Comfort Noise Generation (CNG) is used in vocoders to provide a level of background noise with which people have become accustomed using older telephone services. The absence of signal energy during a conversation can be disconcerting to the person listening to the signal. Due to the long time use of the Plain Old Telephone Service (POTS), people are used to having a certain level of background noise and static. If this low level noise is not present when there is no speech energy, our experience with POTS leads us to believe that the call has been disconnected. Thus our conversations would be full of “Are you still there?”
In these situations, Comfort Noise Generation is used to create the missing noise. CNG detects whether there is signal energy present. If there is, all is good and nothing needs to be done to the decoded signal. On the other hand, if the signal energy falls below a certain threshold, the CNG is activated which generates low level background noise to fill this auditory void. This noise is added to the original signal, which, in the case of a lost packet, can help mask the lost packet. Any discontinuities from the lost packet blend into the random noise.