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+#
+# Copyright 2005,2012 Free Software Foundation, Inc.
+#
+# This file is part of GNU Radio
+#
+# GNU Radio 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.
+#
+# GNU Radio 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 GNU Radio; see the file COPYING. If not, write to
+# the Free Software Foundation, Inc., 51 Franklin Street,
+# Boston, MA 02110-1301, USA.
+#
+
+import math
+from gnuradio import gr
+from gnuradio import filter
+from fm_emph import fm_deemph
+
+try:
+ from gnuradio import analog
+except ImportError:
+ import analog_swig as analog
+
+class nbfm_rx(gr.hier_block2):
+ def __init__(self, audio_rate, quad_rate, tau=75e-6, max_dev=5e3):
+ """
+ Narrow Band FM Receiver.
+
+ Takes a single complex baseband input stream and produces a single
+ float output stream of audio sample in the range [-1, +1].
+
+ Args:
+ audio_rate: sample rate of audio stream, >= 16k (integer)
+ 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)
+
+ quad_rate must be an integer multiple of audio_rate.
+
+ Exported sub-blocks (attributes):
+ squelch
+ quad_demod
+ deemph
+ audio_filter
+ """
+
+ gr.hier_block2.__init__(self, "nbfm_rx",
+ gr.io_signature(1, 1, gr.sizeof_gr_complex), # Input signature
+ gr.io_signature(1, 1, gr.sizeof_float)) # Output signature
+
+ # FIXME audio_rate and quad_rate ought to be exact rationals
+ audio_rate = int(audio_rate)
+ quad_rate = int(quad_rate)
+
+ if quad_rate % audio_rate != 0:
+ raise ValueError, "quad_rate is not an integer multiple of audio_rate"
+
+ squelch_threshold = 20 # dB
+ #self.squelch = analog.simple_squelch_cc(squelch_threshold, 0.001)
+
+ # FM Demodulator input: complex; output: float
+ k = quad_rate/(2*math.pi*max_dev)
+ self.quad_demod = analog.quadrature_demod_cf(k)
+
+ # FM Deemphasis IIR filter
+ self.deemph = fm_deemph(quad_rate, tau=tau)
+
+ # compute FIR taps for audio filter
+ audio_decim = quad_rate // audio_rate
+ audio_taps = filter.firdes.low_pass(1.0, # gain
+ quad_rate, # sampling rate
+ 2.7e3, # Audio LPF cutoff
+ 0.5e3, # Transition band
+ filter.firdes.WIN_HAMMING) # filter type
+
+ print "len(audio_taps) =", len(audio_taps)
+
+ # Decimating audio filter
+ # input: float; output: float; taps: float
+ self.audio_filter = filter.fir_filter_fff(audio_decim, audio_taps)
+
+ self.connect(self, self.quad_demod, self.deemph, self.audio_filter, self)