diff --git a/python/file_c4fm_rx_nogui.py b/python/file_c4fm_rx_nogui.py
index a132d80..082df5e 100755
--- a/python/file_c4fm_rx_nogui.py
+++ b/python/file_c4fm_rx_nogui.py
@@ -102,7 +102,7 @@ class file_c4fm_rx (gr.top_block):
             self.connect(decoder, audio_sink)
         except Exception:
             sink = gr.null_sink(gr.sizeof_float)
-            self.connect(decoder, null);
+            self.connect(decoder, sink)
 
 
     # Adjust the channel offset
diff --git a/python/op25_c4fm_mod.py b/python/op25_c4fm_mod.py
index c38dd6e..0dca929 100644
--- a/python/op25_c4fm_mod.py
+++ b/python/op25_c4fm_mod.py
@@ -1,7 +1,8 @@
 #
 # Copyright 2005,2006,2007 Free Software Foundation, Inc.
 #
-# OP25 4-Level Modulator Block # Copyright 2009, KA1RBI
+# OP25 4-Level Modulator Block
+# Copyright 2009, KA1RBI
 #
 # coeffs for shaping and cosine filters from Eric Ramsey thesis 
 # 
@@ -44,7 +45,7 @@ _def_log = False
 #                           modulator
 # /////////////////////////////////////////////////////////////////////////////
 
-class p25_mod(gr.hier_block2):
+class p25_mod_bf(gr.hier_block2):
 
     def __init__(self,
                  output_sample_rate=_def_output_sample_rate,
@@ -70,11 +71,11 @@ class p25_mod(gr.hier_block2):
         @type reverse: bool
         @param verbose: Print information about modulator?
         @type verbose: bool
-        @param debug: Print modualtion data to files?
+        @param debug: Print modulation data to files?
         @type debug: bool
 	"""
 
-	gr.hier_block2.__init__(self, "p25_fm_mod",
+	gr.hier_block2.__init__(self, "p25_c4fm_mod_bf",
 				gr.io_signature(1, 1, gr.sizeof_char),       # Input signature
 				gr.io_signature(1, 1, gr.sizeof_float)) # Output signature
 
@@ -151,7 +152,7 @@ class p25_mod(gr.hier_block2):
         """
         parser.add_option("", "--excess-bw", type="float", default=_def_excess_bw,
                           help="set RRC excess bandwith factor [default=%default] (PSK)")
-    add_options=staticmethod(add_options)
+        add_options=staticmethod(add_options)
 
 
     def extract_kwargs_from_options(options):
@@ -160,6 +161,7 @@ class p25_mod(gr.hier_block2):
         """
         return modulation_utils.extract_kwargs_from_options(dqpsk_mod.__init__,
                                                             ('self',), options)
+
     extract_kwargs_from_options=staticmethod(extract_kwargs_from_options)
 
 #
diff --git a/python/usrp_c4fm_rx_nogui.py b/python/usrp_c4fm_rx_nogui.py
index 5d5b13a..3fb029a 100755
--- a/python/usrp_c4fm_rx_nogui.py
+++ b/python/usrp_c4fm_rx_nogui.py
@@ -108,7 +108,7 @@ class usrp_c4fm_rx (gr.top_block):
             self.connect(decoder, audio_sink)
         except Exception:
             sink = gr.null_sink(gr.sizeof_float)
-            self.connect(decoder, null);
+            self.connect(decoder, sink)
 
 
     # Adjust the channel offset
diff --git a/python/usrp_c4fm_tx_nogui.py b/python/usrp_c4fm_tx_nogui.py
new file mode 100755
index 0000000..20a89b7
--- /dev/null
+++ b/python/usrp_c4fm_tx_nogui.py
@@ -0,0 +1,124 @@
+#!/usr/bin/env python
+#
+# Copyright 2011 Steve Glass.
+# 
+# This file is part of GNU Radio and part of OP25
+# 
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3, or (at your option)
+# any later version.
+# 
+# It is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+# License for more details.
+# 
+# You should have received a copy of the GNU General Public License
+# along with this; see the file COPYING.  If not, write to
+# the Free Software Foundation, Inc., 51 Franklin Street,
+# Boston, MA 02110-1301, USA.
+# 
+
+import math
+import sys
+import wx
+
+from gnuradio import audio
+from gnuradio import eng_notation
+from gnuradio import gr
+from gnuradio import op25
+from gnuradio import op25_c4fm_mod
+from gnuradio import usrp
+from gnuradio.eng_option import eng_option
+
+
+"""
+Simple tx-from-pcap-file utility.
+"""
+class usrp_c4fm_tx_block(gr.top_block):
+
+    def __init__(self, subdev_spec, freq, subdev_gain, filename, delay, repeat):
+
+        gr.top_block.__init__ (self)
+
+        # important vars (to be calculated from USRP when available
+        dac_rate = 128e6
+        usrp_interp = 400
+        usrp_rate = dac_rate / usrp_interp # 320KS/s
+        sw_interp = 10
+        channel_rate = usrp_rate / sw_interp  # 32 kS/s
+
+        # open the pcap source
+        pcap = op25.pcap_source_b(filename, delay, repeat)
+        # ToDo: consider octet -> symbol unpacking in flow graph
+
+        c4fm = op25_c4fm_mod.p25_mod(output_sample_rate=channel_rate, log=False, verbose=False)
+        self.connect(pcap, c4fm)
+
+        # setup low pass filter + interpolator
+        interp_factor = self.usrp_rate / self.channel_rate
+        low_pass = 2.88e3
+        interp_taps = gr.firdes.low_pass(1.0, self.usrp_rate, low_pass, low_pass * 0.1, gr.firdes.WIN_HANN)
+        interpolator = gr.interp_fir_filter_fff (int(interp_factor), interp_taps)
+        self.connect(c4fm, interpolator)
+
+        # frequency modulator
+        max_dev = 12.5e3
+        k = 2 * math.pi * max_dev / self.usrp_rate
+        adjustment = 1.5   # adjust for proper c4fm deviation level
+        fm = gr.frequency_modulator_fc(k * adjustment)
+        self.connect(interpolator, fm)
+
+        # amplify signal
+        gain = gr.multiply_const_cc(4000)
+        self.connect(fm, gain)
+
+        # # configure USRP
+        u = usrp.sink_c()
+        if subdev_spec is None:
+            subdev_spec = usrp.pick_tx_subdevice(u)
+        u.set_mux(usrp.determine_tx_mux_value(u, subdev_spec))
+        subdev = usrp.selected_subdev(u, subdev_spec)
+        print "Using TX d'board %s" % (subdev.side_and_name(),)
+
+        if gain is None:
+            g = subdev.gain_range()
+            gain = float(g[0] + g[1]) / 2
+        subdev.set_gain(self.subdev.gain_range()[0])
+        u.tune(subdev.which(), subdev, freq)
+        self.connect(gain, u)
+        subdev.set_enable(True)
+
+#        sink = gr.null_sink(gr.sizeof_float)
+#        self.connect(gain, sink)
+
+
+# inject frames
+#
+if __name__ == '__main__':
+
+    from optparse import OptionParser
+    parser = OptionParser (option_class=eng_option)
+    parser.add_option("-T", "--tx-subdev-spec", type="subdev", default=None,
+                      help="select USRP Tx side A or B")
+    parser.add_option("-f", "--freq", type="eng_float", default=None,
+                      help="set Tx frequency to FREQ [required]")
+    parser.add_option("-g", "--gain", type="eng_float", default=None, 
+                      help="set device gain")
+    parser.add_option("-p", "--pcap-file", default="",
+                      help="packet capture file [required]")
+    parser.add_option("-d", "--delay", type="int", default=0,
+                      help="delay before TX of first symbol [default=%default]")
+    parser.add_option("-r","--repeat", action="store_true", default=False,
+                      help="continuously replay input file [default=%default]")
+    (options, args) = parser.parse_args ()
+    if options.freq is None:
+        parser.print_help()
+        sys.exit(1)
+    
+    try:
+        rx = usrp_c4fm_tx_block(options.subdev_spec, options.freq, options.gain, options.pcap_file, options.delay, options.repeat)
+        rx.run()
+    except KeyboardInterrupt:
+        rx.subdev.set_enable(False)