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Re: [Discuss-gnuradio] Filtering USRP FM Reception

From: Francesco B.
Subject: Re: [Discuss-gnuradio] Filtering USRP FM Reception
Date: Mon, 23 Feb 2009 18:07:48 -0800 (PST)

D'oh! Took me long enough to notice that the optfir block generates
coefficients, but isn't the filter itself. Putting it into the pipeline
directly was a bad idea.

So I have transmitter and receiver, and they run without errors... they just
aren't giving me what I expect. The filtered sum of sine waves at 320 Hz,
440 Hz, and 650 Hz, when given a passband of 340 Hz and stopband of 800 Hz
should produce roughly the 320 Hz sine wave (perhaps with some attenuated
elements of the other signals). However, it doesn't appear to be doing so.
When played at a sampling rate of 32000, the speakers produce more of a
popping sound. Which is still periodic, but isn't a sine wave. The output
file is stored as an attachement (which can be viewed in a hex editor or
played with audio_play.py), as is the latest revision of the code.

Any thoughts?

~ Francesco


Francesco B. wrote:
> Hey all,
> I was wondering, is there anything in particular that's peculiar about
> using a filter to isolate one sine wave out of three that are being
> transmitted over FM? I've attempted adding another FIR (optfir) filter
> post-demod, but connecting it to the pipeline produces a long list of
> small floats in the terminal (filter coefficients, perhaps?), and then the
> program terminates. Otherwise, the program is identical to
> usrp_wfm_rcv_nogui.py. It runs as normal when the filter is commented out,
> outputting all signals at a specific FM frequency to the sound card.
> However, I don't accomplish my task with that.
> What's the proper method for filtering signals, then? It looked fairly
> straightforward, but perhaps optfir doesn't function as I was expecting it
> to. My code is attached.
>  http://www.nabble.com/file/p22029608/usrp_wfm_rcv_filt_nogui.py
> usrp_wfm_rcv_filt_nogui.py 

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