diff --git a/src/cnetz/cnetz.c b/src/cnetz/cnetz.c
index 2286e46..0921723 100644
--- a/src/cnetz/cnetz.c
+++ b/src/cnetz/cnetz.c
@@ -216,7 +216,7 @@ int cnetz_init(void)
 static void cnetz_go_idle(cnetz_t *cnetz);
 
 /* Create transceiver instance and link to a list. */
-int cnetz_create(int kanal, enum cnetz_chan_type chan_type, const char *audiodev, int use_sdr, int samplerate, double rx_gain, int auth, int ms_power, int measure_speed, double clock_speed[2], int polarity, int pre_emphasis, int de_emphasis, const char *write_rx_wave, const char *write_tx_wave, const char *read_rx_wave, int loopback)
+int cnetz_create(int kanal, enum cnetz_chan_type chan_type, const char *audiodev, int use_sdr, enum demod_type demod, int samplerate, double rx_gain, int auth, int ms_power, int measure_speed, double clock_speed[2], int polarity, int pre_emphasis, int de_emphasis, const char *write_rx_wave, const char *write_tx_wave, const char *read_rx_wave, int loopback)
 {
 	sender_t *sender;
 	cnetz_t *cnetz;
@@ -290,7 +290,7 @@ int cnetz_create(int kanal, enum cnetz_chan_type chan_type, const char *audiodev
 #endif
 
 	/* init audio processing */
-	rc = dsp_init_sender(cnetz, measure_speed, clock_speed, use_sdr);
+	rc = dsp_init_sender(cnetz, measure_speed, clock_speed, demod);
 	if (rc < 0) {
 		PDEBUG(DCNETZ, DEBUG_ERROR, "Failed to init signal processing!\n");
 		goto error;
diff --git a/src/cnetz/cnetz.h b/src/cnetz/cnetz.h
index b5be755..1856db4 100644
--- a/src/cnetz/cnetz.h
+++ b/src/cnetz/cnetz.h
@@ -127,7 +127,7 @@ int cnetz_channel_by_short_name(const char *short_name);
 const char *chan_type_short_name(enum cnetz_chan_type chan_type);
 const char *chan_type_long_name(enum cnetz_chan_type chan_type);
 int cnetz_init(void);
-int cnetz_create(int kanal, enum cnetz_chan_type chan_type, const char *audiodev, int use_sdr, int samplerate, double rx_gain, int auth, int ms_power, int measure_speed, double clock_speed[2], int polarity, int pre_emphasis, int de_emphasis, const char *write_rx_wave, const char *write_tx_wave, const char *read_rx_wave, int loopback);
+int cnetz_create(int kanal, enum cnetz_chan_type chan_type, const char *audiodev, int use_sdr, enum demod_type demod, int samplerate, double rx_gain, int auth, int ms_power, int measure_speed, double clock_speed[2], int polarity, int pre_emphasis, int de_emphasis, const char *write_rx_wave, const char *write_tx_wave, const char *read_rx_wave, int loopback);
 void cnetz_destroy(sender_t *sender);
 void cnetz_sync_frame(cnetz_t *cnetz, double sync, int ts);
 int cnetz_meldeaufruf(uint8_t futln_nat, uint8_t futln_fuvst, uint16_t futln_rest);
diff --git a/src/cnetz/dsp.c b/src/cnetz/dsp.c
index 00f1ed0..2c2ca4e 100644
--- a/src/cnetz/dsp.c
+++ b/src/cnetz/dsp.c
@@ -83,7 +83,7 @@ static void dsp_init_ramp(cnetz_t *cnetz)
 }
 
 /* Init transceiver instance. */
-int dsp_init_sender(cnetz_t *cnetz, int measure_speed, double clock_speed[2], int use_sdr)
+int dsp_init_sender(cnetz_t *cnetz, int measure_speed, double clock_speed[2], enum demod_type demod)
 {
 	int rc = 0;
 	double size;
@@ -136,7 +136,7 @@ int dsp_init_sender(cnetz_t *cnetz, int measure_speed, double clock_speed[2], in
 	/* reinit the sample rate to shrink/expand audio */
 	init_samplerate(&cnetz->sender.srstate, 8000.0, (double)cnetz->sender.samplerate / 1.1, 3300.0); /* 66 <-> 60 */
 
-	rc = fsk_fm_init(&cnetz->fsk_demod, cnetz, cnetz->sender.samplerate, (double)BITRATE / (1.0 + clock_speed[0] / 1000000.0), (use_sdr) ? FSK_DEMOD_LEVEL : FSK_DEMOD_SLOPE);
+	rc = fsk_fm_init(&cnetz->fsk_demod, cnetz, cnetz->sender.samplerate, (double)BITRATE / (1.0 + clock_speed[0] / 1000000.0), demod);
 	if (rc < 0)
 		goto error;
 
diff --git a/src/cnetz/dsp.h b/src/cnetz/dsp.h
index 7528e18..4e66f26 100644
--- a/src/cnetz/dsp.h
+++ b/src/cnetz/dsp.h
@@ -1,6 +1,6 @@
 
 void dsp_init(void);
-int dsp_init_sender(cnetz_t *cnetz, int measure_speed, double clock_speed[2], int use_sdr);
+int dsp_init_sender(cnetz_t *cnetz, int measure_speed, double clock_speed[2], enum demod_type demod);
 void dsp_cleanup_sender(cnetz_t *cnetz);
 void calc_clock_speed(cnetz_t *cnetz, double samples, int tx, int result);
 void unshrink_speech(cnetz_t *cnetz, sample_t *speech_buffer, int count);
diff --git a/src/cnetz/fsk_demod.c b/src/cnetz/fsk_demod.c
index 99d3dc4..2f56bf0 100644
--- a/src/cnetz/fsk_demod.c
+++ b/src/cnetz/fsk_demod.c
@@ -135,7 +135,7 @@
 #include "dsp.h"
 #include "telegramm.h"
 
-int fsk_fm_init(fsk_fm_demod_t *fsk, cnetz_t *cnetz, int samplerate, double bitrate, enum demod_type demod_type)
+int fsk_fm_init(fsk_fm_demod_t *fsk, cnetz_t *cnetz, int samplerate, double bitrate, enum demod_type demod)
 {
 	int len, half;
 
@@ -146,12 +146,19 @@ int fsk_fm_init(fsk_fm_demod_t *fsk, cnetz_t *cnetz, int samplerate, double bitr
 	}
 
 	fsk->cnetz = cnetz;
-	fsk->demod_type = demod_type;
+	fsk->demod_type = demod;
 
-	if (demod_type == FSK_DEMOD_SLOPE)
+	switch (demod) {
+	case FSK_DEMOD_SLOPE:
 		PDEBUG(DDSP, DEBUG_INFO, "Detecting level change by looking at slope (good for sound cards)\n");
-	if (demod_type == FSK_DEMOD_LEVEL)
+		break;
+	case FSK_DEMOD_LEVEL:
 		PDEBUG(DDSP, DEBUG_INFO, "Detecting level change by looking zero crosssing (good for SDR)\n");
+		break;
+	default:
+		PDEBUG(DDSP, DEBUG_ERROR, "Wrong demod type, please fix!\n");
+		abort();
+	}
 
 	len = (int)((double)samplerate / bitrate + 0.5);
 	half = (int)((double)samplerate / bitrate / 2.0 + 0.5);
diff --git a/src/cnetz/fsk_demod.h b/src/cnetz/fsk_demod.h
index d8d645f..3410268 100644
--- a/src/cnetz/fsk_demod.h
+++ b/src/cnetz/fsk_demod.h
@@ -11,6 +11,7 @@ enum fsk_sync {
 };
 
 enum demod_type {
+	FSK_DEMOD_AUTO, /* auto selection of the demod type below */
 	FSK_DEMOD_SLOPE, /* check for highest slope (good for sound cards) */
 	FSK_DEMOD_LEVEL, /* check for zero crossing (good for SDR) */
 };
diff --git a/src/cnetz/main.c b/src/cnetz/main.c
index 0d4ee87..b4578e8 100644
--- a/src/cnetz/main.c
+++ b/src/cnetz/main.c
@@ -47,6 +47,7 @@ int set_clock_speed = 0;
 const char *flip_polarity = "auto";
 int ms_power = 0; /* 0..3 */
 int auth = 0;
+enum demod_type demod = FSK_DEMOD_AUTO;
 
 void print_help(const char *arg0)
 {
@@ -75,6 +76,15 @@ void print_help(const char *arg0)
 	printf("        Enable authentication on the base station. Since we cannot\n");
 	printf("	authenticate, because we don't know the secret key and the algorithm,\n");
 	printf("	we just accept any card. With this we get the vendor IDs of the phone.\n");
+	printf(" -D --demod auto | slope | level\n");
+	printf("        Adjust demodulation algorithm. Use 'slope' to detect a level change\n");
+	printf("        by finding the highest slope of a bit transition. It is useful, if\n");
+	printf("        the receiver drifts to 0 after a while, due to DC decoupling. This\n");
+	printf("        happens in every analog receiver and in every sound card input.\n");
+	printf("        Use 'level' to detect a level change by passing zero level. This\n");
+	printf("        requires a DC coupled signal, which is produced by SDR.\n");
+	printf("        Use 'auto' to select 'slope' for sound card input and 'level' for SDR\n");
+	printf("        input. (default = '%s')\n", (demod == FSK_DEMOD_LEVEL) ? "level" : (demod == FSK_DEMOD_SLOPE) ? "slope" : "auto");
 	printf("\nstation-id: Give 7 digit station-id, you don't need to enter it for every\n");
 	printf("        start of this program.\n");
 	print_hotkeys_common();
@@ -94,10 +104,11 @@ static int handle_options(int argc, char **argv)
 		{"flip-polarity", 1, 0, 'F'},
 		{"ms-power", 1, 0, 'P'},
 		{"authentication", 0, 0, 'A'},
+		{"demod", 1, 0, 'D'},
 		{0, 0, 0, 0}
 	};
 
-	set_options_common("T:MS:F:N:P:AV", long_options_special);
+	set_options_common("T:MS:F:N:P:AD:", long_options_special);
 
 	while (1) {
 		int option_index = 0, c;
@@ -161,6 +172,19 @@ static int handle_options(int argc, char **argv)
 			auth = 1;
 			skip_args += 1;
 			break;
+		case 'D':
+			if (!strcasecmp(optarg, "auto"))
+				demod = FSK_DEMOD_AUTO;
+			else if (!strcasecmp(optarg, "slope"))
+				demod = FSK_DEMOD_SLOPE;
+			else if (!strcasecmp(optarg, "level"))
+				demod = FSK_DEMOD_LEVEL;
+			else {
+				fprintf(stderr, "Given demodulation type '%s' is illegal, see help!\n", optarg);
+				exit(0);
+			}
+			skip_args += 2;
+			break;
 		default:
 			opt_switch_common(c, argv[0], &skip_args);
 		}
@@ -299,9 +323,20 @@ int main(int argc, char *argv[])
 	if (use_sdr && polarity == 0)
 		polarity = 1; /* SDR is always positive */
 
+	/* demodulation algorithm */
+	if (demod == FSK_DEMOD_AUTO)
+		demod = (use_sdr) ? FSK_DEMOD_LEVEL : FSK_DEMOD_SLOPE;
+	if (demod == FSK_DEMOD_LEVEL && !use_sdr) {
+		fprintf(stderr, "*******************************************************************************\n");
+		fprintf(stderr, "I strongly suggest to use 'slope' demodulation algorithm!!!\n");
+		fprintf(stderr, "Using sound card will cause the DC levels to return to 0. Using 'level' assumes\n");
+		fprintf(stderr, "that a frequency offset never returns to 0. (Use this only with SDR.)\n");
+		fprintf(stderr, "*******************************************************************************\n");
+	}
+
 	/* create transceiver instance */
 	for (i = 0; i < num_kanal; i++) {
-		rc = cnetz_create(kanal[i], chan_type[i], audiodev[i], use_sdr, samplerate, rx_gain, auth, ms_power, (i == 0) ? measure_speed : 0, clock_speed, polarity, do_pre_emphasis, do_de_emphasis, write_rx_wave, write_tx_wave, read_rx_wave, loopback);
+		rc = cnetz_create(kanal[i], chan_type[i], audiodev[i], use_sdr, demod, samplerate, rx_gain, auth, ms_power, (i == 0) ? measure_speed : 0, clock_speed, polarity, do_pre_emphasis, do_de_emphasis, write_rx_wave, write_tx_wave, read_rx_wave, loopback);
 		if (rc < 0) {
 			fprintf(stderr, "Failed to create \"Sender\" instance. Quitting!\n");
 			goto fail;