Interactive · DMR
DMR fits two simultaneous QSOs into a single 12.5 kHz channel by taking turns — so fast that neither operator can tell. Start the clock and watch the channel being shared. Then look at what is actually inside one of those bursts.
| What the 30 ms slot actually carries | Bits | Of the slot | Rate |
|---|---|---|---|
| Voice parameters — 3 × 49 bit AMBE+2 frames | 147 | 51.0% | 2,450 bps |
| Vocoder FEC — 3 × 23 bits | 69 | 24.0% | 1,150 bps |
| Sync word or embedded signalling | 48 | 16.7% | 1,600 bps |
| CACH (outbound) or guard time (inbound) | 24 | 8.3% | 800 bps |
| Total | 288 | 100% | 9,600 bps |
Sources, because these are numbers and not physics. The frame structure is taken from ETSI TS 102 361-1, the standard that defines DMR: a timeslot is 30 ms and numbered 1 or 2; a voice burst is 264 bits (27.5 ms) including a 48 bit synchronisation word; the remaining 2.5 ms is guard time on the inbound channel and carries the CACH — a 24 bit burst — on the outbound; a frame is 60 ms. The vocoder figures — AMBE+2 at 2,450 bps of voice plus 1,150 bps of FEC for 3,600 bps total, a 49 bit frame every 20 ms encoded into 72 bits, three of them per burst for 216 bits of voice — are corroborated by DVSI (who make the codec), the ETSI vocoder socket definition, and several independent open-source implementations and teardowns. The arithmetic closes: 147 + 69 + 48 + 24 = 288 bit-times, and 288 bits in 30 ms is exactly 9,600 bps. The animation speed is not real time — a real DMR frame is over and done in 60 milliseconds, which is precisely why no operator has ever heard it happen. 73 de N6JET