SDR: Use filter to remove harmonics caused by downsampling/upsampling
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.
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@ -115,6 +115,7 @@ void iir_process(iir_filter_t *filter, sample_t *samples, int length)
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b1 = filter->b1;
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b2 = filter->b2;
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/* these are state pointers, so no need to write back */
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z1 = filter->z1;
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z2 = filter->z2;
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@ -131,3 +132,36 @@ void iir_process(iir_filter_t *filter, sample_t *samples, int length)
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}
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}
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void iir_process_baseband(iir_filter_t *filter, float *baseband, int length)
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{
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double a0, a1, a2, b1, b2;
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double *z1, *z2;
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double in, out;
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int iterations = filter->iter;
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int i, j;
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/* get states */
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a0 = filter->a0;
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a1 = filter->a1;
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a2 = filter->a2;
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b1 = filter->b1;
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b2 = filter->b2;
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/* these are state pointers, so no need to write back */
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z1 = filter->z1;
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z2 = filter->z2;
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/* process filter */
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for (i = 0; i < length; i++) {
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in = *baseband;
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for (j = 0; j < iterations; j++) {
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out = in * a0 + z1[j];
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z1[j] = in * a1 + z2[j] - b1 * out;
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z2[j] = in * a2 - b2 * out;
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in = out;
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}
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*baseband = in;
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baseband += 2;
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}
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}
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@ -12,5 +12,6 @@ void iir_highpass_init(iir_filter_t *filter, double frequency, int samplerate, i
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void iir_bandpass_init(iir_filter_t *filter, double frequency, int samplerate, int iterations);
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void iir_notch_init(iir_filter_t *filter, double frequency, int samplerate, int iterations);
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void iir_process(iir_filter_t *filter, sample_t *samples, int length);
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void iir_process_baseband(iir_filter_t *filter, float *baseband, int length);
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#endif /* _FILTER_H */
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@ -47,6 +47,10 @@ enum paging_signal;
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/* enable to debug buffer handling */
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//#define DEBUG_BUFFER
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/* enable to test without oversampling filter */
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//#define DISABLE_FILTER
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typedef struct sdr_thread {
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int use;
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volatile int running, exit; /* flags to control exit of threads */
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@ -56,6 +60,7 @@ typedef struct sdr_thread {
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volatile int in, out; /* in and out pointers (atomic, so no locking required) */
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int max_fill; /* measure maximum buffer fill */
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double max_fill_timer; /* timer to display/reset maximum fill */
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iir_filter_t lp[2]; /* filter for upsample/downsample IQ data */
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} sdr_thread_t;
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typedef struct sdr_chan {
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@ -148,6 +153,10 @@ void *sdr_open(const char __attribute__((__unused__)) *audiodev, double *tx_freq
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return NULL;
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}
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sdr->thread_read.in = sdr->thread_read.out = 0;
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if (oversample > 1) {
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iir_lowpass_init(&sdr->thread_read.lp[0], samplerate / 2.0, sdr_config->samplerate, 2);
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iir_lowpass_init(&sdr->thread_read.lp[1], samplerate / 2.0, sdr_config->samplerate, 2);
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}
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memset(&sdr->thread_write, 0, sizeof(sdr->thread_write));
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sdr->thread_write.buffer_size = sdr->latspl * 2 + 2;
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sdr->thread_write.buffer = calloc(sdr->thread_write.buffer_size, sizeof(*sdr->thread_write.buffer));
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@ -161,6 +170,10 @@ void *sdr_open(const char __attribute__((__unused__)) *audiodev, double *tx_freq
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return NULL;
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}
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sdr->thread_write.in = sdr->thread_write.out = 0;
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if (oversample > 1) {
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iir_lowpass_init(&sdr->thread_write.lp[0], samplerate / 2.0, sdr_config->samplerate, 2);
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iir_lowpass_init(&sdr->thread_write.lp[1], samplerate / 2.0, sdr_config->samplerate, 2);
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}
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}
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/* alloc fm modulation buffers */
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@ -397,6 +410,14 @@ static void *sdr_write_child(void *arg)
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}
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out = (out + 2) % sdr->thread_write.buffer_size;
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}
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sdr->thread_write.out = out;
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#ifndef DISABLE_FILTER
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/* filter spectrum */
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if (sdr->oversample > 1) {
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iir_process_baseband(&sdr->thread_write.lp[0], sdr->thread_write.buffer2, num * sdr->oversample);
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iir_process_baseband(&sdr->thread_write.lp[1], sdr->thread_write.buffer2 + 1, num * sdr->oversample);
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}
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#endif
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#ifdef HAVE_UHD
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if (sdr_config->uhd)
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uhd_send(sdr->thread_write.buffer2, num * sdr->oversample);
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@ -405,7 +426,6 @@ static void *sdr_write_child(void *arg)
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if (sdr_config->soapy)
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soapy_send(sdr->thread_write.buffer2, num * sdr->oversample);
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#endif
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sdr->thread_write.out = out;
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}
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/* delay some time */
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@ -440,6 +460,13 @@ static void *sdr_read_child(void *arg)
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if (count > 0) {
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#ifdef DEBUG_BUFFER
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printf("Thread read %d samples from SDR and writes them to read buffer.\n", count);
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#endif
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#ifndef DISABLE_FILTER
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/* filter spectrum */
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if (sdr->oversample > 1) {
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iir_process_baseband(&sdr->thread_read.lp[0], sdr->thread_read.buffer2, count);
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iir_process_baseband(&sdr->thread_read.lp[1], sdr->thread_read.buffer2 + 1, count);
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}
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#endif
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in = sdr->thread_read.in;
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for (s = 0, ss = 0; s < count; s++) {
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