Field Guide · technology

Also known as: TWELP

TWELP (Time-Warped-Excitation Linear Prediction / Waveform Interpolation) is a family of very-low-bitrate speech vocoders marketed by DSP Group as a competitor to DVSI’s AMBE codecs for digital land-mobile radio.1 It targets the same 2000–4000 bps envelope that IMBE and AMBE+2 occupy, promising comparable intelligibility and noise robustness at those rates while offering an alternative licensing route for radio manufacturers who did not want to depend on a single vocoder supplier.

speech LP model + warped excitation ~2.4 kbps rebuilt
TWELP models each frame as a spectral envelope plus a warped characteristic waveform, coded at a few kbps.

How it works

TWELP is rooted in waveform-interpolation (also called prototype-waveform interpolation) speech modelling combined with linear prediction. Rather than splitting the spectrum into voiced and unvoiced bands the way the multi-band-excitation family does, a waveform-interpolation coder extracts a characteristic excitation waveform once per pitch period, decomposes it into slowly and rapidly evolving components (capturing the voiced and noise-like parts of the sound), and transmits those along with a linear-prediction spectral envelope and pitch track. At the decoder the successive prototype waveforms are interpolated over time to regenerate a continuous excitation, which drives the synthesis filter to reconstruct speech.

Because only a compact set of parameters — envelope, pitch, gain and the evolving excitation description — is sent per frame, TWELP reaches rates around 2400 bps (and lower variants) while remaining reasonably robust in the noisy, mobile environments that radio must survive. Vendors published TWELP profiles aimed specifically at professional and public-safety radio, with error-tolerance and tandem-friendly behaviour comparable to AMBE-class coders.

In practice

TWELP’s practical appeal was as a second source. DVSI’s AMBE and AMBE+2 are the mandated vocoders in several standards, and their licensing and hardware-DSP ecosystem locked manufacturers into one supplier. A codec covering the same rate and quality band gave designers of non-standardised or proprietary radios a way to build low-rate digital voice without that dependency, in the same spirit that the open Codec 2 does for the amateur community — though TWELP itself is a commercial, licensed product rather than an open one.

Relevance to SDR

TWELP sits in the same problem space GopherTrunk cares about — squeezing intelligible voice into a few kbps of a narrowband radio channel — but it is not the vocoder used by the mainstream trunked standards GopherTrunk decodes. P25, DMR and NXDN standardise on the MBE family (IMBE and AMBE+2), so a GopherTrunk receiver following those air interfaces meets AMBE-class bitstreams, not TWELP. TWELP mainly appears in specific vendor products and some non-standard digital radios. GopherTrunk does not implement TWELP decoding; understanding it is useful mostly as context for why the AMBE-versus-alternatives licensing question shaped the digital-voice radio market.

Sources

  1. Vocoder — Wikipedia, background on low-bitrate speech vocoders of the class TWELP belongs to; see also Speech coding. ↩

See also