Re: [Discuss-gnuradio] WBFM receive block and FM demodulation process questions

[Martin Braun]

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



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-- 
Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

Dipl.-Ing. Martin Braun
Research Associate

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KIT -- University of the State of Baden-Württemberg and
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