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Re: [Discuss-gnuradio] General question concerning usrp & audio

From: Alexandru Csete
Subject: Re: [Discuss-gnuradio] General question concerning usrp & audio
Date: Tue, 21 Sep 2010 12:32:17 +0200

On Tue, Sep 21, 2010 at 5:09 AM, Thomas Seidel <address@hidden> wrote:
> hHey!
> I am just playing around with gnuradio and the usrp stuff and want to achieve 
> the following:
> file1.py
> [...]
> self.u = usrp.sink_c(0, 64)
> self.src gr.sig_source_c(48000, gr.GR_SIN_WAVE, 400, 1)
> self.connect(self.src, self.u)
> [...]
> file2.py
> [...]
> self.u = usrp.source_c(0, self.decim_rate)
> self.sndsink = audio.sink(48000, 'plughw:0,0')
> self.blk = gr.complex_to_mag()
> self.connect(self.u, self.blk, (self.sndsink, 0))
> [...]
> in words: file1.py should serve as the sender of sine signal with a frequency 
> of 400Hz and file2.py should receive this signal and emit it on the 
> soundcard. Both excerpts are implemented as a derived class from 
> gr.top_block().
> Running usrp_oscope.py, I can see that a sine is transmitted, however i can 
> not here anything than noise(?). Actually I would have expected something 
> like the output of dial_tone.py. Can somebody tell me why I am wrong and 
> don't receive what i expect.

One of the problems is that you connect the USRP to the audio sink
without any downsampling. Even with max decimation, 256, the USRP will
generate 250 ksps while your audio sink expects only 48 ksps. If you
look at the receiver examples you'll see that all of them use a
channel filter (usually low pass) which also does some downsampling.
By the way, using a channel filter in a receiver is generally a good
idea ;-)

Other problems could be receiver settings (gain, frequency, antenna
selection, etc.).


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