VoIP Voice and Fax Signal Processing

1.5 SIGNIFICANCE OF POWER LEVELS ON LISTENING

Table 1.2 is given for different power levels starting from +3.17 dBm and down to −66 dBm in a few suitable steps. In relation to Table 1.2, summary points are given here [ITU-Handbook (1992)]. These levels are mainly mapped to the telephone interfaces and voice chain processing built for VoIP. In VoIP, power levels play a role in operation of some voice processing operations such as voice activity detection (VAD)/comfort noise generation (CNG), automatic gain control (AGC), echo level, and ideal channel noise. PSTN systems do not have any power-sensitive module operations such as VAD/CNG. In voltage scale, a 0.775-Volt RMS sine wave is the same as 0 dBm, terminated in 600-Ω impedance. The usual speech conversation level is −16 dBm in power. Some members may speak at a low amplitude below −24 dBm, which can also happen because of positioning of acoustic interfaces during conversation. This problem demands more effort by the listener. Automatic gain control (AGC), if employed, starts working from this power level. As per the ITU-T-G.169 (1999) recommendation, a maximum gain of 15 dB is used on low-level speech signals. A peak-to-average (PAR) level of speech is 15 to 18 dB [ITU-Handbook (1992)] for 0.1% of the active speech time. An average speech level of −12 dBm with 15 dB peak to average can preserve the speech as undistorted.

In practice, speech signals are gain controlled up to speech levels of −42 to −40 dBm. Below −42 dBm, it ...