Interactive · Vocoders

Three Families

Every digital voice mode on the air is running one of three vocoder lineages — and they are not interchangeable. Pick a mode and see what it is really spending its bits on, who owns the algorithm, and how a 1990s codec that needed 7,200 bps got replaced by one that does the job in half that.

Every figure here was checked before it was drawn. IMBE (P25 Phase 1): 7,200 bps; 144 bits per 20 ms frame, divided into 87 MBE parameter bits, 56 FEC bits (Golay and Hamming) and 1 synchronisation bit — from the APCO Project 25 Vocoder Description as quoted in DVSI's own patent filings, and corroborated by DVSI's published vocoder papers. AMBE+ (D‑STAR): 2,400 bps speech + 1,200 bps FEC = 3,600 bps — this is a line in DVSI's own AMBE‑2020 chip manual rate table, and is corroborated independently by KB9MWR, a TAPR DCC paper, and D‑STAR Info. AMBE+2 half‑rate (DMR, P25 Phase 2): 2,450 bps speech + 1,150 bps FEC = 3,600 bps; 49 speech bits per 20 ms encoded into 72 — confirmed by DVSI, an ETSI vocoder-socket definition, and several open-source teardowns. DVSI describe it as a 2‑for‑1 capacity improvement over the 7,200 bps full‑rate IMBE. Codec 2 (M17): 3,200 bps full‑rate — from the official M17 protocol specification, which also gives the 4FSK physical layer at 4,800 symbols/s and 384‑bit frames every 40 ms. Codec 2's error correction lives in the M17 protocol (Golay, punctured convolutional coding, interleaving) rather than inside the codec, which is why its FEC bar is empty here — the protection is real, it is just accounted for elsewhere. Deliberately absent: System Fusion and NXDN also sit in the AMBE family, but I did not verify their exact bit splits to the same standard, so they are not shown rather than guessed at. 73 de N6JET