(version 5.0) enables voice communication in extremely challenging conditions where it was previously impossible, delivering speech quality and intelligibility comparable to that of MELPe at 600 bps, even while operating at an exceptionally low bit rate.

For Digital HF Radio 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 significantly better speech quality than other well-known technologies—including AMBE+2, MELPe, ACELP, and others—at equivalent bit rates ranging from 300 bps to 4800 bps and beyond.

Additionally, unlike other low-bitrate vocoders (such as MELPe, for example), TWELP delivers much higher quality for non-speech signals, including sirens, background music, and similar audio.

Note: 
We have updated the speech database by minimizing inter-speech pauses to eliminate their impact on the evaluation results. Therefore, the numbers obtained from the quality measurements using this updated speech database differ from those previously obtained with the original speech database, where speech pauses were not removed.

Speech Samples (WAV-files). 
A few independent experts compared the TWELP 300 bps vocoder with the MELPe 600 bps vocoder using the preference method.
All experts noted difficulties in recognizing unfamiliar speech in both cases, and none expressed a clear preference for one vocoder over the other.
Despite this, independent experts with HF communication experience observed satisfactory speech intelligibility with the TWELP 300 bps vocoder under conditions of very low (negative) SNR, where analog communication is nearly impossible due to high noise levels completely masking the speech signal.
Many users described the quality of voice communication with this vocoder as “impressive”.

You can play and listen to short samples of the source speech, as well as the speech processed by the MELPe 600 bps vocoder and the TWELP 300 bps vocoder, which operates at half the bit rate, using the links in the table below.

For the best listening experience, we recommend using high-quality headphones or premium audio equipment to hear the nuances and differences in the vocoder sound more clearly.

You can also download the complete set of P.50 samples as zip files for all languages simultaneously by using the links in the 'Downloads' section at the bottom of the page.

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.

Note: 
Of course, at such an ultra-low bitrate 300 bps, the quality of non-speech signal processing is much worse than at higher bitrates and is essentially not much better than that of MELPe 600 bps.

The fact that the difference in speech intelligibility between the MELPe and TWELP vocoders is smaller for noisy speech (3.15%) than for clean speech (5.87%) indicates better resistance of the TWELP vocoder to acoustic noise.

The NCSE integrated into the TWELP vocoder is described in more detail on the webpage for our standalone product, 'NCSE-AGC Preprocessor'.

For all users of our non-robust vocoder versions, we offer the following recommendations.