Put a DMR operator and a D-STAR operator on the same reflector and, left to themselves, they'll hear each other as noise. Their radios digitize speech with different, mutually unintelligible vocoders. To let them talk, something in the middle has to take the audio apart and rebuild it in the other mode's language. That's transcoding — and thanks to the licensing story behind AMBE, it's the reason a lot of reflectors have little USB sticks plugged into them. (If you haven't read AMBE and Codec2 yet, start there — it's the why behind everything below.)
The tempting shortcut is to imagine a gateway that simply relabels the packets — take a DMR stream, stamp it “D-STAR,” and send it on. It cannot work. The bits coming off a DMR radio are AMBE+2 parameters; a D-STAR radio expects the older AMBE generation; an M17 radio expects Codec2. These aren't dialects of one language, they're separate codebooks. Even the two AMBE modes aren't bit-compatible with each other — they're different vocoder generations describing the vocal tract in incompatible ways. Feed one mode's compressed stream into another mode's decoder and you get gibberish. The only thing every mode agrees on is uncompressed audio.
So transcoding takes the long way around, on purpose. It fully decodes the incoming stream all the way back to plain digital audio — ordinary PCM samples, the same uncompressed representation your sound card uses — and then re-encodes that audio into the outgoing mode's vocoder. Uncompressed audio is the neutral hub language: every mode can be reached from it, and every mode can be decoded down to it. Decode to PCM, re-encode to the target. Do it fast enough that nobody notices the round trip, and the DMR user and the D-STAR user never know there was a translator standing between them.
Here the AMBE licensing wall from the previous page bites. To decode or encode AMBE, you need DVSI's vocoder — in practice, the chip. So any transcoder that touches an AMBE mode needs AMBE hardware: a dongle such as the ThumbDV or the DVStick 30 (built on the single-channel AMBE3000) or the DVStick 33 (built on the AMBE3003, a three-channel chip that transcodes three streams at once). Codec2, being open, needs no hardware at all — the M17 and analog sides run purely in software. So the shape of a bridge depends on what it connects: a DMR-to-M17 bridge is one hardware AMBE decode plus one software Codec2 encode, while a DMR-to-D-STAR bridge is one AMBE decode and one AMBE re-encode, both riding on the chip.
Transcoding tends to live in one of two homes. On the reflector: a multimode server such as urfd can host a transcoder module that gives every connected mode a common meeting point — decode everyone down to PCM, re-encode for each listener — so a call arriving on one mode goes back out correctly shaped on all the others. On a bridge: the DVSwitch suite — Analog_Bridge and MMDVM_Bridge, fed by AMBEserver talking to the dongle — stitches one mode to another between otherwise separate systems. Same principle either way; the difference is whether the translator sits at a hub everyone joins or on a link between two networks.
On this network
URFJET's transcoded modules do exactly this. A pair of AMBE3003 dongles on the transcoder host decode and re-encode among D-STAR, DMR, System Fusion, and the rest, so a call entering on any one mode leaves correctly shaped on all of them — a working example of the decode-to-PCM-and-back principle running full time.
Transcoding is not free of consequence: every decode-and-re-encode is a fresh lossy pass through a vocoder, so a heavily transcoded path sounds slightly rougher than a native one. That's the price of admission for letting incompatible modes share a conversation, and for nearly everyone it's a bargain — a touch of vocoder texture in exchange for a DMR handheld and a D-STAR handheld and an M17 radio all sitting in the same net.
The big idea
Transcoding never translates compressed bits directly. It decodes all the way down to plain audio — the one language every mode shares — then re-encodes for the destination. Because AMBE lives behind a licensed chip, any transcode touching an AMBE mode needs a hardware dongle; Codec2 rides along in software. That single fact shapes the plumbing of every multimode reflector in the hobby.
A noncommercial hobby reference compiled by N6JET, gathered from public sources and shared freely for anyone interested in amateur digital voice.