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 mode:

• 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) validation,
• callsign decoder

It also supports packet mode (TX only for now).

There's no support for any encryption yet.

Building

Simply cd to the directory of interest and make

Capabilities

Two 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.

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.