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

[Tom Golden]

Wow - magic!! Works like a champ on my latest data as well!  So how did you figure out the descrambler settings?  I was running a program attempting all permutations this morning and so far, couldn't create one that worked.

Thanks!!

-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
>