This is an Erlang port program written in plain C for decoding AX.25 frames sent using 1200 baud AFSK (Audio Frequency Shift Keying). This is most frequently (no pun intended) used today for APRS (Automatic Packet Reporting System).
The decoded frames are written raw to stdout in the {packet,2} format.
The idea here is to take advantage of Erlangs excellent facilities for parsing binary data into packets.
I have not yet incorporated the code for reading real time samples from audio hardware. I'm testing the program using recorded APRS traffic recorded by WA8LMF. More info below.
Right now, to build this early version of the project, you need Xcode and libsndfile. I'm planning a Linux version.
I have been testing this program with the excellent test data provided by WA8LMF Stephen Smith.
Stephen has created a TNC test CD that you can download here.
The downloaded .zip file contains a .bin and .cue file for burning to CD. You can convert it to wav files that are more practical these days, using the excellent bchunk found here.
Convert to wave files:
$ bchunk -w TNC_Test_Ver-1.1.bin TNC_Test_Ver-1.1.cue TNC_Test
Remove one channel to make it mono, downsample to 24k/s and trim the first 4.75s of speech / silence.
$ sox TNC_Test01.wav TNC_Test01-mono24.wav remix 1 rate 24k trim 4.75
Then give the path to one of these files as the first argument of the program. The program will try to decode any afsk ax.25 frames.
- AFSK AX.25 code heavily inspired by multimon-ng
- libsndfile
- CRC code from WAMPES by Dieter Deyke
I'd be happy to receive ideas and suggetions!
- Albin Stigö - Initial work - albinstigo.me
This project is licensed under the MIT License - see the LICENSE.md file for details
- WA8LMF Stephen Smith for his excellent test CD.
- The ring buffer code was inspired by Juho Snellman
- The DSP code was inspired by The Scientist and Engineer's Guide to Digital Signal Processing, an amazing book!