A Multimode USRP Fanout Bridge is a way to tie several different digital voice modes into a single shared conversation. It's a companion piece to the XLX and URF explainers — but it works on a completely different principle. Where a reflector is one server with rooms inside it, this is a bridge that joins several separate reflectors and networks together, so a transmission on any one of them is heard on all the others. The name describes exactly what it does: it links many modes (multimode), carries their audio over a common transport (USRP), distributes that audio one-to-all (fanout), and joins otherwise separate systems (bridge).
The bridge connects two or more independent digital voice systems — each running its own mode — into a single linked group. Key up on any mode, and everyone on the others hears you. It scales naturally: a two-leg bridge links two modes, a six-leg bridge links six, and the design is the same either way.
This is a different idea from an XLX or URF reflector. Those are single servers with internal "module" rooms; you pick a room and talk to whoever else is in it. A fanout bridge has no rooms of its own. Instead, it reaches out to several different reflectors and networks — one for each mode — and stitches them together behind the scenes.
Here's the clever part, and the thing that lets the bridge work without a wall of transcoding hardware. All of its legs meet in the middle as plain digital audio — the uncompressed audio-over-IP format (USRP) that AllStar and the DVSwitch tools use. At the center sits a small program called a fanout.
The common-ground trick
Each leg's job is to convert between its own mode and plain audio. When someone transmits, their audio arrives at the fanout as ordinary uncompressed sound — no longer "DMR" or "D-Star," just audio. The fanout then copies that audio out to every other leg, and each leg re-encodes it into its own mode. The meeting point is uncompressed audio, not any single codec.
That's the heart of the design. A reflector like XLX has to transcode directly from one codec to another (D-Star's AMBE to DMR's AMBE+2, for example), which is what forces it to buy hardware vocoders. A fanout bridge sidesteps that by never converting one codec straight into another — every mode only ever converts to and from plain audio at its own leg. The fanout in the middle just distributes that audio to everyone. Add a leg or remove one and nothing else changes; the hub simply has one more or one fewer destination to copy to.
The N6JET bridge is a six-leg example, joining the full spread of amateur digital voice. Each mode connects through its own leg — a link to a reflector or network that already serves that mode:
| Leg | Mode | Connects to |
|---|---|---|
| 1 | DMR | TGIF talkgroup 95110 |
| 2 | YSF (Fusion) | URFJET, reflector 95111 |
| 3 | NXDN | Reflector 9511 |
| 4 | P25 | Reflector 9511 |
| 5 | M17 | M17-JET, Module Q |
| 6 | D-Star | XLXJET, Module Q |
From a user's seat, none of this plumbing is visible. A D-Star operator, a Fusion operator, a DMR operator on TGIF 95110, and the M17, P25, and NXDN operators are all simply in the same conversation. Someone transmitting on any one mode is heard, in real time, by everyone on the other five. The lobby behaves like one big room that happens to be reachable from six different kinds of radio.
A fanout bridge is assembled from open-source building blocks rather than a single piece of reflector software. Each leg uses DVSwitch bridge components (Analog_Bridge and MMDVM_Bridge) or a mode-specific bridge such as USRP2M17 for an M17 leg, all feeding a central usrp-fanout program that does the audio distribution. The bridge runs independently of the reflectors it links to — separate software, separate ports — so the bridge and the reflectors never interfere with one another.
Credits
The foundations here are other people's work. Meeting several digital modes in plain USRP audio is what the DVSwitch suite was built to do. The mode legs are DVSwitch's Analog_Bridge and MMDVM_Bridge, by Steve Zingman (N4IRS) and Mike Zingman (N4IRR), with MMDVM_Bridge built on Jonathan Naylor's (G4KLX) MMDVM; the M17 leg uses USRP2M17 from the nostar / N7TAE project. The central relay that copies audio across the six legs is a small custom script written for this setup; the reflectors and networks each leg connects to are separate community systems. See github.com/DVSwitch and github.com/nostar/MMDVM_CM.
XLX and URF are reflectors: one server, internal rooms, codec-to-codec transcoding. A Multimode USRP Fanout Bridge is a bridge: several independent reflectors and networks, joined through a central audio hub where everything meets as plain sound. It's a different answer to the same goal that runs through all of these projects — letting people on different digital modes talk to one another as if the mode never mattered — and because it scales from two legs to many, the same design serves a small two-mode link or a full six-mode lobby.
Built on DVSwitch (Steve Zingman N4IRS & Mike Zingman N4IRR), G4KLX's MMDVM, and USRP2M17 (nostar / N7TAE); see Credits above. A noncommercial hobby project, shared freely for anyone interested in amateur digital voice.