Re: [Discuss-gnuradio] Figured out descrambler (Re: Ettus N210 GMSK 9600)

[Andy Walls]

On Sun, 2016-03-27 at 12:26 -0600, Tom Golden wrote:
> Wow - magic!! Works like a champ on my latest data as well!

Yay!

>   So how did you figure out the descrambler settings?

I translated the description of the scramble from these sources:

http://www.amsat.org/amsat/articles/g3ruh/109.html
http://www.amsat.org/amsat/articles/g3ruh/109/fig03.gif
https://www.rcscomponents.kiev.ua/datasheets/cnsms00930-1.pdf

Into what the source code here was expecting:

https://github.com/gnuradio/gnuradio/blob/master/gr-digital/include/gnuradio/digital/lfsr.h#L133

That code expects to be told a length of 16 for a 17 bit shift register.
It also shifts data into bit 16 and out of bit 0, which is the opposite
direction of what the above sources show in pictures.  So the tap-mask
sense is in the opposite order as well.


>   I was running a program attempting all permutations this morning and
> so far, couldn't create one that worked.
> 
> 
> Thanks!!

You're welcome.

Now I have more uninteresting work to finish up - on a Holiday. :(

-Andy

> -Tom

> 
> On Sun, Mar 27, 2016 at 12:20 PM, Andy Walls
> <address@hidden> wrote:
>         On Sun, 2016-03-27 at 14:03 -0400, Andy Walls wrote:
>         > On Sun, 2016-03-27 at 12:00 -0400,
>         address@hidden
>         > wrote:
>         > >
>         > > Message: 5
>         > > Date: Sat, 26 Mar 2016 15:05:02 -0400
>         > > From: Andy Walls
>         >
>         > > >                              From:
>         > > > Tom Golden
>         > > >                           Subject:
>         > > > Re: [Discuss-gnuradio] Ettus N210
>         > > > GMSK 9600
>         > > >                              Date:
>         > > > Sat, 26 Mar 2016 10:12:18 -0600
>         > > >
>         > > > I've been staring at the de-modulator and I really think
>         it's
>         > > correct.
>         > > > Unfortunately amateur packet radio isn't well documented
>         (at least
>         > > for
>         > > > me).
>         > > >
>         > > >
>         > > > I think there's a scambler I'm not taking into account.
>         Does anyone
>         > > > know the specifics of this?  Or can point me to how to
>         what the
>         > > > parameters of the descrambler block mean and/or how to
>         determine
>         > > their
>         > > > settings (my best guess is it's a 17-bit LFSR with a
>         polynomial of 1
>         > > > +x^12+x^17)?
>         > >
>         > >
>         > > I can't help with the descrambler, sorry.  There are some
>         AX.25 pages
>         > > out there that talk about it; you've probably seen them
>         already.
>         >
>         > I was able to figure out the descrambler. :)
>         >
>         > The attached flowgraph now has the HDLC deframer spewing out
>         test data
>         > packets full of 0x44's.
>         
>         
>         Oopsie.  Flowgraph attached for real this time; not just the
>         generated
>         python.
>         
>         -Andy
>         
>         > The radio appears create a pretty long idle stream (repeated
>         HDLC flags)
>         > at the start of a burst.
>         >
>         > Regards,
>         > Andy
>         >
>         
>         
> 
>