2channelrx.py

#!/usr/bin/env python2

from gnuradio import gr
import sys

#
# return a gr.flow_graph
#
def build_graph (freq1, freq2):
    input_rate = 1e6
    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
        50e3,        # 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:])