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Re: [Discuss-gnuradio] Help decoding Bell 202

From: Tim K
Subject: Re: [Discuss-gnuradio] Help decoding Bell 202
Date: Sun, 27 Dec 2015 08:55:26 -0500

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.


- 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
I am new to GNU Radio and signal processing, and constructed this flow graph
by searching previous topics on AFSK:

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

2. After NBFM receive

3. After quadrature demod

4. Final

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
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.


En Shih

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