On Tue, Jul 19, 2011 at 5:22 PM, concernedconsumer
<address@hidden> 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 ), 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
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