Listing 2. Simple Broadcast FM Receiver

#!/usr/bin/env python

# simple broadcast FM receiver

from GnuRadio import *

#
# return a gr_FlowGraph
#
def build_graph (IF_freq):
    input_rate = 20e6

    CFIR_decimate = 125
    RFIR_decimate = 5
    fm_demod_gain = 2200

    quad_rate = input_rate / CFIR_decimate
    audio_rate = quad_rate / RFIR_decimate

    volume = 1.0

    src = GrHighSpeedADCSourceS (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_filter = \
      GrFreqXlatingFIRfilterSCF (CFIR_decimate,
                                 channel_coeffs,
                                 IF_freq)
    # input: complex; output: float
    fm_demod = \
      GrQuadratureDemodCF (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: short
    audio_filter = \
      GrFIRfilterFSF (RFIR_decimate, audio_coeffs)

    final_sink = GrAudioSinkS ()

    fg = gr_FlowGraph ()

    fg.connect (src, chan_filter)
    fg.connect (chan_filter, fm_demod)
    fg.connect (fm_demod, audio_filter)
    fg.connect (audio_filter, final_sink)

    return fg

if __name__ == '__main__':

    # connect to RF front end
    rf_front_end = microtune_eval_board ()
    if not rf_front_end.board_present_p ():
        raise IOError, 'RF front end not found'

    # set gain and radio station frequency
    rf_front_end.set_AGC (300)
    rf_front_end.set_RF_freq (100.1e6)

    IF_freq = rf_front_end.get_output_freq ()
    fg = build_graph (IF_freq)
    fg.start ()        # fork thread(s) and return
    raw_input ('Press Enter to quit: ')
    fg.stop ()