10 files changed, 44 insertions, 61 deletions
diff --git a/docs/exploring-gnuradio/fm_tx.grc b/docs/exploring-gnuradio/fm_tx.grc
index be8fe771df..bb13417a6b 100644
--- a/docs/exploring-gnuradio/fm_tx.grc
+++ b/docs/exploring-gnuradio/fm_tx.grc
@@ -793,7 +793,7 @@
     </param>
     <param>
       <key>fh</key>
-      <value>0.0</value>
+      <value>0.925 * tx_rate/2.0</value>
     </param>
     <param>
       <key>affinity</key>
diff --git a/gr-analog/examples/fmtest.py b/gr-analog/examples/fmtest.py
index 22448e6428..7ed08cafbe 100755
--- a/gr-analog/examples/fmtest.py
+++ b/gr-analog/examples/fmtest.py
@@ -48,8 +48,8 @@ class fmtx(gr.hier_block2):
                                 gr.io_signature(1, 1, gr.sizeof_float),
                                 gr.io_signature(1, 1, gr.sizeof_gr_complex))
 
-        fmtx = analog.nbfm_tx(audio_rate, if_rate, max_dev=5e3, tau=75e-6,
-	                                                        fh=0.0)
+        fmtx = analog.nbfm_tx(audio_rate, if_rate, max_dev=5e3,
+	                      tau=75e-6, fh=0.925*if_rate/2.0)
 
         # Local oscillator
         lo = analog.sig_source_c(if_rate,            # sample rate
diff --git a/gr-analog/grc/analog_fm_preemph.xml b/gr-analog/grc/analog_fm_preemph.xml
index e5a8c35ce6..0e8928c3b5 100644
--- a/gr-analog/grc/analog_fm_preemph.xml
+++ b/gr-analog/grc/analog_fm_preemph.xml
@@ -23,7 +23,7 @@
 	<param>
 		<name>High Corner Freq</name>
 		<key>fh</key>
-		<value>0.0</value>
+		<value>0.925*samp_rate/2.0</value>
 		<type>real</type>
 	</param>
 	<sink>
diff --git a/gr-analog/grc/analog_nbfm_tx.xml b/gr-analog/grc/analog_nbfm_tx.xml
index 3e91e9c4e1..d496a18a1e 100644
--- a/gr-analog/grc/analog_nbfm_tx.xml
+++ b/gr-analog/grc/analog_nbfm_tx.xml
@@ -46,7 +46,7 @@
   <param>
     <name>Preemphasis High Corner Freq</name>
     <key>fh</key>
-    <value>0.0</value>
+    <value>0.925*float(quad_rate)/2.0</value>
     <type>real</type>
   </param>
 
diff --git a/gr-analog/grc/analog_wfm_tx.xml b/gr-analog/grc/analog_wfm_tx.xml
index cd67d6a48a..dccfeb0767 100644
--- a/gr-analog/grc/analog_wfm_tx.xml
+++ b/gr-analog/grc/analog_wfm_tx.xml
@@ -40,7 +40,7 @@
 	<param>
 		<name>Preemphasis High Corner Freq</name>
 		<key>fh</key>
-		<value>0.0</value>
+		<value>0.925*float(quad_rate)/2.0</value>
 		<type>real</type>
 	</param>
 	<check>($quad_rate)%($audio_rate) == 0</check>
diff --git a/gr-analog/python/analog/fm_emph.py b/gr-analog/python/analog/fm_emph.py
index 7637743d91..bfa4742ace 100644
--- a/gr-analog/python/analog/fm_emph.py
+++ b/gr-analog/python/analog/fm_emph.py
@@ -251,69 +251,52 @@ class fm_preemph(gr.hier_block2):
     """
     FM Preemphasis IIR filter.
     """
-    def __init__(self, fs, tau=75e-6, fh=0.0):
+    def __init__(self, fs, tau=75e-6, fh=-1.0):
         """
 
         Args:
             fs: sampling frequency in Hz (float)
             tau: Time constant in seconds (75us in US, 50us in EUR) (float)
-            fh: High frequency at which to flatten out; 0.0 means none (float)
+            fh: High frequency at which to flatten out (< 0 means default of 0.925*fs/2.0) (float)
         """
         gr.hier_block2.__init__(self, "fm_preemph",
                                 gr.io_signature(1, 1, gr.sizeof_float),  # Input signature
                                 gr.io_signature(1, 1, gr.sizeof_float))  # Output signature
 
-	if fh > 0.0 and fh < (fs / 2.0):
-		# Digital corner frequencies
-		w_cl = 1.0 / tau
-		w_ch = 2.0 * math.pi * fh
+	# Set fh to something sensible, if needed.
+	# N.B. fh == fs/2.0 or fh == 0.0 results in a pole on the unit circle
+	# at z = -1.0 or z = 1.0 respectively.  That makes the filter unstable
+	# and useless.
+	if fh <= 0.0 or fh >= fs/2.0:
+		fh = 0.925 * fs/2.0
 
-		# Prewarped analog corner frequencies
-		w_cla = 2.0 * fs * math.tan(w_cl / (2.0 * fs))
-		w_cha = 2.0 * fs * math.tan(w_ch / (2.0 * fs))
+	# Digital corner frequencies
+	w_cl = 1.0 / tau
+	w_ch = 2.0 * math.pi * fh
 
-		# Resulting digital pole, zero, and gain term from the bilinear
-		# transformation of H(s) = (s + w_cla) / (s + w_cha) to
-		# H(z) = b0 (1 - z1 z^-1)/(1 - p1 z^-1)
-		kl = -w_cla / (2.0 * fs)
-		kh = -w_cha / (2.0 * fs)
-		z1 = (1.0 + kl) / (1.0 - kl)
-		p1 = (1.0 + kh) / (1.0 - kh)
-		b0 = (1.0 - kl) / (1.0 - kh)
+	# Prewarped analog corner frequencies
+	w_cla = 2.0 * fs * math.tan(w_cl / (2.0 * fs))
+	w_cha = 2.0 * fs * math.tan(w_ch / (2.0 * fs))
 
-		# Since H(s = infinity) = 1.0, then H(z = -1) = 1.0 and
-		# this filter  has 0 dB gain at fs/2.0.
-		# That isn't what users are going to expect, so adjust with a
-		# gain, g, so that H(z = 1) = 1.0 for 0 dB gain at DC.
-		w_0dB = 2.0 * math.pi * 0.0
-		g =        abs(1.0 - p1 * cmath.rect(1.0, -w_0dB))  \
-		   / (b0 * abs(1.0 - z1 * cmath.rect(1.0, -w_0dB)))
+	# Resulting digital pole, zero, and gain term from the bilinear
+	# transformation of H(s) = (s + w_cla) / (s + w_cha) to
+	# H(z) = b0 (1 - z1 z^-1)/(1 - p1 z^-1)
+	kl = -w_cla / (2.0 * fs)
+	kh = -w_cha / (2.0 * fs)
+	z1 = (1.0 + kl) / (1.0 - kl)
+	p1 = (1.0 + kh) / (1.0 - kh)
+	b0 = (1.0 - kl) / (1.0 - kh)
 
-		btaps = [ g * b0 * 1.0, g * b0 * -z1 ]
-		ataps = [          1.0,          -p1 ]
+	# Since H(s = infinity) = 1.0, then H(z = -1) = 1.0 and
+	# this filter  has 0 dB gain at fs/2.0.
+	# That isn't what users are going to expect, so adjust with a
+	# gain, g, so that H(z = 1) = 1.0 for 0 dB gain at DC.
+	w_0dB = 2.0 * math.pi * 0.0
+	g =        abs(1.0 - p1 * cmath.rect(1.0, -w_0dB))  \
+	   / (b0 * abs(1.0 - z1 * cmath.rect(1.0, -w_0dB)))
 
-	else:
-		# Just use H(s) = (s + 1/RC)/(1/RC) as the transfer function
-
-		# Digital corner frequencies
-		w_cl = 1.0 / tau
-
-		# Prewarped analog corner frequencies
-		w_cla = 2.0 * fs * math.tan(w_cl / (2.0 * fs))
-
-		# Resulting digital pole, zero, and gain term from the bilinear
-		# transformation of H(s) = (s + w_cl)/w_cl to
-		# H(z) = b0 (1 - z1 z^-1)/(1 - p1 z^-1)
-		kl = -w_cla / (2.0 * fs)
-		z1 = (1.0 + kl) / (1.0 - kl)
-		p1 = -1.0
-		b0 = (1.0 - kl) / -kl
-
-		# Since H(s = 0) = 1.0, then H(z = 1) = 1.0 and
-		# has 0 dB gain at DC
-
-		btaps = [ b0 * 1.0, b0 * -z1 ]
-		ataps = [      1.0,      -p1 ]
+	btaps = [ g * b0 * 1.0, g * b0 * -z1 ]
+	ataps = [          1.0,          -p1 ]
 
         if 0:
             print "btaps =", btaps
diff --git a/gr-analog/python/analog/nbfm_tx.py b/gr-analog/python/analog/nbfm_tx.py
index cd11c2f49d..aa6c1eccc7 100644
--- a/gr-analog/python/analog/nbfm_tx.py
+++ b/gr-analog/python/analog/nbfm_tx.py
@@ -29,7 +29,7 @@ except ImportError:
     import analog_swig as analog
 
 class nbfm_tx(gr.hier_block2):
-    def __init__(self, audio_rate, quad_rate, tau=75e-6, max_dev=5e3, fh=0.0):
+    def __init__(self, audio_rate, quad_rate, tau=75e-6, max_dev=5e3, fh=-1.0):
         """
         Narrow Band FM Transmitter.
 
@@ -41,7 +41,7 @@ class nbfm_tx(gr.hier_block2):
             quad_rate: sample rate of output stream (integer)
             tau: preemphasis time constant (default 75e-6) (float)
             max_dev: maximum deviation in Hz (default 5e3) (float)
-            fh: high frequency at which to flatten preemphasis; 0.0 means none (float)
+            fh: high frequency at which to flatten preemphasis; < 0 means default of 0.925*quad_rate/2.0 (float)
 
         quad_rate must be an integer multiple of audio_rate.
         """
diff --git a/gr-analog/python/analog/wfm_tx.py b/gr-analog/python/analog/wfm_tx.py
index 7363ccd836..a1b589350d 100644
--- a/gr-analog/python/analog/wfm_tx.py
+++ b/gr-analog/python/analog/wfm_tx.py
@@ -30,7 +30,7 @@ except ImportError:
     import analog_swig as analog
 
 class wfm_tx(gr.hier_block2):
-    def __init__(self, audio_rate, quad_rate, tau=75e-6, max_dev=75e3, fh=0.0):
+    def __init__(self, audio_rate, quad_rate, tau=75e-6, max_dev=75e3, fh=-1.0):
         """
         Wide Band FM Transmitter.
 
@@ -42,7 +42,7 @@ class wfm_tx(gr.hier_block2):
             quad_rate: sample rate of output stream (integer)
             tau: preemphasis time constant (default 75e-6) (float)
             max_dev: maximum deviation in Hz (default 75e3) (float)
-            fh: high frequency at which to flatten preemphasis; 0.0 means none (float)
+            fh: high frequency at which to flatten preemphasis; < 0 means default of 0.925*quad_rate/2.0 (float)
 
         quad_rate must be an integer multiple of audio_rate.
         """
diff --git a/gr-filter/examples/synth_to_chan.py b/gr-filter/examples/synth_to_chan.py
index 7a295eab47..88fb080a65 100755
--- a/gr-filter/examples/synth_to_chan.py
+++ b/gr-filter/examples/synth_to_chan.py
@@ -54,7 +54,7 @@ def main():
     fmtx = list()
     for fi in freqs:
         s = analog.sig_source_f(fs, analog.GR_SIN_WAVE, fi, 1)
-        fm = analog.nbfm_tx(fs, 4*fs, max_dev=10000, tau=75e-6, fh=0.0)
+        fm = analog.nbfm_tx(fs, 4*fs, max_dev=10000, tau=75e-6, fh=0.925*(4*fs)/2.0)
         sigs.append(s)
         fmtx.append(fm)
 
diff --git a/gr-uhd/examples/python/fm_tx4.py b/gr-uhd/examples/python/fm_tx4.py
index ffc74abf82..516033dae1 100755
--- a/gr-uhd/examples/python/fm_tx4.py
+++ b/gr-uhd/examples/python/fm_tx4.py
@@ -63,8 +63,8 @@ class pipeline(gr.hier_block2):
             sys.exit(1)
 
         print audio_rate, if_rate
-        fmtx = analog.nbfm_tx(audio_rate, if_rate, max_dev=5e3, tau=75e-6,
-	                                                        fh=0.0)
+        fmtx = analog.nbfm_tx(audio_rate, if_rate, max_dev=5e3,
+	                      tau=75e-6, fh=0.925*if_rate/2.0)
 
         # Local oscillator
         lo = analog.sig_source_c(if_rate,            # sample rate