Complete Communications Engineering

VOCAL provides ITU, GSM, and wideband vocoders including G.721, G.711 PCM μ-law or A-law, G.723, G.726, G.726, and G.728 for customized VoIP solutions. Our software is optimized for execution on ANSI C and leading DSP architectures (TI, ADI, AMD, ARM, MIPS, CEVA, LSI Logic ZSP, etc.). These libraries are modular and can be executed as a single task under a variety of operating systems or standalone with its own microkernel. Contact us to discuss your VoIP application requirements.

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G.711, G.721, G.726, and G.728

G.711 is a 64 kbps compressed stream and is a common reference for speech compression quality. G.711 encoding/decoding is usually done within codecs although a technique to convert linear samples to PCM (A-law or μ-law) is straightforward.

G.721 produces toll quality speech using a 32 kbps Adaptive Differential Pulse Code Modulation (ADPCM) speech compression algorithm. Under error free transmission, G.721 has a slightly worse perceived quality than G.711. The quality of G.721 significantly deteriorates when several such links are used in tandem. However with transmission error rates higher than 10.4, the perceived quality of G.721 is better than G.711.

G.726 extends the G.721 ADPCM to include 40, 24 and 16 kbps, as well as 32 kbps. G.726 at 40 kbps performs comparable to G.711.

G.728 Low-Delay Code Excited Linear Prediction (LD-CELP) compression is a 16 kbps compression. Compared with G.721, G.728 tends to score worse in objective, but better in subjective testes. A characteristic of CELP algorithms is that they tend to perform more poorly than ADPCM in the presence of background noise.



The G.723 dual rate speech coder was developed for use in multimedia platforms, in particular those specified by the H.32x series recommendations. Relative to the G.729/G.729A coders, the G.723 speech coders pass DTMF tones through with less distortion.



G.729 is an 8 kbps Conjugate-Structure Algebraic-Code-Excited Linear Prediction (CS-ACELP) speech compression algorithm. G.729 Annex A is a reduced complexity version of the G.729 coder developed for use in multimedia simultaneous voice and data applications like DSVD.


Example Resource Requirements