#!/usr/bin/env python

from gnuradio import gr, eng_notation
from gnuradio import audio
from gnuradio import usrp
from gnuradio.eng_option import eng_option
from optparse import OptionParser
import sys
import math


#
#  Tune a demodulate a narrow band FM audio signal from an IF stream.
#
#  @param fileName file that contains 256ksps complex IF data in gr format
#  @param channelOffset frequency to tune where 0 is center of sampled IF
#
#  @return flow_graph
#
def build_graph (fileName, channelOffset):
    adc_rate = 64e6
    usrp_decim = 250
    if_rate = adc_rate / usrp_decim               # 256 kHz
    if_decim = 4
    demod_rate = if_rate / if_decim        # 64 kHz
    audio_decimation = 2
    audio_rate = demod_rate / audio_decimation  # 16 kHz


    fg = gr.flow_graph ()
    

    src = gr.file_source (gr.sizeof_gr_complex, fileName)

    (head, tail) = build_pipeline (fg, if_rate, if_decim, audio_rate, audio_decimation, channelOffset)

    # sound card as final sink
    audio_sink = audio.sink (int (audio_rate))

    # now wire it all together
    fg.connect (src, head)
    fg.connect (tail, (audio_sink, 0))
    
    return fg

def build_pipeline (fg, if_rate, if_decim, audio_rate, audio_decimation, channelOffset):

    # Filter around FM modulated channel (16KHz wide) = 2 * (5KHz Deviation + 3KHz Max Audio Freq)
    # Note the FM modulated bandwidth is wider than the audio pass band.
    # This filter gives us our adjacent channel rejection.
    channel_coeffs = gr.firdes.low_pass (1.0, if_rate, 10e3, 6e3, gr.firdes.WIN_HANN)

    # Select desired channel
    ddc = gr.freq_xlating_fir_filter_ccf (if_decim, channel_coeffs, channelOffset, if_rate)

    # Demod and scale to prevent overflow.
    fm_demod = gr.quadrature_demod_cf (0.2)

    # Filter the output audio.
    # We use a band pass to get rid of the 60Hz AC hum on the signal.
    # Some of the NOAA equipment is very old.
    # The real audio band pass is 150 - 5000Hz because we decimate down 2:1.
    audio_coeffs = gr.firdes.band_pass (1.0, if_rate, 300, 10000, 1000, gr.firdes.WIN_HANN)

    audio_filter = gr.fir_filter_fff (audio_decimation, audio_coeffs)

    # Connect the chain
    fg.connect (ddc, fm_demod)
    fg.connect (fm_demod, audio_filter)

    return ((ddc, 0), (audio_filter, 0))

def main (args):
    if len(args) != 2:
        sys.stderr.write ('usage: nfm_rcv_file filename tuning(KHz)\n')
        sys.exit (1)

    fg = build_graph (args[0], float(args[1]) * 1000)

    fg.start ()        # fork thread(s) and return
    raw_input ('Press Enter to quit: ')
    fg.stop ()

if __name__ == '__main__':
    main (sys.argv[1:])