2. Stream transport in the user plane: codecs and protocols

2.1 Audio codecs

Digital sound transport is a discipline closely linked to the physiological characteristics of the human listener. In this case, there are two frequency domains to consider:

• that of the ear, which perceives a spectrum of frequencies ranging from 20 Hz to 20 kHz;

• that of the vocal system, which produces sounds with frequencies essentially between 100 and 6,000 Hz.

It's this last area that is of interest to telephony, and in particular signals between 300 and 3,000 Hz are sufficient for a speaker to be understandable with limited distortion.