Transmitting M17 with rpitx

rpitx is an amazing piece of software by F5OEO that allows you to experiment with transmitting RF using only a Raspberry Pi. rpitx includes many different modes, such as:

FM Carrier (Transmit an unmodulated FM carrier)
Chirp (Sweep a signal through a range of frequencies)
Spectrum (display a picture in the waterfall)
RfMyFace (take a picture from RPi camera, display on waterfall)
FM with RDS (transmit stereo wideband FM with RDS information)
SSB (transmit single-sideband audio)
SSTV (transmit a photo using SSTV)
POCSAG (transmit pager tones)
FreeDV (transmit voice and data using an open-source codec)
Opera (transmit an Opera beacon)

This document will help you set up a Raspberry Pi for rpitx, convert a sound file to M17 baseband, and then transmit it from the Raspberry Pi.

You will need the following:

Raspberry Pi with SSH access - I am using a Raspberry Pi 3B+
- Preferably a Raspberry Pi OS Lite install, there's no need for a full desktop
(Optional) A short piece of wire to act as an antenna
(Optional) A RTL-SDR or other SDR dongle to receive your signal

That's pretty much it!

First, log into your Raspberry Pi via SSH. We will be doing all this work via SSH, due to a few users stating that the HDMI output has a tendency to disappear when transmitting. I have not experienced this myself, due to only working on a headless RPi, but all of the programs are run from the command line anyway, so there's no need for a full Raspberry Pi OS desktop install.

Next, we need to install some tools. We'll start with codec2.

Set Up Your Environment

Get a few tools for getting source code and compiling

sudo apt install git build-essential cmake

Install sox - will need this later for generating baseband audio

sudo apt install sox

Create a working folder for source code

mkdir git && cd git

Compile and Install codec2

git clone https://github.com/drowe67/codec2.git
cd codec2
mkdir build_linux && cd build_linux
cmake ..
make
sudo make install
cd ../..
sudo ldconfig

Compile and Install m17-cxx-demod

sudo apt install libboost-program-options1.67-dev libgtest-dev
git clone https://github.com/mobilinkd/m17-cxx-demod.git
cd m17-cxx-demod
mkdir build && cd build
cmake ..
make
make test
sudo make install
cd ../..

Compile and Install rpitx

This install will take a while, it pulls in many dependencies and self-installs. Get yourself a cup of coffee while you wait.

git clone https://github.com/F5OEO/rpitx
cd rpitx
./install.sh

When finished compiling, you will see this prompt:

In order to run properly, rpitx need to modify /boot/config.txt. Are you sure (y/n)

Be sure to accept (press Y and Enter) in order for the proper boot options to be set. Then, finally reboot the Raspberry Pi

sudo reboot

Cut a wire to a length that's appropriate for the frequency you wish to transmit on. rpitx reportedly can transmit on frequencies from 5 KHz up to 1500 MHz. For example, I used a spare jumper wire with female Dupont connectors on it. As I hold a license for ham radio, I cut the wire for the 70cm band, about 150cm-160cm long. If you do not have a ham radio license, cut the wire for an acceptable ISM band for your location.

CAVEAT

While the Raspberry Pi may not have a lot of transmitting power using rpitx, I strongly suggest staying off any licensed land mobile or broadcast frequencies. Better safe then sorry.

Now that we have all the tools and programs installed, it's time to generate a file that contains M17 baseband audio. We will use m17-mod for this.

From the m17-cxx-demod GitHub README:

m17-cxx-mod
—-
This program reads in an 8k sample per second, 16-bit, 1 channel raw audio stream from STDIN and writes out an M17 4-FSK baseband stream at 48k SPS, 16-bit, 1 channel to STDOUT.

Using this information, you can either create your own audio file or use an existing audio file that fits the requirements. I use Audacity to create files for my use. You could also download a speech sample file from the Open Speech Repository.

Let's create a working directory to use for playing with M17 stuff.

cd
mkdir m17 && cd m17

Place your audio file in the working directory. We will now use m17-mod in order to generate the M17 baseband audio.

sox <filename> -t raw - | m17-mod -S <callsign> > m17baseband.wav

Replace <filename> with the name of the original .wav audio file and <callsign> with your ham radio callsign. If you do not have a ham radio license, you can use something like TEST for the callsign instead.

The command as entered does the following:

Play the original wav file as raw to STDOUT
Pipe the STDOUT from sox into m17-mod
m17-mod attaches the callsign to the M17 metadata
Output the generated M17 baseband to a new .wav file

The process of generating the baseband audio may take a few seconds to a minute or two, depending on the length of the original audio file.

Great, you should now have a new file with the generated baseband audio. Now, it's time to send that through rpitx. Run the following command:

cat m17baseband.wav | csdr convert_i16_f | csdr gain_ff 4200 | csdr convert_f_samplerf 20833 | sudo rpitx -i- -m RF -f <frequency>

Change <frequency> to the frequency you want to transmit at. For example, I chose a frequency in the 70cm ham band of 438.850MHz, which would look like -f 438.850e3 in the command above.

FURTHER EXPLANATION OF COMMANDS NEEDED HERE

This command will take the baseband audio that you generated, do some DSP magic on it, and then transmit it all through GPIO 4 (Pin 7) on the Raspberry Pi.

Congratulations, you've transmitted M17 using only a Raspberry Pi!

Use an RTL-SDR dongle or similar SDR receiver to listen to the baseband audio, or pipe the received baseband into m17-demod to diagnose, test, and listen to your transmission!