Re: [Discuss-gnuradio] Help decoding Bell 202

[Albin Stigö]

En Shih,

> 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.

Packet uses NRZI (non-return to zero inverted) encoding, which means
that a 0 is encoded as a change in tone, and a 1 is encoded as no
change in tone. You are probably looking for the change in bit rather
than the actual bits, that could explain why your data is a bit off.

There's a good summary here: http://n1vg.net/packet/

--Albin

On Mon, Dec 28, 2015 at 10:54 AM, Albin Stigö <address@hidden> wrote:
> For anyone working with APRS Stephen (wa8lmf) has put together a
> downloadable cd image with a lot of test data. Not in quadrature, but
> that's maybe something Hilbert can take care of.
>
> http://wa8lmf.net/TNCtest/
>
> # Get it with wget.
> $ wget http://www.argentdata.com/files/tnc_test_cd_ver1.0.zip
>
> # Convert the tracks to wave format with the bchunk utility (search
> your packet manager or homebrew).
> $ bchunk -w TNC_Test_Ver-1.1.bin TNC_Test_Ver-1.1.cue TNC_Test_Ver-1.1.iso
>
>
> --Albin
>
> On Mon, Dec 28, 2015 at 2:07 AM, En Shih <address@hidden> wrote:
>> Thanks, Tim. That really clarified a lot of confusion that I had!
>>
>> I have uploaded the files here:
>> https://drive.google.com/folderview?id=0B5T4PFVUKZgmcXlTWnV0VzZFQTg&usp=sharing
>> <https://drive.google.com/folderview?id=0B5T4PFVUKZgmcXlTWnV0VzZFQTg&usp=sharing>
>> -"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
>> corrupted)
>> -"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
>> http://edge.rit.edu/edge/P09141/public/FSK.pdf)
>>
>> Regards,
>> En Shih
>>
>>
>>
>>
>> --
>> View this message in context: 
>> http://gnuradio.4.n7.nabble.com/Help-decoding-Bell-202-tp57483p57503.html
>> Sent from the GnuRadio mailing list archive at Nabble.com.
>>
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