wfm_rcv.py

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#
# Copyright 2005 Free Software Foundation, Inc.
# 
# This file is part of GNU Radio
# 
# GNU Radio is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2, or (at your option)
# any later version.
# 
# GNU Radio is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
# 
# You should have received a copy of the GNU General Public License
# along with GNU Radio; see the file COPYING.  If not, write to
# the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
# Boston, MA 02111-1307, USA.
# 

from gnuradio import gr
from gnuradio.blksimpl.fm_emph import fm_deemph
import math

class wfm_rcv(gr.hier_block):
    def __init__ (self, fg, quad_rate, audio_decimation):
        """
        Hierarchical block for demodulating a broadcast FM signal.
        
        The input is the downconverted complex baseband signal (gr_complex).
        The output is the demodulated audio (float).
        
        @param fg: flow graph.
        @type fg: flow graph
        @param quad_rate: input sample rate of complex baseband input.
        @type quad_rate: float
        @param audio_decimation: how much to decimate quad_rate to get to audio.
        @type audio_decimation: integer
        """
        volume = 20.

        max_dev = 75e3
        fm_demod_gain = quad_rate/(2*math.pi*max_dev)
        audio_rate = quad_rate / audio_decimation
        

        # We assign to self so that outsiders can grab the demodulator 
        # if they need to.  E.g., to plot its output.
        #
        # input: complex; output: float
        self.fm_demod = gr.quadrature_demod_cf (fm_demod_gain)

        # input: float; output: float
        self.deemph = fm_deemph (fg, quad_rate)
        
        # compute FIR filter taps for audio filter
        width_of_transition_band = audio_rate / 32
        audio_coeffs = gr.firdes.low_pass (volume,         # gain
                                           quad_rate,      # sampling rate
                                           audio_rate/2 - width_of_transition_band,
                                           width_of_transition_band,
                                           gr.firdes.WIN_HAMMING)
        # input: float; output: float
        self.audio_filter = gr.fir_filter_fff (audio_decimation, audio_coeffs)

        fg.connect (self.fm_demod, self.deemph, self.audio_filter)

        gr.hier_block.__init__(self,
                               fg,
                               self.fm_demod,       # head of the pipeline
                               self.audio_filter)   # tail of the pipeline