M17_Implementations

Various implementations

Woj's implementation (/SP5WWP)

Overview

Written in C, it has all the components described by the protocol's specification of the stream and packet modes:

• convolutional encoder with soft Viterbi decoder (utilizing fixed point arithmetic),
• Golay encoder with soft decoder (fixed point),
• bit interleaver and randomizer,
• cyclic redundancy check (CRC) calculation (both LSF and arbitrary input),
• callsign encoder and decoder

There's no support for any encryption yet.

Building

First, build the shared object libm17.so:

cd M17_Implementations/libm17
make
make install
sudo ldconfig

Then, cd back up to the directory of interest and make again.

Capabilities

Four executables are available:

• m17-coder-sym is used to convert a raw binary data bitstream to symbols. Symbol stream has to be filtered with an appropriate filter before transmission, see the specification document for details.
• m17-decoder-sym decodes a stream of floats at stdin, one sample per symbol. After a valid syncword is detected, decoding process starts. The program expects a stream of synchronized symbols at the input. See the /grc/symbol_recovery.grc file for details.
• m17-packet-encode is a handy tool for generating baseband (or a symbol stream, if needed) for M17 packets. The program expects a limited stream of raw data at the stdin. The number of bytes is set with the -n parameter, range 1 to 800.
• m17-packet-decode decodes incoming packets.

Testing

Stream mode

Both the encoder and the decoder can be tested simultaneously. The test setup should look as follows:
GRC flowgraph -> fifo1 -> m17-coder-sym -> fifo2 -> m17-decoder-sym -> stdout
To perform a simple test, GNURadio 3.10 is required.

Run the following commands:

mkfifo fifo1
mkfifo fifo2

This should create 2 named pipes: fifo1 and fifo2. The first one is used for the "transmitted" raw bitstream from GNURadio. The other one is used for the "receiver" part - the symbol stream.

Start gnuradio-companion, open the .grc file included in this repo (/grc/m17_streamer.grc) and change the name of the named pipe to fifo1 (at the File Sink block - the rightmost one). Change the location of it if needed.

Open up 2 terminals and run:
Terminal 1:

cat fifo1 | ./m17-coder-sym > fifo2

Terminal 2:

cat fifo2 | ./m17-decoder-sym

Hit the Execute the flow graph button in GNURadio and watch it roll.

Terminal 2 should show similar results, with the Frame Number advancing each frame:

Packet mode

Packet encoding is available with m17-packet-encoder. Its input parameters are shown below.

-S - source callsign (uppercase alphanumeric string) max. 9 characters
-D - destination callsign (uppercase alphanumeric string) or ALL for broadcast
-C - Channel Access Number (0..15, default - 0)
-n - number of bytes (1 to 798)
-o - output file path/name
-x - binary output (M17 baseband as a packed bitstream)
-r - raw audio output - default (single channel, signed 16-bit LE, +7168 for the +1.0 symbol, 10 samples per symbol)
-s - signed 16-bit LE symbols output

Input data is passed over stdin. Example command:

echo -en "\x05Testing M17 packet mode." | ./m17-packet-encode -S N0CALL -D ALL -C 0 -n 25 -o baseband.rrc

-en parameter for echo suppresses the trailing newline character and enables the use of \ within the message. \x05 sets the packet data content and stands for text message (as per M17 Specification document, chapter 3.2 - Packet Application). If a WAVE file format is required for the baseband, sox can be used:

sox -t raw -r 48000 -b 16 -c 1 -L -e signed-integer baseband.rrc baseband.wav

This line converts .rrc to .wav. SDRangel successfully decoding a packet:

The two characters at the end of the message are probably CRC bytes erroneously decoded by SDRangel as a part of the text message.