Try using a single pole IIR filter. If you don't know what an IIR is, wiki it, better explanation there than from me. Keep pushing the frequency cutoff to a lower frequency until the static gets better. If you go to far, your audio should disappear.
On Tue, Jul 19, 2011 at 5:22 PM, concernedconsumer < concernedconsu...@verizon.net> wrote: > Hello, > Thank you so much for the helpful responses! I really appreciate them. > > I have now successfully calculated the instantaneous frequencies, and am > able to hear the station. However, there is much static present. From this > website ( http://radioware.nd.edu/**documentation/basic-gnuradio/** > exploring-the-fm-receiver<http://radioware.nd.edu/documentation/basic-gnuradio/exploring-the-fm-receiver>), > I see that I should perform FM de-emphasis to reduce the strengths of the > higher frequency parts of the signal. I have tried to do this using a FIR > filter (using 21 taps calculated by the sinc function) with a cutoff at 2122 > Hz (because the tau constant is 75e-6 which gives a cutoff of 2122 Hz in an > RC filter). However, this did not do much to reduce the static. I have also > tried an 11 point least squares quadratic filter. This, also, did not help > much. > > What type of filter, with what tap values, should be used for FM > de-emphasis? I have attached a flowchart with my process (the pdf file), > along with a screenshot of the gnuradio flowgraph I am following. Also, are > there other ways I can reduce the amount of static in the signal? > > > Thanks, > J > > > > > > > On 07/10/2011 09:07 PM, concernedconsumer wrote: > >> Hello, >> >> I am a high school student and am very interested in the AM and FM >> demodulation processes. I have been using gnu radio in combination with a >> USRP to receive radio signals, demodulate, and listen. This past year, I >> worked through the details of AM demodulation in gnu radio by computing >> mixing, low-pass filtering, decimating, interpolating, and normalizing >> algorithms/. /This helped me better understand the AM demodulation process, >> and especially what was being done in each of the gnu radio blocks. I am now >> trying to do the same thing with FM demodulation and have implemented the >> following; frequency x-lating FIR filtering, calculating the instantaneous >> frequencies, and decimating and interpolating routines (as performed in the >> rational resampler). >> >> My question is about the instantaneous frequencies in FM demodulation. I >> know that the frequency of the carrier varies with the strength of the >> transmitted signal. The instantaneous frequencies must then be the deviation >> of the modulated signal from the carrier (which is at baseband) and denote >> changes in the strength of the transmitted signal. Still, I must be missing >> some steps that are performed in the WBFM block. After I calculate the >> instantaneous frequencies, what do I have to do to complete the demodulation >> process and be able to play the file back through the audio sink? I have >> attached a screenshot of the gnuradio-companion flow graph I am following, >> along with a pdf of the process I am implementing. >> >> What does the WBFM block do, mathematically, to the signal? I would also >> appreciate feedback on the process I am using. >> >> Thanks, >> J >> > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >
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