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Re: [Discuss-gnuradio] Some nice HUM
From: |
mgray |
Subject: |
Re: [Discuss-gnuradio] Some nice HUM |
Date: |
Tue, 22 Feb 2005 12:54:46 -0700 (MST) |
Some NBFM voice signals also include a sub-audible tone for squelch
control. These are typically in the range 67Hz to 255Hz. You might be
hearing these.
>From additional information:
http://www.eecis.udel.edu/~dra/pl.html
On Tue, 22 Feb 2005, James Cooley wrote:
> Hi all,
>
> OK, Got FM radio and FM stereo to work using USRP + Basic RX + microtune
> 4937 frontend.
>
> I'm hearing a nice hum along with whatever I tune into. Anyone else?
>
> This is not 60Hz, but, purely guessing here, it *could* be 120Hz. Could
> this be an image of the 60Hz somehow? Also, how do I get rid of it?
>
> I tried constructing a notch filter after Chuck's example from NBFM....
> Something like
>
> # insert an audio notch filter for 120 Hz
> taps = [1] # original signal, no delay
> for i in range(1,133): # 4.17mSec ( 1/2 cycle at 120Hz ) =
> 132/16000
> taps = taps + [0]
> taps = taps + [1] # signal delayed 4.17mSec cancels self at 120Hz
> notch0 = gr.fir_filter_fff (1, taps)
> notch1 = gr.fir_filter_fff (1, taps)
>
> And stuck it in-between the demod/dsp graph and the audio sink.
>
> Doesn't really work too well. It's too noticeable both because it's
> within audible range and because the delay is slow enough so that you
> can hear phase difference (kind of a slight flanger effect). Of course
> the latter is true, it's 4.17ms delay.
>
> -jamie
>
>
>
>
>
>
>
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