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.

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.

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.

High Robustness To The Channel Errors. 
The diagram below illustrates the sensitivity of bits at the output of the vocoder to communication channel errors.
Essentially, the diagram shows by what percentage speech quality is reduced when a specific bit is distorted. The first bits in order cause catastrophic distortions, while the latter bits have significantly less impact on quality.

We strongly recommend using FEC (Forward Error Correction) with unequal protection of the bits in strong accordance with their sensitivity to errors and utilizing 'Soft Decisions' decoding. This will provide the highest robustness of the vocoder against errors in the channel. The first bits in order cause catastrophic distortions, whereas the latter bits have almost no effect on quality.

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. 

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:

* 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.

Downloads:

• Datasheet (pdf)
• ITU-T P.50 source speech samples (zip)
• MELPe 600 bps speech samples (zip)
• TWELP 300 bps speech samples (zip)
• P.862 and STOI/ESTOI utilities
• PC-evaluation package (zip) — on request
• User's Guide document (pdf) — on request