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[Discuss-gnuradio] Amateur Radio APRS AX.25 A-FSK

From: mgray
Subject: [Discuss-gnuradio] Amateur Radio APRS AX.25 A-FSK
Date: Fri, 14 Jan 2005 16:22:57 -0700 (MST)

I'm starting an effort to implement transmit/receive functions for the 
amateur radio APRS (AX.25) standard using 1200 bps A-FSK.  I've provided 
background information on the standard at the end of this message.  The 
project is designed to learn about GNU Radio as well as contribute 
something to the project.  Now on to the questions/comments:

Is anyone else working on this effort?  (I don't want to spend time 
inventing yet another wheel).

This is an Eric type question, what is the best way to implement this 
function?  Here is the demod path:

NCO Tuner (select frequency) > 
Filter to channel bandwidth (16KHz typically) >
FM Demod >
Filter for each tone (1200Hz / 2200Hz) >
Integrate/Dump (1200bps) >
NRZI decode >
Search for start symbol >
Decode bit stream >
Check and generate complete message

How many of these are already in the GNU library?  A search of the 
discussion board shows information on a data slicer and VrFSKDemod, but 
neither one are in the core CVS.

Was the thought of GNU core primitives to take a sampled baseband signal 
and generate a data stream output?

Can the same primitives operate on the stream and generate a complete 
message that could be sent to a log file or to other processes via a 
TCP/IP socket?  For example, received APRS messages could be logged to a 

As background, A-FSK is an audio FSK system where the symbols are a 1200Hz 
and 2200 Hz set of tones.  These are the same as the old 300/1200 baud 
Bell 1xx/2xx standard MODEMs and US/Canada Bellcore Caller ID.  The audio 
tones are sent over the air as a standard narrow band FM audio signal.  
The AX.25 protocol is a standard for encoding the data that includes a 
message header, station callsign, destination station, message, and a 
simple CRC-16.  The system is widely used for position reporting of 
amateur stations.

Should be fun!

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