Compandor noise advantage
From Figure 20.16 it can be seen that while no speech is transmitted the noise power will be subject to an attenuation of 25dB at the expandor output.
The theoretical noise advantage will therefore be given by Equation 20.38.
However, the circuit noise while speech power is transmitted is virtually unaffected by the compression/expansion process and there is no advantage. During talking, therefore, the conversation is subject to the full effect of the system noise. The effect of this noise produces an impairment to speech intelligibility that detracts from the companding advantage by an estimated 5dB.
Overall advantage is therefore given by Equation 20.39.