Re: [Discuss-gnuradio] Quadrature demodulation of FM

[Philip Balister]

I think you want to look at Richard Lyon's book on DSP,  Fig 13-61(b).
The book should be on the recommended reading list for GNU Radio. I
think this is an implementation of the algorithm you are trying to
implement.

I've used this method successfully with the OSSIE SCA code from VT.
Unfortunately that code is behind a silly click through system, but
I'm copying this code to an open repository next week. If you have
more questions, email me next week.

Philip

On Thu, Mar 27, 2008 at 5:55 PM, Jonathon Pendlum <address@hidden> wrote:
> Before I ask my question, I'd like to thank everyone that's been helping me
> out. I (and my peers at Purdue) are just starting out with the USRP and
> really appreciate the support.
>
> On to my question, I wrote a demodulation block for FM that uses a method
> different than the provided quadrature_demod_cf block. Here is a block
> diagram of what I'm attempting:
> http://pico.iis.u-tokyo.ac.jp/media/9/20060626-SQD-princ.jpg. I can record
> data from the USRP and successfully demodulate FM radio in MATLAB using this
> technique. However, I cannot accomplish this with my signal processing block
> I wrote. To test my block, I use the example wideband FM receiver code,
> usrp_wfm_rcv.py, and basically insert my demodulation block in place of the
> provided demodulation block.
>
> Here is the work code I made, note it is just a small modification of
> quadrature_demod_cf.cc:
>
> int gr_quad_demod_baseband::work (int noutput_items,
>         gr_vector_const_void_star &input_items,
>         gr_vector_void_star &output_items)
>  {
>     gr_complex *in = (gr_complex *) input_items[0];
>     float *out = (float *) output_items[0];
>     in++;        // ensure that in[-1] is valid
>
>     for (int i = 0; i < noutput_items; i++){
>         float I_1 = real(in[i-1]);
>          float I_2 = real(in[i]);
>         float Q_1 = imag(in[i-1]);
>         float Q_2 = imag(in[i]);
>
>         float dI = I_1 - I_2;
>         float dQ = Q_1 - Q_2;
>
>         float dQxI = dQ*I_1;
>          float dIxQ = dI*Q_1;
>
>         out[i] = d_gain*(dIxQ - dQxI);
>     }
>
>     return noutput_items;
> }
>
>
> The demodulation is really simple and if you do not get it at first, please
> check out the link to the block diagram. On a final note, my block is also
> running very slowly. I continuously get overrun notifications (I believe
> that is the meaning of uO). I think I am quite close to the answer, I just
> need a little guidance. Anyone have any suggestions? Thanks in advance!
>
>
>
>
> Jon Pendlum
> Purdue University Undergraduate Student
>
>
>
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