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Re: [Discuss-gnuradio] Help decoding Bell 202

From: Tim K
Subject: Re: [Discuss-gnuradio] Help decoding Bell 202
Date: Mon, 28 Dec 2015 17:08:59 -0500


Ah! Sitting and playing with it greatly helped my understanding of the problem.

1. Your capture seems to have some distortion on it. Fortunately, this doesn't affect FM too bad, but you may want to crank your gain back.
2. Remember, Bell 202 uses 1200 Hz and 2200 Hz. You probably don't want 1200 Hz as the corner for your LPF. FSK Demod already has one as well, I just removed it.
3. I listened to the transmissions, and looked at the data. It does sound like Bell 202, and appears to have HDLC frames... but it doesn't sound like APRS. May I ask where you got this?

What I got after removing the LPF: http://i.imgur.com/Z3EbIgU.png

- Tim

On Mon, Dec 28, 2015 at 12:07 AM, En Shih <address@hidden> wrote:
Thanks, Tim. That really clarified a lot of confusion that I had!

I have uploaded the files here:
-"bell202new.grc" is the current flow graph.
-"dump" is how the input signal I'm trying to decode looks like.
-"decoded" is the bytes of a successfully decoded signal, which was decoded
previously using some hardware equipment. (some parts of the data may be
-"imet.pdf" explains the data packet.
-"tcp.py" explains the data packet.

As far as I know, at the physical layer the data is modulated using Bell
202. The data framing is not AX.25, but conforms to UART, although I'm
unsure about the parity and stop bits. I tried looking for the data frames
by looking for start/stop bits, the header. I've also tried flipping the
bits. However, it still doesn't look quite right.

Correct me if I'm wrong: your FSK demod block uses tone detection to
demodulate, which is passing the signal through two band pass filters in
parallel, and determine which comes through (as in p.9 of

En Shih

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