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


From: Andy Walls
Subject: [Discuss-gnuradio] Figured out descrambler (Re: Ettus N210 GMSK 9600)
Date: Sun, 27 Mar 2016 14:03:15 -0400

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.

The radio appears create a pretty long idle stream (repeated HDLC flags)
at the start of a burst.

Regards,
Andy

Attachment: gmsk9600_test2.py
Description: Text Data


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