Problem using fm_demod.py

GNU Radio
1

Hi everyone,

I recently started playing around with gnuradio, and being the beginner
that I am, wanted to begin by listening to a close by radio station. I
basically followed the instruction on
http://www.trondeau.com/gr-tutorial/
and used the uhd_rx_cfile with the following command:

uhd_rx_cfile -a “addr=192.168.10.2” -g 0 -f 97.1M --samp-rate=500k -N
5000000 radio_sample_97.1Mhz.32fc
Below you can find the image of the IQ data and the PSD that I got when
I
used *gr_plot_psd_c *
http://imgur.com/kFpqK25

But when I used the fm_demod.py to playback the captured data, it comes
out
completely scrambled. Am I missing something here?

2

Hi everyone,

I recently started playing around with gnuradio, and being the beginner
that I am, wanted to begin by listening to a close by radio station. I
basically followed the instructions on
http://www.trondeau.com/gr-tutorial/
and used the uhd_rx_cfile with the following command:

uhd_rx_cfile -a “addr=192.168.10.2” -g 0 -f 97.1M --samp-rate=500k -N
5000000 radio_sample_97.1Mhz.32fc
Below you can find the image of the IQ data and the PSD that I got when
I
used *gr_plot_psd_c *
http://imgur.com/kFpqK25

But when I used the fm_demod.py to playback the captured data, it comes
out
completely scrambled. Am I missing something here?

3

On Thu, Jun 11, 2015 at 7:17 PM, Nick H. [email protected]
wrote:

5000000 radio_sample_97.1Mhz.32fc*
Below you can find the image of the IQ data and the PSD that I got when I
used *gr_plot_psd_c *
http://imgur.com/kFpqK25

But when I used the fm_demod.py to playback the captured data, it comes
out completely scrambled. Am I missing something here?

What do you mean by “scrambled”? That can mean a handful of different
things in this domain.

Also, you set the gain to 0; you might want to increase that, though it
looks like the PSD is showing a decent amount of signal.

When plotting the image the way you did, skip into the file a few
thousand
samples (pass -s 4000 to the gr_plot_psd_c script) to skip the start-up
transients, which are obscuring the signal.

You also are bringing the signal in at 500 ksps. Make sure that the
script
to play back the FM signal is set up for that (it should be based on
that
tutorial page, but you’ll want to verify).

Tom

4

Hey Tom,

I guess “scrambled” wasn’t really the best word to describe the problem.
:slight_smile:
What I basically meant was that there was a lot of static, and I
couldn’t
really differentiate the radio signal from the noise when I played it
back.
Apparently, I hadn’t set up the script to the correct sampling rate, but
once I got that fixed, the script worked like a charm.

Thank you!