Provides very high speech quality - better than GSM AMR 4.75 kbps and close to GSM AMR 5.15 kbps that operate on much higher bit rates.

For Digital HF Radio, Digital Mobile Radio (DMR) and other markets.

TWELP Technology Features. The vocoder is based on newest technology of speech coding called "Tri-Wave Excited Linear Prediction" (TWELP) that was developed by experts of DSPINI. 

TWELP technology is a new class of vocoders that differs from any other LPC-based vocoders by:

  • advance reliable method of pitch estimation
  • pitch-synchronous analysis
  • advance tri-wave model of excitation
  • newest quantization schemes
  • pitch-synchronous synthesis

Thanks to these unique features, TWELP technology provides much better speech quality in comparison with any well-known technologies, including AMBE+2, MELPe, ACELP, etc. on the same bit rate in range from 300 bps up to 9600 bps and more. Moreover, in contrast to other LBR vocoders (like MELPe, etc.) TWELP provides much better quality for non-speech signals like sirens, background music, etc.

Superiority In Speech Quality. Here is a comparison with GSM AMR vocoders, working on much higher bit rates, in noiseless channel. TWELP 3600 bps vocoder, GSM AMR 4.75 kbps and 5.15 kbps vocoders were tested, using ITU-T P.50 speech base for 20 different languages. ITU-T P.862 utility was used for estimation of the speech quality in PESQ terms:

A diagram demonstrates a superiority of the TWELP 3600 bps over AMR 4750 bps and a closeness (and even superiority for a few languages) to AMR 5150 bps vocoder of the speech quality in clear channel. Exact numbers are presented in the table below.
LanguageAMR 5150TWELP 3600AMR 4750
American 3.395 3.414 3.351
Arabic 3.334 3.322 3.277
British 3.363 3.283 3.272
Chinese 3.293 3.377 3.267
Danish 3.345 3.366 3.311
Dutch 3.172 3.181 3.089
Finnich 3.208 3.132 3.166
French 3.358 3.395 3.277
German 3.385 3.264 3.321
Greek 3.269 3.344 3.206
Hindi 3.348 3.349 3.286
Hungarian 3.410 3.344 3.305
Italian 3.522 3.442 3.462
Japanese 3.441 3.467 3.369
Norwegian 3.344 3.350 3.267
Polish 3.333 3.343 3.263
Portuguese 3.421 3.447 3.377
Russian 3.260 3.276 3.186
Spanish 3.392 3.381 3.343
Swedish 3.449 3.338 3.391

Superiority of the TWELP 3600 bps vocoder over AMR 4750 bps is on average 0.052 PESQ
and just -0.011 PESQ in comparison with AMR 5150 bps

Speech Samples (WAV-files). A few independent experts listened TWELP 3600 bps vocoder in comparison with GSM AMR 4750 bps vocoder and GSM AMR 5150 bps vocoder, using method of preferences. A majority of the experts preferred TWELP to the AMR 4750, having noted much more clear sounding of voice in the TWELP vocoder. Also, a majority of the experts haven't given preferences to AMR 5150 or TWELP 3600 vocoder.
You can play and listen short samples of the source speech as well as the speech processed by these vocoders for any of 20 languages, using links in the table below.
Also, you can download full set of the P.50 samples as zip-files for all languages simultaneously, using the links in the "Downloads" para in a bottom of the page.

LanguageSource speechAMR 4750 bpsTWELP 3600 bpsAMR 5150 bps

Superiority In Quality Of The Non-speech Signals. In contrast to other LBR vocoders (MELPe, AMBE+2, etc.), TWELP vocoders provide high quality of non-speech signals, including police, ambulance, fire sirens, etc. This feature in conjunction with high quality natural human-sounding of voice makes TWELP vocoders well suitable for replacement of analog radio by digital radio and also for other applications where high quality transmitting of non-speech signals is relevant along with high quality transmitting of speech signals.

Source typeSource signalAMR 4750 bpsTWELP 3600 bps
Siren only
With voice

High Robustness To Acoustic Noise. In contrast to other LBR vocoders, TWELP vocoders are well robust to acoustic noise thanks to robust reliable method of pitch estimation and other features of TWELP technology.

Moreover, vocoder includes in-built Noise Cancellation—Speech Enhancement (NCSE) functionality that improves speech quality in noisy acoustic environment.

NCSE ModeSource signalAMR 4750 bpsTWELP 3600 bps

High Robustness To The Channel Errors. The diagram and table below show a dependence of the averaged speech quality for AWGN-noisy channel for different BER in comparison with other vocoders.

Special "robust" versions of the TWELP vocoders include FEC that are integrated with vocoder on base of "joint source-channel coding" approach that provides high speech quality simultaneously in noisy channel as well as in noiseless channel. FEC can operate with "soft decisions" as well as with "hard decisions" from a modem. "Soft decisions" mode provides much better robustness in comparison with the "hard decisions" mode.

BER %TWELP 3600TWELP 3600 RobustTWELP 4800 Robust
0.00 3.341 3.244 3.341
0.10 3.216 3.243 3.330
0.20 3.132 3.242 3.329
0.30 3.056 3.240 3.325
0.40 2.976 3.239 3.322
0.50 2.929 3.238 3.317
0.60 2.845 3.237 3.312
0.70 2.790 3.236 3.307
0.80 2.732 3.234 3.302
0.90 2.679 3.233 3.297
1.00 2.622 3.232 3.292

Additional Functionalities. The following additional functionalities are developed by DSPINI and integrated into TWELP vocoders:

  • Automatic Gain Control (AGC),
  • Noise Cancellation for Speech Enhancement (NCSE)
  • Voice Activity Detector (VAD),
  • Tone Detection/Generation (Single tones and Dual tones). The tones are transmitted by the vocoder facilities.

Each functionality has unique features, performance and characteristics, providing significant superiority over any well-known implementations on the market.

Technical Characteristics And Resource Requirements:

Technical characteristics
Bit Rate
AlgorithmFrame size
Algorithmic delay
(including frame size)
Sampling rate
Signal formatBit stream format
3600 TWELP 20 40 8 Linear
Additional functionalities
NameFunctionalityTechnical characteristics
AGC Automatic Gain 
Control range: 0 ... +20 dB
NCSE Noise Canceller -
Speech Enhancer
SNR increasing > 6 dB
Speech quality
> 0.1 PESQ
Single/Dual tones 
In accordance with international standards
Single/Dual tones 
Special generator, kept continuity of signal 
(phase and amplitude of signal of previous frame)
VAD Voice Activity 
Reliable detection speech 
in background noise
CNG Comfort Noise 
Type of noise "white"
Level - 60 dB

Resources for ARM Cortex-M4 platform
Memory (KBytes)
Voice Encoder 60.2 60 71 4.8 3.7 2.1
NCSE 5.8
AGC 0.4
Voice Decoder 14.6
Voice Encoder +
Voice Decoder
Total 81.0

Resources for TI's C64 DSP platform
Memory (KBytes)
Voice Encoder 19.2 71 72 4.8 3.7 2.1
NCSE 2.7
AGC 0.2
Voice Decoder 5.4
Voice Encoder +
Voice Decoder
Total 27.5

Resources (estimated) for TI's C55 DSP platform
Memory (KBytes)
Voice Encoder 36.0 24 71 4.5 3.7 2.1
NCSE 7.0
AGC 0.2
Voice Decoder 13.0
Voice Encoder +
Voice Decoder
Total 56.2

* DSPINI continues optimization of the TWELP algorithm and code in order to minimize computational complexity of the vocoder.

Vulnerability / Security. DSPINI guarantees ABSOLUTE cleanliness of the software from any undocumented features, undeclared capabilities, etc. All our customers can be sure that any our software/ code doesn't contain any secret functions and features hidden from user. We are ready to provide source codes of our software products for an appropriate certification if need.

Guarantee And Support.  DSPINI guarantees a quality and accordance of all technical characteristics of the product to requirement of current specifications. Testing and other method of quality control are used for guarantee support.

Any Platforms.  DSPINI can port this vocoder software into any other DSP, RISC or general- purposes platform inshort time: 1-2 months.

Licensing Terms.  To use the vocoder, customer should obtain a license from DSPINI only.

Customization.  The vocoder can be customized under any specific requirements- other bit rate, frame size, any other robustness to channel errors, etc. Please contact with us for details.

Prospects.  DSPINI is impoving and developing continuously a set of new vocoders with range from 300 bps up to 9600 bps, based on TWELP technology.

Related Software.  This vocoder may be effectively used in a bundle with other DSPINI's products:

  • Linear and acoustic echo cancellers,
  • Multichannel noise cancellers (including two-microphone adaptive array),
  • Wired or radiomodems for any types of channels and bitrates,
  • Other products.