Hi J, a couple of thoughts: - Your flow graph seems correct. From your mail, I wasn't quite sure if it's working or not. If not, the place I'd check is the input filter. - The frequency does not strictly depend on the *strength* of the signal, nor is it demodulated by from signal strength deviations. It's rather the *phase* changes which are relevant. To figure out what the WBFM block does, check out gnuradio-core/src/python/gnuradio/blks2impl/wfm_rcv.py. The heart of this block is gr_quadrature_demod_cf, which does exactly that--output the difference of phases. Mathematically, if \phi(k) denotes the phase of the k-th sample, the output is (\phi(k) - \phi(k-1)) * gain. The latter factor describes the frequency deviation.
Hope this helps... good luck with your stuff! I wish some of my students had spent time in high school this way :) MB On Sun, Jul 10, 2011 at 09:07:09PM -0400, 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 -- Karlsruhe Institute of Technology (KIT) Communications Engineering Lab (CEL) Dipl.-Ing. Martin Braun Research Associate Kaiserstraße 12 Building 05.01 76131 Karlsruhe Phone: +49 721 608-43790 Fax: +49 721 608-46071 www.cel.kit.edu KIT -- University of the State of Baden-Württemberg and National Laboratory of the Helmholtz Association
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