M17 is the newcomer, and the only one of the bunch built by hams, for hams, with nothing proprietary inside. Every other digital voice mode leans on a patented vocoder chip; M17 throws that out and uses an open codec instead. The result is a fully open, royalty-free digital voice mode you can study, build, and modify from end to end. This is a plain-language look at how it works on the air and what makes it different.
M17 began around 2019 as a community project, started by Wojciech Kaczmarski (SP5WWP) and grown by an open group of contributors. Its founding goal was independence: a digital voice mode with no proprietary parts — open protocol, open hardware designs, and most importantly an open voice codec — so that no license fee or closed chip stands between a ham and the mode. It is the clean-sheet, free-software answer to a landscape built on commercial standards.
M17's defining choice is its vocoder. Where DMR, Fusion, NXDN and D-STAR all use a flavor of the proprietary, licensed AMBE codec — which requires either a licensed hardware chip or paid software — M17 uses Codec 2, an open-source voice codec created by David Rowe (VK5DGR). (P25 is the partial exception: its IMBE codec is proprietary too, but the patent has expired, so it can be implemented in free software.) Codec 2 is free to use, free to inspect, and runs in plain software on an ordinary microcontroller. That single decision is what makes M17 fully open.
On the air, M17 transmits with 4-level FSK at 9600 bps, occupying roughly 9 kHz — comfortably inside a 12.5 kHz channel. Because its codec is different from AMBE, linking M17 to the AMBE modes requires transcoding, the same decode-and-re-encode step D-STAR needs to reach the AMBE+2 family.
M17's channel-access code is the CAN — Channel Access Number, a value from 0 to 15. Like a color code or PL tone, it lets groups share a frequency while ignoring each other's traffic. CAN sits alongside Color Code, RAN, DG-ID, and NAC in the access-codes guide.
M17 addresses traffic by callsign, like D-STAR — but it does it efficiently. Each callsign is packed into a compact numeric form (a base-40 encoding) that rides inside every frame, so the source and destination travel with the audio and no central ID registry is needed. A destination can be another callsign, a broadcast, or a reflector with a module letter.
M17 links over the internet through reflectors run by the mrefd daemon, each divided into modules labeled A–Z that act as separate rooms. Hotspots and repeaters connect to a reflector and join a module, exactly the way they do on the other modes. M17 support in hotspot software has been unsettled since July 2025, when MMDVM dropped the mode: Pi-Star V4.2.3_18 still carries M17, WPSD no longer does, and the community maintains a WPSD-M17 fork alongside M17-native packages such as MSpot.
M17 is young and growing, carried along by the open-source spirit that created it. There is open hardware (the Module17 board and radios running the OpenRTX firmware), open software, and an active community refining the protocol in the open. It is the smallest of the modes by user count today, but it is the one you can take apart completely — which, for experimenters and software-defined-radio enthusiasts, is the entire appeal.
Beyond ideology, openness has practical consequences. There is no royalty buried in every radio, no closed chip to source, and nothing hidden about how the mode works — so anyone can write a new implementation, fix a bug, or build a feature without asking permission. M17 also carries metadata and supports optional features in a transparent, documented way. For a hobby built on experimentation, a mode you are free to modify is a natural fit.
| M17 | |
|---|---|
| Origin | Open community project (amateur) |
| Modulation | 4-level FSK (~9 kHz, 9600 bps) |
| Vocoder | Codec 2 (open, royalty-free) |
| Access code | CAN (0–15) |
| Addressing | Callsign (base-40 encoded) |
| Bridging | Needs transcoding to reach AMBE modes |
M17 is the open-source digital voice mode: a 4-level FSK signal carrying open Codec 2 voice, gated by a CAN, addressed by callsign, and linked through mrefd reflectors and modules. It gives up nothing technically, and its one trade — needing transcoding to reach the AMBE world — is the price of being free of proprietary parts. For hams who value being able to see and change everything inside the mode they run, M17 is in a class of its own.