Re: [Discuss-gnuradio] Some nice HUM

[mgray]

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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