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 > > > > > > > > _______________________________________________ > Discuss-gnuradio mailing list > [email protected] > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
