Re: [Discuss-gnuradio] Streaming Funcube Dongle Pro+ over UDP

[Iain Young, G7III]

On 08/08/13 17:52, Vanush Vaswani wrote:

> So the receiver begins with a sample rate of 48 KHz? So how is it
> possible to FM demodulation this case?

No, that's my transmit chain (assuming you are referring to my
GUI_TRX_JACK flowgraph)

The (Simplified) FM RX chain in the same flowgraph goes through
the following blocks:

1) UHD_Source : This is the HW receiver. In my case, a USRP running at 
250k. In your case the FCDPP, running at 192k

2) A FIR Filter for tuning. If you just want the centre frequency that 
you've configured on the FCDPP then you can safely remove this block

3) A BPF, which does two things:
   a) Implements a Band Pass Filter of ~10k [can be changed at runtime 
in my case, set the width appropriately for your signal
   b) Decimates [IE down samples] at a 5:1 ratio, so 250k becomes 50k.

You would need to modify this, probably easiest to make the decimation
1, and then modify the rational resampler as appropriate. I did it this
way to reduce the workload of the resampler.

4) A rational resampler, which resamples from 50k to 48k. In your case,
I'd modify that to resample from 192k to whatever your soundcard supports

5) An NBFM receive block. Note the Audio and Quadrature rates shown, and
their relationship to the audio device's rate, and the output from the 
resampler

6) The audio sink device, that happens to run at 48k, since my 
"soundcards" (actually the jack audio system) run at 48k. Yours may
want 44k1

In my case, it is narrow band FM, so I had no need to receive bandwidths
of >50k (Heck, >10k on the BPF would mean someone was over deviating!),
so I used the fact that the BPF could decimate as well as filter.


I urge you again to google for "gnuradio grc FM Receiver example". There
are plenty out there. Yes, you will need to replace whatever source
block they use with the dongle source block. You will also need to
adjust any resampling appropriately.

(And as Marcusj ust  mentioned, if you dont have any GUI elements [they
can usually be safely removed from the flow graph], just select the non
GUI option in the "Options", build the flow graph, and run it from the
command line)

Look at http://hal.g7iii.net/GRC/Examples/Simple_Multimode_RX.png again.
It does the FM receive at 192k (I was being lazy, it was only an
example, and a "learning test"), but then resamples at the "real" level
before sending on to the audio device.

Implement just the FM bits (switch to WBFM if you want]. It will show
you the effects of the BPF. You can even add another waterfall *AFTER*
the resampling, so you can see the effect of that.


Iain