Quickstart
modem73 is a free, open source software modem. It has configs for any setup, from line of sight UHF links over FM to HF propagation. This guide gets you running in under 5 minutes.
modem73 is a free, open source software modem. It has configs for any setup, from line of sight UHF links over FM to HF propagation. This guide gets you running in under 5 minutes.
Step 1
The first step is to download either the source or compiled binary for your system from modem73.app.
If you're building from source, follow along with the instructions in the README or run install.sh if you are on a Debian or Arch based system.
For a precompiled build, find the release that matches your OS and architecture:
arm64Install the package with apt or your system's package manager:
sudo apt install ./modem73_<version>_<arch>.deb
Grab the latest build from the modem73-win releases. The Windows build is a separate fork, ported to Win32 APIs and PDCurses with LLM assisted tooling, so treat it as experimental. If you do not need serial PTT, running the upstream build under WSL is the more stable path.
To build it yourself, everything is vendored under deps/ (aicodix DSP, PDCurses, hidapi, miniaudio, cJSON) and the exe is statically linked, so there is nothing external to install at build or run time. CM108 USB PTT is always enabled.
Native on Windows, from a MinGW64 shell after installing MSYS2:
pacman -S --needed make mingw-w64-x86_64-gcc make CXX=g++ CC=gcc
Or cross compile from Linux:
# Debian/Ubuntu
sudo apt install make g++-mingw-w64-x86-64-posix
make
Either way you end up with modem73.exe.
Step 2
Once installed, you can launch modem73 in any terminal.
modem73
By default it will start in UI mode where you can configure all of the settings.
To navigate the program you can click with the mouse or use the shortcut keys:
On Windows you will want to call the .exe instead:
# Start in UI mode modem73.exe # Start in headless mode modem73.exe --headless # See all options modem73.exe --help
Step 3
The first tab you will want to head over to is CONFIG. Under config, you'll want to configure your:
Input is what the modem decodes. Set this to whatever your radio or sound interface shows up as on the output side.
Output goes the other way. Set this to your radio's input.
Most interfaces present both under the same name. An all in one audio cable shows up as AIOC, and similarly HF rigs like the ICOM-7300 that have internal soundcards will have their own name.
Push to talk is what keys up the radio to transmit. There are five options:
| Option | Use case |
|---|---|
none | Over the air (speaker/mic) or PTT handled elsewhere |
rigctl | HF rigs with CAT control |
VOX | Radios keyed by an audio tone |
COM (serial) | Serial RTS/DTR keying, including the AIOC and Digirig |
CM108 | GPIO keying on CM108 based sound interfaces |
COM (serial) is the straightforward path for HT interfaces. Point it at the serial device your cable enumerates as and pick RTS or DTR to match the cable.
rigctl comes from the Hamlib library. Install it with the libhamlib-utils package or your distro's equivalent:
sudo apt install libhamlib-utils
modem73 connects to rigctl over TCP, so start rigctld with your rig's options first. For an IC-7300:
rigctld -m 3073 -r /dev/ttyUSB0 -s 115200
Run rigctl -l to find the model number for your radio. rigctld runs in the background, listens on port 4532, and gives you both PTT and control of the rig from modem73 itself, including ALC tuning under the RIG tab when rigctl PTT is selected.
Hamlib ships the rigctld.exe binary. Download it from the Hamlib 4.7.2 release, then add its bin folder to your PATH so you can start rigctld from any terminal.
An example that lets modem73 key up and change frequency on an ICOM-7300:
rigctld.exe -r COM3 -m 3073 -s 19200
Replace COM3 with whatever your radio actually enumerates as. -m is the model number, and rigctl.exe -l lists them all.
To check the connection before pointing modem73 at it, drop the d and run rigctl.exe on its own, then type f to read back the current frequency.
Serial PTT takes a COM port rather than a device path. Check Device Manager for the number, and expect it to change after a reboot or a replug.
modem73.exe --ptt com --com-port COM4 --com-line rts
For the All In One Cable, set PTT to COM, pick your COM port, set PTT line to BOTH, and set invert to NORMAL. Note that this differs from Linux, where the AIOC wants invert set to INVERT RTS.
CM108 keying works the same as it does everywhere else:
modem73.exe --ptt cm108 --cm108-gpio 3
Restart modem73 after changing audio or PTT settings.
If audio is working you will see it coming in on the waterfall. To confirm the transmit side, go to UTILS and hit Send random data, or press 2. Your radio should key up and transmit.
Step 4
modem73 has three families of data modems, and all of them decode simultaneously. Between them they cover poor HF propagation through to clean line of sight FM links.
Three things matter on this screen: the modem itself, which you change with the left and right arrow keys, its settings, and the info panel on the right, which updates to show the details of whatever mode and options you have selected.
Use OFDM for anything over FM, and for stable HF SSB links keep modulation under 8PSK.
Modulation runs from BPSK up to QAM4096. Each step up the ladder buys throughput and costs you required SNR.
Coding rate is the balance between data and redundancy. 1/4 carries the most redundancy, 5/6 the least. On a link with a high noise floor or fading, take the lower rate and give up the throughput.
Frame size is how many bytes go out in one transmission: short, normal, or long frames. It also sets how long the modem spends on air.
Postamble adds a second sync marker at the end of the frame. Worth turning on for high noise, busy channels, or HF fading. It costs 0.4 seconds of extra transmit time.
The RDM modes are built for daily HF use and run from RDM-1200 down to RDM-300.
| Mode | Decodes down to | Survives |
|---|---|---|
| RDM-1200 | 5 dB SNR | 0.5 second of deep fading |
| RDM-600 | 1 dB SNR | 2 seconds of deep fading |
These are the modes for long distance HF DXing and NVIS.
MFSK is non-coherent, so it decodes without a preamble or sync. Modes run from 32R down to 8.
Keep these around as your weak signal backup.
Every mode decodes at once, which costs CPU. On a Pi Zero 2 or anything else with limited headroom, disable the ones you aren't using under the decoders section.
Step 5
Carrier-sense multiple access (CSMA) simply means listening before we transmit. When CSMA is enabled, modem73 can work concurrently with multiple stations at once in the same shared channel, unlike other software like VARA.
modem73 implements several CSMA strategies that can be configured in the config. They are Threshold, Sync, and Ranked.
Threshold means simply listening for the level of audio coming in. This is useful for quiet FM links where we know our signal will always be louder than the noise floor.
Sync waits and listens for a frame coming in. Instead of measuring how loud the audio is, it looks for the start of a real modem73 signal. This matters on HF, where the noise floor moves around all day. With Threshold on a noisy band you can end up waiting forever for a quiet that never comes, because the band itself is never quiet. Sync only waits for actual stations.
Sync still picks a random wait before it transmits. That wait is called the contention window, and its size is worked out from how many stations we have heard recently. With two stations on frequency the window stays short. With six or more it opens up, because more stations need more room to avoid landing on the same moment. Picking at random is simple and it always works, but it is a gamble. Two stations can draw the same moment, and a lost frame is expensive.
Ranked builds on Sync and takes the guessing out. Every transmission already starts with a short signature tone, and that tone carries a 16 bit station ID. Every station listening keeps a list of the IDs it has heard. Each one sorts that list the same way, so all of them work out the same order without sending anything extra. Your turn is a time slot, your position out of the stations on frequency. Whoever transmitted last moves to the back of the line, so nobody takes two turns while someone else is still waiting.
Because the order is worked out instead of drawn at random, stations that can hear each other stop colliding. Two stations passing traffic back and forth settle into clean alternation, and the reply comes back at the start of the next slot rather than after a random wait. On a channel with several users this is where most of the speed comes from.
Ranked has two requirements. Every station needs to be running it, with Lead Tone and the presence tone that goes out every 45 to 90 seconds while a station is idle, or the order will not form. Selecting Ranked turns both on.
And every station needs to hear every other one. Ranked does not solve the hidden station problem. Good fits for Ranked are an HF NVIS net in one region, VHF simplex, or everyone on a shared repeater.
Step 6
Fragmentation handles incoming packets that exceed your frame size by chunking them automatically.
TX blanking stops you from hearing your own packets.
Turn Fragmentation ON when another external application like Winlink is sending out PACLEN frames that exceed the modems frame size or your incoming frame size is unknown. It is ideal to have fragmentation OFF, but enable it where neccessary.
Settings, presets, and the performance log are stored in %APPDATA%\modem73\.
If you are running modem73interface for Reticulum, the interface file goes in %USERPROFILE%\.reticulum\interfaces\.
Step 7
modem73 exposes two TCP ports. Applications send and receive data on the KISS port, and read or change the modem's settings on the control port.
Standard KISS framing on TCP 127.0.0.1:8001. Point any KISS capable application at it and the bytes you write go out over the air, while decoded frames come back to every connected client.
modem73 --port 8001
A JSON protocol on TCP 127.0.0.1:8073, each message prefixed with its length as a 4 byte big endian integer. Use it to read the current mode, SNR and channel state, change the modem settings, or pass a command straight through to rigctl. The control port is very useful for application developers who want to make the most use of MODEM73.
modem73 --control-port 8073
Send {"cmd": "get_config"} to read the current settings, including payload_size, which tells you the largest frame the current mode can carry. {"cmd": "set_config", "robust_mode": 6} changes the mode on the fly, and the modem pushes an event to every connected client whenever anything changes.
Full command list in CONTROL_PORT.md.
modem73interface is a Reticulum interface that reads the frame size from the control port and switches frame sizes to match the traffic.
Step 8
Signal to noise ratio. This is how far your signal sits above the noise floor in dB. A higher SNR is always better and combined with bit error rate determines link quality.
The total percentage of raw bit errors before forward error correction. modem73 works it out by re-encoding the frame once it decodes, then counting how many of the received bits disagreed. A frame can come through perfectly clean at 15% BER, because fixing those errors is the entire job of the FEC. What BER tells you is how much margin is left before frames start failing.
How many bytes go out in one transmission. OFDM frames run from 256 to 6144 bytes depending on modulation and code rate (SHORT, NORMAL or LONG), ROBUST frames are 510, 170 or speciality modes like RDM-QB, and the info panel shows the exact number for the mode you have picked.
A bigger frame wastes less time on sync and overhead but spends longer on air, and one deep fade, or collision, can take the whole frame with it. Longer frames are less of an issue on line of sight FM.
Packets bigger than the frame are split up by fragmentation when enabled.
What carrier, number of carriers, and how many bits we send at once. Higher carriers (like QAM4096) require a better signal, or SNR. Lower modulation orders like BPSK require a lot less.
modem73 has 3 modes: OFDM, ROBUST, or MFSK.
OFDM The fast family. Hundreds of carriers side by side in 2400 Hz, each carrying a PSK or QAM symbol. From about 790 bps at BPSK to over 13 kbps at QAM4096. Use it for anything over FM, and on good HF SSB paths at 8PSK or below.
ROBUST Built for fading HF such as 40 and 80 meter NVIS. QPSK on widely spaced carriers with a guard interval between symbols, so Doppler spread and multipath echoes do not smear one symbol into the next. RDM-1200 (about 1150 bps) decodes down to 5 dB SNR and RDM-600 near 0 dB. The RDMN modes are 600 Hz wide versions, RDMN-300 and RDMN-150, for narrow filters and crowded bands.
MFSK One tone at a time out of 8, 16 or 32. The receiver only has to find the loudest tone, with no phase tracking, which is why it decodes below the noise floor (MFSK-8 to about -9 dB) and why it is slow: 34 bps for MFSK-8, 99 bps for MFSK-32R. Keep it as the weak signal backup.
Start with the lowest mode first. Then, go up. Don't pick something like QAM256 right out of the box. Check your SNR and BER and step up one notch at a time while frames keep decoding. BER is the number to watch.
Low means you have margin to go faster, climbing means you are near the edge and the next step up will start dropping frames. SNR tells you roughly where you will land. modem73 shows it green above 10 dB and yellow between 5 and 10, and the higher modulations need the green. When frames start failing, step back down one notch and stay there.
If you're on HF, start with the ROBUST modes. RDM-600 while the band is fading, RDM-1200 once it decodes cleanly with SNR over 5 dB. Only move to OFDM when the path is steady, and keep it at 8PSK or below. If nothing decodes at all, drop to MFSK.
Remember that two stations can use different settings, and as long as they have the RX decoder on, can hear you. This enables setups with asymmetric conditions
You should always make sure your audio input and output are tuned properly. Use the TX level and check to see if your packets are distorted or overmodulated. If you use Hamlib or Rigctl PTT, there is an auto-ALC tune feature under RIG.