We had requests for a comparison of our vocoders with open-source CODEC2 vocoder.

Here, we present a comparison of the CODEC2 (ver 0.8) vocoder with TWELP vocoder at 2400 bps bit rate.
We added to some tests MELPe 2400 bps vocoder - just for additional comparison with this well-known standard vocoder.

Technology Features. The TWELP vocoders are based on the 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. at the same bit rate in range from 300 bps up to 9600 bps and more. Unlike other LBR vocoders (MELPe, etc.) TWELP also provides much better quality for non-speech signals like sirens, background music, etc.

CODEC2 is based on the older and simpler technology SHC (Sinusoidal Harmonic Coding), that was popular 30 years ago.

Speech Quality. Here is a comparison of the speech quality in a noiseless channel. TWELP 2400 bps vocoder and CODEC2 2400 bps vocoder 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 significant superiority of the TWELP as well as MELPe vocoders over CODEC2 vocoder in speech quality. Exact numbers of the PESQ-estimations are presented in the table below.
LanguageTWELP 2400MELPe 2400CODEC2 2400
American 3.330 3.077 2.777
Arabic 3.253 3.053 2.684
British 3.181 3.019 2.596
Chinese 3.307 2.970 2.748
Danish 3.275 3.022 2.735
Dutch 3.114 2.830 2.463
Finnich 3.049 2.791 2.397
French 3.325 3.106 2.767
German 3.183 2.998 2.641
Greek 3.275 3.004 2.625
Hindi 3.246 3.089 2.670
Hungarian 3.279 3.086 2.921
Italian 3.363 3.226 2.889
Japanese 3.407 3.188 2.846
Norwegian 3.286 3.032 2.817
Polish 3.267 3.029 2.594
Portuguese 3.350 3.146 2.832
Russian 3.165 2.952 2.674
Spanish 3.294 3.048 2.958
Swedish 3.269 3.147 2.713

Superiority of the TWELP 2400 and MELPe 2400 over CODEC2 2400 is on average 0.544 and 0.324 PESQ apropriately

Speech Samples (WAV-files). A few independent experts listened CODEC2 2400 bps vocoder in comparison with TWELP 2400 bps vocoder, using method of preferences. All experts preferred TWELP to CODEC2, having noted much better intellegibility and much more natural human-sounding of voice in the TWELP vocoder.

You can listen to 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 speechMELPe 2400 bpsCODEC2 2400 bpsTWELP 2400 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 signalMELPe 2400 bpsCODEC2 2400 bpsTWELP 2400 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. Here, we used our NCSE-preprocessor for CODEC2 too.

NCSE ModeSource signalMELPe 2400 bpsCODEC2 2400 bpsTWELP 2400 bps

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 performs a highly optimized porting of the vocoder for any other DSP, RISC or general-purpose platform in short time: 1-2 months.

Licensing Terms. DSPINI is the exclusive owner of the rights to the TWELP vocoder software, a customer should obtain a license from DSPINI only.

Customization.  DSPINI can customize any vocoder under specific requirements- other bit rate, frame size, any other robustness to channel errors, etc. Please contact us for the details.

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

Related Software.  Any 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.


Conclusion.   The open-source CODEC2 vocoder is a very good achievement for a free codec at low bit rates, and if zero cost for your codec is a must for you, it's worth trying it in your application.

However, based on an older technology, Codec2 has lower speech quality, robustness to noise and not as effective with non-speech signals. We recommend taking into consideration all the factors in your specific application, consult with your engineers and make your decision what codec to use.

We'd be happy to provide you the detailed info on TWELP vocoder, consult you and help make a decision if it's worth buying for your application, please shoot us an email.