_read.lp[1], samplerate / 2.0, sdr_config->samplerate, 2);
}
memset(&sdr->thread_write, 0, sizeof(sdr->thread_write));
sdr->thread_write.buffer_size = sdr->latspl * 2 + 2;
sdr->thread_write.buffer = calloc(sdr->thread_write.buffer_size, sizeof(*sdr->thread_write.buffer));
@@ -161,6 +170,10 @@ void *sdr_open(const char __attribute__((__unused__)) *audiodev, double *tx_freq
return NULL;
}
sdr->thread_write.in = sdr->thread_write.out = 0;
if (oversample > 1) {
iir_lowpass_init(&sdr->thread_write.lp[0], samplerate / 2.0, sdr_config->samplerate, 2);
iir_lowpass_init(&sdr->thread_write.lp[1], samplerate / 2.0, sdr_config->samplerate, 2);
}
}
/* alloc fm modulation buffers */
@@ -397,6 +410,14 @@ static void *sdr_write_child(void *arg)
}
out = (out + 2) % sdr->thread_write.buffer_size;
}
sdr->thread_write.out = out;
#ifndef DISABLE_FILTER
/* filter spectrum */
if (sdr->oversample > 1) {
iir_process_baseband(&sdr->thread_write.lp[0], sdr->thread_write.buffer2, num * sdr->oversample);
iir_process_baseband(&sdr->thread_write.lp[1], sdr->thread_write.buffer2 + 1, num * sdr->oversample);
}
#endif
#ifdef HAVE_UHD
if (sdr_config->uhd)
uhd_send(sdr->thread_write.buffer2, num * sdr->oversample);
@@ -405,7 +426,6 @@ static void *sdr_write_child(void *arg)
if (sdr_config->soapy)
soapy_send(sdr->thread_write.buffer2, num * sdr->oversample);
#endif
sdr->thread_write.out = out;
}
/* delay some time */
@@ -440,6 +460,13 @@ static void *sdr_read_child(void *arg)
if (count > 0) {
#ifdef DEBUG_BUFFER
printf("Thread read %d samples from SDR and writes them to read buffer.\n", count);
#endif
#ifndef DISABLE_FILTER
/* filter spectrum */
if (sdr->oversample > 1) {
iir_process_baseband(&sdr->thread_read.lp[0], sdr->thread_read.buffer2, count);
iir_process_baseband(&sdr->thread_read.lp[1], sdr->thread_read.buffer2 + 1, count);
}
#endif
in = sdr->thread_read.in;
for (s = 0, ss = 0; s < count; s++) {