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From: | Tom Golden |
Subject: | Re: [Discuss-gnuradio] Ettus N210 GMSK 9600 |
Date: | Sat, 26 Mar 2016 11:36:29 -0600 |
To un-do the NRZI coding, send the NRZI data into one input of anHI Tom - AX.25 is NRZI coded. What that means is that theencoder toggles the state of a flip flop when the input data is zero(i.e. a JK flip flop). The output of that flipflop is the NRZI encoded
data stream.
AX.25 FM Radio AFSK audio is not scrambled.
Some 9600 baud FSK (not audio) links may be scrambled.
exclusive-or gate and the D input of a D flip flop at the same time.
The Q-bar output of the flipflop is exclusive-or'ed with the NRZI data
(XOR the D-in and Q-bar output of the FF).-- Tom, N5EGOn Sat, Mar 26, 2016 at 9:13 AM, Tom Golden <address@hidden> wrote:_______________________________________________Thanks - I'll give it a try.On Sat, Mar 26, 2016 at 10:09 AM, Andy Walls <address@hidden> wrote:Hi Tom,
The corr_est block needs to match against the NRZI encoded symbols, so yes I used 7 ones for the hdlc flag.
The HDLC flag has two possible NRZI encodings depending on the initial state:
01111110 - HDLC flag byte
1 00000001 - NRZI encoded HDLC flag byte, initial state 1
01111110 - HDLC flag byte
0 11111110 - NRZI encoded HDLC flag byte, initial state 0
So encoding your two byte sequence:
01111110 00100001
1 00000001 01101011
or
0 11111110 10010100
You should build your corr_est preamble samples based on one of these.
Regards,
Andy
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