Problem with hearing FM transmission

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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