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<?xml version="1.0" encoding="ISO-8859-1"?>
<example id="fm_demod_ex"><title>Broadcast FM Receiver</title>
<programlisting>
#!/usr/bin/env python
from gnuradio import gr
from gnuradio import audio
from gnuradio import mc4020
import sys
def high_speed_adc (fg, input_rate):
# return gr.file_source (gr.sizeof_short, "dummy.dat", False)
return mc4020.source (input_rate, mc4020.MCC_CH3_EN | mc4020.MCC_ALL_1V)
#
# return a gr.flow_graph
#
def build_graph (freq1, freq2):
input_rate = 20e6
cfir_decimation = 125
audio_decimation = 5
quad_rate = input_rate / cfir_decimation
audio_rate = quad_rate / audio_decimation
fg = gr.flow_graph ()
# use high speed ADC as input source
src = high_speed_adc (fg, input_rate)
# compute FIR filter taps for channel selection
channel_coeffs = \
gr.firdes.low_pass (1.0, # gain
input_rate, # sampling rate
250e3, # low pass cutoff freq
8*100e3, # width of trans. band
gr.firdes.WIN_HAMMING)
# input: short; output: complex
chan_filter1 = \
gr.freq_xlating_fir_filter_scf (cfir_decimation,
channel_coeffs,
freq1, # 1st station freq
input_rate)
(head1, tail1) = build_pipeline (fg, quad_rate, audio_decimation)
# sound card as final sink
audio_sink = audio.sink (int (audio_rate))
# now wire it all together
fg.connect (src, chan_filter1)
fg.connect (chan_filter1, head1)
fg.connect (tail1, (audio_sink, 0))
return fg
def build_pipeline (fg, quad_rate, audio_decimation):
'''Given a flow_graph, fg, construct a pipeline
for demodulating a broadcast FM signal. The
input is the downconverted complex baseband
signal. The output is the demodulated audio.
build_pipeline returns a two element tuple
containing the input and output endpoints.
'''
fm_demod_gain = 2200.0/32768.0
audio_rate = quad_rate / audio_decimation
volume = 1.0
# input: complex; output: float
fm_demod = gr.quadrature_demod_cf (volume*fm_demod_gain)
# compute FIR filter taps for audio filter
width_of_transition_band = audio_rate / 32
audio_coeffs = gr.firdes.low_pass (1.0, # gain
quad_rate, # sampling rate
audio_rate/2 - width_of_transition_band,
width_of_transition_band,
gr.firdes.WIN_HAMMING)
# input: float; output: float
audio_filter = gr.fir_filter_fff (audio_decimation, audio_coeffs)
fg.connect (fm_demod, audio_filter)
return ((fm_demod, 0), (audio_filter, 0))
def main (args):
nargs = len (args)
if nargs == 1:
# get station frequency from command line
freq1 = float (args[0]) * 1e6
else:
sys.stderr.write ('usage: fm_demod freq\n')
sys.exit (1)
# connect to RF front end
rf_front_end = gr.microtune_4937_eval_board ()
if not rf_front_end.board_present_p ():
raise IOError, 'RF front end not found'
# set front end gain
rf_front_end.set_AGC (300)
# determine the front end's "Intermediate Frequency"
IF_freq = rf_front_end.get_output_freq () # 5.75e6
# Tell the front end to tune to freq1.
# I.e., freq1 is translated down to the IF frequency
rf_front_end.set_RF_freq (freq1)
# build the flow graph
fg = build_graph (IF_freq, None)
fg.start () # fork thread(s) and return
raw_input ('Press Enter to quit: ')
fg.stop ()
if __name__ == '__main__':
main (sys.argv[1:])
</programlisting>
</example>
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