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.

src/common/iir_filter.c

@@ -115,6 +115,7 @@ void iir_process(iir_filter_t *filter, sample_t *samples, int length)
 	b1 = filter->b1;
 	b2 = filter->b2;
 
+	/* these are state pointers, so no need to write back */
 	z1 = filter->z1;
 	z2 = filter->z2;
 
@@ -131,3 +132,36 @@ void iir_process(iir_filter_t *filter, sample_t *samples, int length)
 	}
 }
 
+void iir_process_baseband(iir_filter_t *filter, float *baseband, int length)
+{
+	double a0, a1, a2, b1, b2;
+	double *z1, *z2;
+	double in, out;
+	int iterations = filter->iter;
+	int i, j;
+
+	/* get states */
+	a0 = filter->a0;
+	a1 = filter->a1;
+	a2 = filter->a2;
+	b1 = filter->b1;
+	b2 = filter->b2;
+
+	/* these are state pointers, so no need to write back */
+	z1 = filter->z1;
+	z2 = filter->z2;
+
+	/* process filter */
+	for (i = 0; i < length; i++) {
+		in = *baseband;
+		for (j = 0; j < iterations; j++) {
+			out = in * a0 + z1[j];
+			z1[j] = in * a1 + z2[j] - b1 * out;
+			z2[j] = in * a2 - b2 * out;
+			in = out;
+		}
+		*baseband = in;
+		baseband += 2;
+	}
+}
+

src/common/iir_filter.h

@@ -12,5 +12,6 @@ void iir_highpass_init(iir_filter_t *filter, double frequency, int samplerate, i
 void iir_bandpass_init(iir_filter_t *filter, double frequency, int samplerate, int iterations);
 void iir_notch_init(iir_filter_t *filter, double frequency, int samplerate, int iterations);
 void iir_process(iir_filter_t *filter, sample_t *samples, int length);
+void iir_process_baseband(iir_filter_t *filter, float *baseband, int length);
 
 #endif /* _FILTER_H */

src/common/sdr.c

@@ -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++) {