VoIP Voice and Fax Signal Processing

4.3 COMFORT NOISE PAYLOAD FORMAT

As explained in the previous section, two popular types of VAD/CNG schemes are included in G.711. The noise power levels in both VAD/CNG methods are expressed in −dBov to interoperate with each other. The unit “dBov” is the dB level relative to the overload of the system. It is not a Volts reference for dBV. For example, in the case of μ-law-based coding, the maximum sine wave signal power without distortion is 3.17 dBm with amplitude ±8159. Square wave power is 3 dB more than sine wave power. The maximum possible power of signal from a square wave with an amplitude of ±8159 is 0 dBov as reference, which corresponds to a 6.17-dBm power level in the μ-law system. Hence, 0 dBov = 6.17 dBm is used in μ-law system. Sine and square wave power levels and signal amplitudes are shown in Fig. 4.1(c). Representation relative to the overload point of a system is particularly useful for digital implementations, because one does not need to know the relative calibration of the analog circuitry.

Figure 4.1. VAD/CNG representations. (a) VAD/CNG operation with G.729AB built-in VAD/CNG. (b) VAD/CNG with external VAD/CNG. (c) Power-level mapping with sine and square wave. (d) 1-byte power payload. (e) 11 bytes of payload.

Noise power levels in VAD are comparable from −40 to −60 dBm. The noise power is represented as −dBov with input power ranging from 0 to −127 dBov. ...