[Discuss-gnuradio] Problem with hearing FM transmission

[meggahertz]

Hello All,

I am trying to receive only a small portion of different FM station
frequencies stored in a string. To do this I modified the
usrp_wfm_rcv_nogui.py. The way I am doing this is to feed the incoming
stream from usrp.source_c() to vector of a fixed length and then listen to
that vector. My problem is when I try to sink the vector into the audio sink
all I hear is noise. But when I use usrp.source_c() directly it plays fine
but ofcourse I end up listening to a continuous transmission instead of a
short segment. The following is the code (modified version of
usrp_wfm_rcv_nogui.py. I hope i made my issue clear enough. PLease ask me if
you need any clarification. I really need help at this point. Thank you.

#!/usr/bin/env python
#
# Copyright 2005,2006,2007 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 3, 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., 51 Franklin Street,
# Boston, MA 02110-1301, USA.
# 

from gnuradio import gr, gru, eng_notation, optfir
from gnuradio import audio
from gnuradio import usrp
from gnuradio import blks2
from gnuradio.eng_option import eng_option
from optparse import OptionParser
from usrpm import usrp_dbid
import sys
import math,time,re

def pick_subdevice(u):
    """
    The user didn't specify a subdevice on the command line.
    Try for one of these, in order: TV_RX, BASIC_RX, whatever is on side A.

    @return a subdev_spec
    """
    return usrp.pick_subdev(u, (usrp_dbid.TV_RX,
                                usrp_dbid.TV_RX_REV_2,
                                usrp_dbid.TV_RX_REV_3,
                                usrp_dbid.BASIC_RX))


class wfm_rx_block (gr.top_block):

    def __init__(self):
        gr.top_block.__init__(self)

        parser=OptionParser(option_class=eng_option)
        parser.add_option("-R", "--rx-subdev-spec", type="subdev",
default=None,
                          help="select USRP Rx side A or B (default=A)")
        parser.add_option("-f", "--freq", type="str", default=100.1e6,
                          help="set frequency to FREQ", metavar="FREQ")
        parser.add_option("-g", "--gain", type="eng_float", default=None,
                          help="set gain in dB (default is midpoint)")
        parser.add_option("-O", "--audio-output", type="string", default="",
                          help="pcm device name.  E.g., hw:0,0 or surround51
or /dev/dsp")

        (options, args) = parser.parse_args()
        if len(args) != 0:
            parser.print_help()
            sys.exit(1)
        
        self.vol = 3
        self.state = "FREQ"
        self.freq = 0

        # build graph
        
        self.u = usrp.source_c()                    # usrp is data source

        adc_rate = self.u.adc_rate()                # 64 MS/s
        usrp_decim = 200
        self.u.set_decim_rate(usrp_decim)
        usrp_rate = adc_rate / usrp_decim           # 320 kS/s
        chanfilt_decim = 1
        demod_rate = usrp_rate / chanfilt_decim
        audio_decimation = 10
        audio_rate = demod_rate / audio_decimation  # 32 kHz


        if options.rx_subdev_spec is None:
            options.rx_subdev_spec = pick_subdevice(self.u)

        self.u.set_mux(usrp.determine_rx_mux_value(self.u,
options.rx_subdev_spec))
        self.subdev = usrp.selected_subdev(self.u, options.rx_subdev_spec)
        print "Using RX d'board %s" % (self.subdev.side_and_name(),)


        chan_filt_coeffs = optfir.low_pass (1,           # gain
                                            usrp_rate,   # sampling rate
                                            80e3,        # passband cutoff
                                            115e3,       # stopband cutoff
                                            0.1,         # passband ripple
                                            60)          # stopband
attenuation
        #print len(chan_filt_coeffs)
        chan_filt = gr.fir_filter_ccf (chanfilt_decim, chan_filt_coeffs)

        self.guts = blks2.wfm_rcv (demod_rate, audio_decimation)

        self.volume_control = gr.multiply_const_ff(self.vol)

        # sound card as final sink
        audio_sink = audio.sink(int(audio_rate),
                                options.audio_output,
                                False)  # ok_to_block
        
        # now wire it all together
        src=self.u
        c2f_vec = gr.stream_to_vector(gr.sizeof_gr_complex,1)
        snk_vec = gr.vector_sink_c()
        self.connect(src,snk_vec)
        self.run()
        vec_data = snk_vec.data()
        
        self.disconnect_all()
        
        if options.gain is None:
            # if no gain was specified, use the mid-point in dB
            g = self.subdev.gain_range()
            options.gain = float(g[0]+g[1])/2


        # set initial values

        self.set_gain(options.gain)
        
        
        Y=options.freq
        c_freqs_temp=re.findall(r"(-\d+\.\d+|\d+\.\d+|\d+)",Y)
        c_freqs=[]
        for ndx0 in range(len(c_freqs_temp)):
                c_freqs_temp_1=float(c_freqs_temp[ndx0])
                c_freqs.append(c_freqs_temp_1)
        print c_freqs,len(c_freqs)
        
        for ndx1 in range(0,1):
                        self.set_freq(c_freqs[ndx1])
                        vec_src=gr.vector_source_c(vec_data,False)      
                        self.connect (vec_src, self.guts, self.volume_control, 
audio_sink)

                
    def set_vol (self, vol):
        self.vol = vol
        self.volume_control.set_k(self.vol)
        self.update_status_bar ()

    def set_freq(self, target_freq):
        """
        Set the center frequency we're interested in.

        @param target_freq: frequency in Hz
        @rypte: bool

        Tuning is a two step process.  First we ask the front-end to
        tune as close to the desired frequency as it can.  Then we use
        the result of that operation and our target_frequency to
        determine the value for the digital down converter.
        """
        r = self.u.tune(0, self.subdev, target_freq)
        
        if r:
            self.freq = target_freq
            self.update_status_bar()
            self._set_status_msg("OK", 0)
            return True


        self._set_status_msg("Failed", 0)
        return False
        
    def set_gain(self, gain):
        self.subdev.set_gain(gain)

    def update_status_bar (self):
        msg = "Freq: %s  Volume:%f  Setting:%s" % (
            eng_notation.num_to_str(self.freq), self.vol, self.state)
        self._set_status_msg(msg, 1)
        
    def _set_status_msg(self, msg, which=0):
        print msg

    
if __name__ == '__main__':
    tb = wfm_rx_block()
    try:
        tb.run()
                
    except KeyboardInterrupt:
        pass

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