A comfort noise generator (CNG) is a program used to generate background noise for voice communications during periods of silence that occur during the course of conversation.
CNG is part of the silence suppression or voice activity detection (VAD) handling for VoIP (Voice over IP) technology. The purpose of VAD and CNG is to maintain an acceptable perceived quality of service (QoS) while simultaneously keeping transmission costs and bandwidth usage as low as possible.
CNG, in conjunction with VAD algorithms, quickly determines when periods of silence occur and inserts artificial noise until voice activity resumes. The insertion of artificial noise gives the illusion of a constant transmission stream, so that background sound is consistent throughout the call and the listener does not think the line has gone dead. Most conversations include about 50% silence. VAD software allows a data network carrying voice traffic over the Internet to detect the absence of audio and conserve bandwidth by preventing the transmission of "silent packets."
CNG uses special algorithms to create artificial noise that matches the actual background noise it detects on a call. The generation of background noise helps avoid noise modulation that might otherwise occur if voice transmission were completely switched off during silent intervals. There are a number of reasons to suppress noise modulation. For one thing, noise modulation may be so different from natural background noise that it is unpleasant for calling parties. Furthermore, it may reduce the intelligibility of speech when spoken communications resume.
The use of VADs and CNG has been estimated to reduce bandwidth requirements for a group of voice channels by as much as 50 percent.