[Discuss-gnuradio] Some nice HUM

[James Cooley]

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