diff --git a/.gitignore b/.gitignore
index 42ad00d..4c188b3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -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
diff --git a/src/cnetz/scrambler.c b/src/cnetz/scrambler.c
index 461fe76..2b49335 100644
--- a/src/cnetz/scrambler.c
+++ b/src/cnetz/scrambler.c
@@ -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 */
diff --git a/src/cnetz/scrambler.h b/src/cnetz/scrambler.h
index 757940b..0aea77d 100644
--- a/src/cnetz/scrambler.h
+++ b/src/cnetz/scrambler.h
@@ -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);
diff --git a/src/common/filter.c b/src/common/filter.c
index f568f1b..3aea327 100644
--- a/src/common/filter.c
+++ b/src/common/filter.c
@@ -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;
diff --git a/src/common/filter.h b/src/common/filter.h
index 09a379b..42f051b 100644
--- a/src/common/filter.h
+++ b/src/common/filter.h
@@ -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 */
diff --git a/src/common/samplerate.c b/src/common/samplerate.c
index 8113055..f554cf0 100644
--- a/src/common/samplerate.c
+++ b/src/common/samplerate.c
@@ -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++) {
diff --git a/src/common/samplerate.h b/src/common/samplerate.h
index 27955de..d8c9d41 100644
--- a/src/common/samplerate.h
+++ b/src/common/samplerate.h
@@ -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;
diff --git a/src/common/sdr.c b/src/common/sdr.c
index 2644968..db46906 100644
--- a/src/common/sdr.c
+++ b/src/common/sdr.c
@@ -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]);
diff --git a/src/test/Makefile.am b/src/test/Makefile.am
index 25211d2..eace0dc 100644
--- a/src/test/Makefile.am
+++ b/src/test/Makefile.am
@@ -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 = \
diff --git a/src/test/test_filter.c b/src/test/test_filter.c
new file mode 100644
index 0000000..8c82292
--- /dev/null
+++ b/src/test/test_filter.c
@@ -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;
+}
+