VoIP Voice and Fax Signal Processing

3.9 VOICE QUALITY OF Codecs

Mean opinion score (MOS) is used to measure the voice quality performance of the compression scheme or end-to-end voice call. Different methods like E-model [ITU-T-G.107 (2005)], perception based on listening, and perceptual evaluation of speech quality (PESQ) are used for MOS reporting [URL (DSLAII), URL (Opticom-PESQ)]. To present MOS in this chapter, PESQ and rating (R) factor are used. Some more details on voice quality measuring procedures are given in Chapter 20.

PESQ is an active measurement conducted mainly on instruments. In a PESQ-based measurement, the instrument sends a reference waveform from one end and compares the distortions at the other end with the original. PESQ is an external measure. The PESQ and R-factor are mapped to MOS. PESQ-based MOS differs from R-factor MOS.

MOS is presented in the range of 1 to 5, but it is limited to 4.5 in actual usage. The PESQ-based MOS of G.711 is 4.3 to 4.4. Worldwide G.711 in PSTN is treated as delivering toll voice quality. MOS of greater than or equal to 4.0 is considered as toll voice quality. To achieve toll quality in VoIP, G.711 and G.726 at higher bit rates and wideband codecs have to be used.

In the narrowband case, G.729 and G.729A are popularly used. G.729A gives a PESQ MOS of 3.75 to 3.8. The main G.729 codec gives better quality at about 3.9, but it requires more processing. These are not toll (MOS more than 4.0) quality codecs, but they are accepted worldwide with voice quality broadly ...