Also known as: AMBE
AMBE (Advanced Multi-Band Excitation) is a family of low-bitrate speech vocoders from DVSI, building on the MBE model.1 It is used by D-STAR and EDACS ProVoice, and is the direct ancestor of the more efficient AMBE+2 used by DMR and P25 Phase 2. AMBE improved on the earlier IMBE by delivering comparable quality at lower net bit rates, which mattered as digital systems pushed toward narrower channels and multi-slot TDMA.
How it works
Like every MBE coder, AMBE decomposes each 20 ms frame into a pitch estimate, a set of harmonic spectral magnitudes, and a voiced/unvoiced decision per band. The decoder rebuilds voiced bands as a sum of pitch harmonics and unvoiced bands as shaped noise. What distinguishes AMBE from the older IMBE is a more efficient parameter representation — vector-quantized spectral magnitudes and smarter bit allocation — that reaches similar intelligibility with fewer bits, plus integrated forward error correction that DVSI tuned to the frame’s perceptual sensitivity.
D-STAR’s DV mode, for example, carries a 3600 bps AMBE stream (2400 bps voice + 1200 bps FEC) alongside a low-rate data channel inside its 6.25 kHz-equivalent bandwidth. The exact bit split and FEC scheme vary by system, but the analysis/synthesis core is the same MBE machinery.
Variants
AMBE spans several generations. The original AMBE chips (AMBE-1000/2000) served D-STAR, EDACS/ProVoice, Iridium satphones, and Inmarsat. The refined AMBE+2 (AMBE-3000 series) added better quality at low rate and explicit half-rate modes for DMR, NXDN, and P25 Phase 2. All are proprietary to DVSI and share the MBE analysis model but are not bit-compatible with one another or with IMBE.
In practice
Because AMBE is patent-encumbered, commercial radios embedded DVSI’s DSP chips and early computer decoders needed a hardware “dongle.” Independent software reimplementations exist and are what let a general-purpose SDR render AMBE voice today. The trade AMBE makes — proprietary, licensed, but very efficient — is exactly what motivated the open Codec 2 alternative adopted by M17.
Relevance to SDR
GopherTrunk implements AMBE-family decoding in pure Go to render clear digital voice without proprietary hardware. It applies where D-STAR and ProVoice appear, and the same codebase covers the closely related AMBE+2 used by the mainstream DMR/NXDN/P25 Phase 2 systems. As always, encryption sits above the vocoder: GopherTrunk can follow an encrypted AMBE call but cannot resynthesize its scrambled frames without the key.
Sources
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Multi-Band Excitation — Wikipedia, on the MBE vocoder family that includes AMBE. ↩