VoIP Voice and Fax Signal Processing

1.4 POWER LEVELS AND DIGITAL QUANTIZATION FOR G.711 μ/A-LAW

Telephone line electrical signal power levels are presented in dBm units. Power level expressed in dBm is the power in decibel (dB) with reference to one milliwatt (mW). Power 0 dBm represents one milliwatt of power. One mW is a strong speech level for listening. Usual telephone conversion levels are close to −16 dBm (25 μW; μW is microwatt). ITU-T-G.711 and ITU-T-G.168 (2004) recommendations listed speech power levels and corresponding digital quantization mapping for 8-bit A-law and μ-law. A-law and μ-law (pronounced as Mu) are logarithmic compression methods explained in Chapter 3 under G.711 compression. Telephone interfaces have SLIC and SLAC for converting TIP-RING signals to digital samples. SLIC converts a signal from two-wire TIP-RING interface to a four-wire interface to SLAC. In recent literature, the hardware CODEC name is used instead of SLAC. CODEC consists of a COder, DECoder hardware with an analog-to-digital converter (ADC) and a digital-to-analog converter (DAC) that samples the signals at 8 or 16 kHz. Sampled signals are given to the processor for additional processing. In this chapter, signal levels and the mapping to 16-bit numbers in A-law and μ-law, are given with reference to dB and dBm scale.

1.4.1 μ-Law Power Levels and Quantization

The maximum μ (read as Mu)-law undistorted power level for the sine wave is 3.17 dBm. Power of 3.17 dBm is 2.0749 mW in 600 Ohms (Ω) of telephone impedance. Impedance ...