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[Discuss-gnuradio] AM RECEIVER


From: shijesh a.k
Subject: [Discuss-gnuradio] AM RECEIVER
Date: Tue, 11 Sep 2007 15:59:22 +0100 (BST)

Hello friends,
 
I(shijesh) made debute to USRP & GNUradio a few days back.
I have a querry as mentioned after the environment observations given below
 
My environment is :
            O.S     ::    Ubuntu 7.04 Feisty fawn
            CPU    ::     Core2 DUO Intel
            gcc      ::     4.2.0
            SWIG   ::     1.3.31
  GNU CODE VERSION ::     3.0.4
 
when i tried to receive AM with the program "am_rcv.py" , it didn't work. Later on when i went through the code i realised that, it did not feature the option for changing the SOUND CARD RATE  , so i modified the program as shown below, but even then it was all in vain
 
*********###### program for receiving AM ########************
#!/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
from gnuradio.wxgui import stdgui, fftsink
import wx
class am_rx_graph (stdgui.gui_flow_graph):
    def __init__(self,frame,panel,vbox,argv):
        stdgui.gui_flow_graph.__init__ (self,frame,panel,vbox,argv)
       
######modified for including option for changing the sound card rate ####### 
       
        parser=OptionParser(option_class=eng_option)
        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.frame = frame
        self.panel = panel
       
        self.vol = 0
        self.state = "FREQ"
        self.freq = 0
       
###################### modification ends ######################       
        station = parseargs(argv[1:])
        offset_freq = 30e3
        IF_freq = offset_freq - station
        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
       
        # usrp is data source
        src = "" (0, usrp_decim)
        src.set_rx_freq (0, IF_freq)
        actual_IF_freq =src.rx_freq(0)
        actual_offset = actual_IF_freq + station
       
        #print actual_IF_freq
        #print actual_offset
 
        src.set_pga(0,20)
        # sound card as final sink
        audio_sink = audio.sink (int (audio_rate), options.audio_output, False)
       
        channel_coeffs = \
                       gr.firdes.low_pass (1.0,           # gain
                                           if_rate,   # sampling rate
                                           9e3,         # low pass cutoff freq
                                           10e3,         # width of trans. band
                                           gr.firdes.WIN_HANN)
        ddc =  gr.freq_xlating_fir_filter_ccf (if_decim,channel_coeffs,-actual_offset,if_rate)
        magblock = gr.complex_to_mag()
        volumecontrol = gr.multiply_const_ff(.003)
        # Deemphasis.  Is this necessary on AM?
        TAU  = 75e-6  # 75us in US, 50us in EUR
        fftaps = [ 1 - math.exp(-1/TAU/if_rate), 0]
        fbtaps= [ 0 , math.exp(-1/TAU/if_rate) ]
       
        deemph = gr.iir_filter_ffd(fftaps,fbtaps)
        # compute FIR filter taps for audio filter
        width_of_transition_band = audio_rate / 8
        audio_coeffs = gr.firdes.low_pass (1.0,            # gain
                                           if_rate,      # sampling rate
                                           9e3,         #audio_rate/2 - width_of_transition_band,
                                           4e3,         # width_of_transition_band,
                                           gr.firdes.WIN_HANN)
       
        # input: float; output: float
        audio_filter = gr.fir_filter_fff (audio_decimation, audio_coeffs)
 
       
        print len(channel_coeffs)
        print len(audio_coeffs)
       
        # now wire it all together
        self.connect (src, ddc)
        self.connect (ddc, magblock)
        self.connect (magblock, volumecontrol)
        self.connect (volumecontrol,deemph)
        self.connect (deemph,audio_filter)
        self.connect (audio_filter, (audio_sink, 0))
        if 1:
            pre_demod = fftsink.fft_sink_c (self, panel, title="Pre-Demodulation", fft_size=128, sample_rate=if_rate)
            self.connect (src, pre_demod)
            vbox.Add (pre_demod.win, 1, wx.EXPAND)
        if 0:
            post_demod = fftsink.fft_sink_c (self, panel, title="Post Demodulation", fft_size=256, sample_rate=demod_rate)
            self.connect (ddc, post_demod)
            vbox.Add (post_demod.win, 1, wx.EXPAND)
        if 0:
            post_filt = fftsink.fft_sink_f (self, panel, title="Post Filter", fft_size=512, sample_rate=audio_rate)
            self.connect (magblock,post_filt)
            vbox.Add (post_filt.win, 1, wx.EXPAND)
       
def parseargs (args):
    nargs = len (args)
    if nargs == 1:
        freq1 = float (args[0]) * 1e3
    else:
        sys.stderr.write ('usage: am_rcv freq1\n')
        sys.exit (1)
    return freq1
if __name__ == '__main__':
    app = stdgui.stdapp (am_rx_graph, "AM RX")
    app.MainLoop ()
############################program endss########################
 
I have used the following command for the execution
###############################################################
address@hidden:/home/sabu# GNUradio/gnuradio-3.0.4/gnuradio-examples/python/usrp/am_rcv.py 1164000 -o plughw:0,0
#############################################################
I know that it is a silly question for all of u friends, but for me it really will be  worth,  Please extend your helping hand.
 
SHIJESH AK
INDIA
 


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