VoIP Voice and Fax Signal Processing

3.1 COMPRESSION Codecs

Voice compression is broadly classified as waveform, vocoders, and hybrid codecs [Kondoz (1999)]. Waveform-based codecs of G.711, and G.726 encode–decode voice on an actual signal without making an assumption on speech models. They work on any input that is supported by sampling without any significant distortions.

A vocoder is a voice coder that makes use of a vocal tract voice production model. Vocal tract-based compression achieves a better compression ratio than waveform-based codecs. Vocoder-based codecs generate a set of parameters that represent the speech production models. On the receive side, voice is synthetically reproduced based on the parameters. These codecs take very low bandwidth (very high compression) and deliver lower quality. They are of a moderate complexity for processing. Linear predictive coding-10 (LPC-10) codec uses 2.4 and 4.8 kbps [Kondoz (1999)], which is much lower than waveform and hybrid codecs. The main benefit of vocoders is a lower bit rate.

Hybrid codecs achieve acceptable compression and quality. Compared with vocoders, hybrid codecs deliver better quality, and a wideband version of hybrid codecs can exceed waveform-based codec quality. Several extended techniques in hybrid codecs are beyond the techniques used in vocoders and waveform-based codecs. Hybrid codecs analyze the signal in frequency and time domain. Frequency-domain compressions are known as sub-band or transform-domain coding. Time-domain compression makes ...