diff --git a/src/direwolf.c b/src/direwolf.c
index 327978e..8855367 100644
--- a/src/direwolf.c
+++ b/src/direwolf.c
@@ -193,7 +193,6 @@ int main (int argc, char *argv[])
 	//int eof;
 	int j;
 	char config_file[100];
-	int xmit_calibrate_option = 0;
 	int enable_pseudo_terminal = 0;
 	struct digi_config_s digi_config;
 	struct cdigi_config_s cdigi_config;
@@ -231,6 +230,8 @@ int main (int argc, char *argv[])
 	float e_recv_ber = 0.0;		/* Receive Bit Error Rate (BER). */
 	int X_fx25_xmit_enable = 0;	/* FX.25 transmit enable. */
 
+	int x_opt = 0;		/* "-x N" option for transmitting calibration tones. */
+
 	strlcpy(l_opt_logdir, "", sizeof(l_opt_logdir));
 	strlcpy(L_opt_logfile, "", sizeof(L_opt_logfile));
 	strlcpy(P_opt, "", sizeof(P_opt));
@@ -388,7 +389,7 @@ int main (int argc, char *argv[])
 
 	  /* ':' following option character means arg is required. */
 
-          c = getopt_long(argc, argv, "hP:B:gjJD:U:c:pxr:b:n:d:q:t:ul:L:Sa:E:T:e:X:A",
+          c = getopt_long(argc, argv, "hP:B:gjJD:U:c:px:r:b:n:d:q:t:ul:L:Sa:E:T:e:X:A",
                         long_options, &option_index);
           if (c == -1)
             break;
@@ -490,9 +491,22 @@ int main (int argc, char *argv[])
             }
             break;
 
-          case 'x':				/* -x for transmit calibration tones. */
+          case 'x':				/* -x N for transmit calibration tones. */
+
+      	     switch (*optarg) {
+      	      case 'a':  x_opt = 1; break; // Alternating tones
+      	      case 'm':  x_opt = 2; break; // Mark tone
+      	      case '1':  x_opt = 2; break; // Mark tone alternative
+      	      case 's':  x_opt = 3; break; // Space tone
+      	      case '2':  x_opt = 3; break; // Space tone alternative
+      	      case 'p':  x_opt = 4; break; // Set PTT only
+      	      default:
+      	    	text_color_set(DW_COLOR_ERROR);
+      	    	dw_printf ("Invalid option for -x. \n");
+      	    	exit (EXIT_FAILURE);
+      	    	break;
+      	     }
 
-	    xmit_calibrate_option = 1;
             break;
 
           case 'r':				/* -r audio samples/sec.  e.g. 44100 */
@@ -907,30 +921,70 @@ int main (int argc, char *argv[])
 	xmit_init (&audio_config, d_p_opt);
 
 /*
- * If -x option specified, transmit alternating tones for transmitter
+ * If -x N option specified, transmit calibration tones for transmitter
  * audio level adjustment, up to 1 minute then quit.
+ * N = a: Alternating mark/space tones
+ * N = m: (or 1): Mark tone (e.g. 1200Hz)
+ * N = s: (or 2): Space tone (e.g. 2200Hz)
+ * N = p: Set PTT only.
  * TODO:  enhance for more than one channel.
  */
 
-	if (xmit_calibrate_option) {
+	if (x_opt != 0) {
+		if (audio_config.achan[0].mark_freq
+				&& audio_config.achan[0].space_freq) {
+			int max_duration = 60;
+			int n = audio_config.achan[0].baud * max_duration;
+			int chan = 0;
 
-	  int max_duration = 60;  /* seconds */
-	  int n = audio_config.achan[0].baud * max_duration;
-	  int chan = 0;
-	
-	  text_color_set(DW_COLOR_INFO);
-	  dw_printf ("\nSending transmit calibration tones.  Press control-C to terminate.\n");
+			text_color_set(DW_COLOR_INFO);
+			ptt_set(OCTYPE_PTT, chan, 1);
 
-	  ptt_set (OCTYPE_PTT, chan, 1);
-	  while (n-- > 0) {
+			switch (x_opt) {
+			case 1:  // Alternating tones: -x a
+				dw_printf(
+						"\nSending alternating mark/space calibration tones (%d/%dHz).\nPress control-C to terminate.\n",
+						audio_config.achan[0].mark_freq,
+						audio_config.achan[0].space_freq);
+				while (n-- > 0) {
+					tone_gen_put_bit(chan, n & 1);
+				}
+				break;
+			case 2:  // "Mark" tone: -x m
+				dw_printf(
+						"\nSending mark calibration tone (%dHz).\nPress control-C to terminate.\n",
+						audio_config.achan[0].mark_freq);
+				while (n-- > 0) {
+					tone_gen_put_bit(chan, 0);
+				}
+				break;
+			case 3:  // "Space" tone: -x s
+				dw_printf(
+						"\nSending space calibration tone (%dHz).\nPress control-C to terminate.\n",
+						audio_config.achan[0].space_freq);
+				while (n-- > 0) {
+					tone_gen_put_bit(chan, 1);
+				}
+				break;
+			case 4:  // Silence - set PTT only: -x p
+				dw_printf(
+						"\nSending silence (Set PTT only).\nPress control-C to terminate.\n");
+				sleep(max_duration);
+				break;
+			}
 
-	    tone_gen_put_bit (chan, n & 1);
+			ptt_set(OCTYPE_PTT, chan, 0);
+			exit(0);
 
-	  }
-	  ptt_set (OCTYPE_PTT, chan, 0);
-	  exit (0);
+		} else {
+			text_color_set(DW_COLOR_ERROR);
+			dw_printf(
+					"\nSpace/mark frequencies not defined. Cannot calibrate using this modem type.\n");
+			exit(EXIT_FAILURE);
+		}
 	}
 
+
 /*
  * Initialize the digipeater and IGate functions.
  */
@@ -1508,6 +1562,10 @@ static void usage (char **argv)
 	dw_printf ("    -p             Enable pseudo terminal for KISS protocol.\n");
 #endif
 	dw_printf ("    -x             Send Xmit level calibration tones.\n");
+	dw_printf ("       a             a        = Alternating mark/space tones.\n");
+	dw_printf ("       m             m (or 1) = Steady mark tone (e.g. 1200Hz).\n");
+	dw_printf ("       s             s (or 2) = Steady space tone (e.g. 2200Hz).\n");
+	dw_printf ("       p             p        = Silence (Set PTT only).\n");
 	dw_printf ("    -u             Print UTF-8 test string and exit.\n");
 	dw_printf ("    -S             Print symbol tables and exit.\n");
 	dw_printf ("    -T fmt         Time stamp format for sent and received frames.\n");