http://www.linuxjournal.com/article/7505
This article is a useful one I've studied to understand FM demodulation
In the block of code that implements (albeit I haven't tried to run it
because it's not made to run with a USRP, it's more pedagogical), to
demodulate two frequencies, it averages them and then centers them around an
IF_freq.
My guess is that this is the frequency that serves as a "0" post
demodulation. Ie by reading frequency 5.75e6, you are reading the 0
frequency. But I'm not sure.
If this is the case, if I wanted to implement something similar using the
USRP and TVRX, is there a similar "center" that I need to use? Is it 5.75e6?
or is it simply 0hz?
I've attached the code.
Thanks
Listing 3. Multi-channel FM Receiver
#!/usr/bin/env python
from gnuradio import gr
import sys
#
# 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 = gr.high_speed_adc (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 = gr.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))
# two stations at once?
if freq2:
# Extract the second station and connect
# it to a second pipeline...
# input: short; output: complex
chan_filter2 = \
gr.freq_xlating_fir_filter_scf (
cfir_decimation,
channel_coeffs,
freq2, # 2nd station freq
input_rate)
(head2, tail2) = \
build_pipeline (fg, quad_rate,
audio_decimation)
fg.connect (src, chan_filter2)
fg.connect (chan_filter2, head2)
fg.connect (tail2, (audio_sink, 1))
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 downconverteed 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
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:
freq1 = float (args[0]) * 1e6
freq2 = None
elif nargs == 2:
freq1 = float (args[0]) * 1e6
freq2 = float (args[1]) * 1e6
else:
sys.stderr.write (
'usage: fm_demod freq1 [freq2]\n')
sys.exit (1)
# connect to RF front end
rf_front_end = gr.microtune_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)
IF_freq = rf_front_end.get_output_freq ()
IF_freq = 5.75e6
if not freq2: # one station
rf_front_end.set_RF_freq (freq1)
fg = build_graph (IF_freq, None)
else: # two stations
if abs (freq1 - freq2) > 5.5e6:
raise IOError, 'freqs too far apart'
target_freq = (freq1 + freq2) / 2
actual_freq = \
rf_front_end.set_RF_freq (target_freq)
fg = build_graph (IF_freq+freq1-actual_freq,
IF_freq+freq2-actual_freq)
fg.start () # fork thread(s) and return
raw_input ('Press Enter to quit: ')
fg.stop ()
if __name__ == '__main__':
main (sys.argv[1:])
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