wfm_rcv.py

#
# Copyright 2005,2007,2012 Free Software Foundation, Inc.
#
# This file is part of GNU Radio
#
# SPDX-License-Identifier: GPL-3.0-or-later
#
#

from __future__ import absolute_import
from __future__ import division
from __future__ import unicode_literals

import math

from gnuradio import gr, filter

from . import analog_swig as analog
from .fm_emph import fm_deemph


class wfm_rcv(gr.hier_block2):
    def __init__ (self, 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).

        Args:
            quad_rate: input sample rate of complex baseband input. (float)
            audio_decimation: how much to decimate quad_rate to get to audio. (integer)
        """
        gr.hier_block2.__init__(self, "wfm_rcv",
                                gr.io_signature(1, 1, gr.sizeof_gr_complex), # Input signature
                                gr.io_signature(1, 1, gr.sizeof_float))      # Output signature

        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 = analog.quadrature_demod_cf(fm_demod_gain)

        # input: float; output: float
        self.deemph = fm_deemph(audio_rate)

        # compute FIR filter taps for audio filter
        width_of_transition_band = audio_rate / 32
        audio_coeffs = filter.firdes.low_pass(1.0,            # gain
                                              quad_rate,      # sampling rate
                                              audio_rate / 2 - width_of_transition_band,
                                              width_of_transition_band,
                                              filter.firdes.WIN_HAMMING)
        # input: float; output: float
        self.audio_filter = filter.fir_filter_fff(audio_decimation, audio_coeffs)

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