Re: [Discuss-gnuradio] Help decoding Bell 202

[Tim K]

Just some obsesrvations...

 - Why do WBFM to Audio? Shouldn't that be NBFM?

 - Your post NBFM receive looks good.

 - Your QD output looks a little strange. Perhaps look at the FFT of your signal before the QD?

 - In my experience the MM Clock Recovery block likes using ~4 sps. Perhaps use the Rational Resampler to decimate by 10 first?

 - 2 MHz / 40 = 50 kbps... did you mean to be working with 1200 baud?

I've done my own "1200 baud AFSK" receiver, you may be interested in it.

https://github.com/tkuester/gr-bruninga

- Tim

On Sun, Dec 27, 2015 at 7:52 AM, En Shih <address@hidden> wrote:

Hi all,

I am trying to decode a Bell 202 signal from a RTL dongle source using GNU
Radio Companion 3.7.8.1.
I am new to GNU Radio and signal processing, and constructed this flow graph
by searching previous topics on AFSK:

<http://gnuradio.4.n7.nabble.com/file/n57483/2015-12-27_20-51-40.png>
first by passing through a NBFM and then a quadrature demod, but I can't
recover the expected signal, which should have bytes 01 04 or 01 01 at the
start of a packet.

This is how the signal looks like:

1. RTL-SDR source
<http://gnuradio.4.n7.nabble.com/file/n57483/signal1.png>

2. After NBFM receive
<http://gnuradio.4.n7.nabble.com/file/n57483/3.png>

3. After quadrature demod
<http://gnuradio.4.n7.nabble.com/file/n57483/5.png>

4. Final
<http://gnuradio.4.n7.nabble.com/file/n57483/6.png>

Does it look like I am on the right track?

The grc file, a dump of the original signal, and a decoded file is stored
here:
https://drive.google.com/folderview?id=0B5T4PFVUKZgmcXlTWnV0VzZFQTg&usp=sharing
<https://drive.google.com/folderview?id=0B5T4PFVUKZgmcXlTWnV0VzZFQTg&usp=sharing>
The decoded file was done using a physical modem, and its contents does not
match the dump but is in the same format.

I have also tried using  multimon-ng
<https://github.com/EliasOenal/multimon-ng>   with the following command:
rtl_fm -f 402000000 -s 22050 | multimon-ng -a AFSK1200 -t raw /dev/stdin
but to no avail.

Thanks.

Regards,
En Shih



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