Comfort noise

Comfort noise (or comfort tone) is artificial background noise used in radio and wireless communications to fill the silence in a transmission resulting from voice activity detection or from the audio clarity of modern digital lines.

Some modern telephone systems (such as wireless and VoIP) use voice activity detection (VAD), a form of squelching where low volume levels are ignored by the transmitting device. In digital audio transmissions, this saves bandwidth of the communications channel by transmitting nothing when the source volume is under a certain threshold, leaving only louder sounds (such as the speaker's voice) to be sent. However, improvements in background noise reduction technologies can occasionally result in the complete removal of all noise. Although maximizing call quality is a primary importance, exhaustive removal of noise may not properly simulate the typical behavior of terminals on the PSTN system.

The result of receiving total silence, especially for a prolonged period, has a number of unwanted effects on the listener, including the following:

• the listener may believe that the transmission has been lost, and therefore hang up prematurely.
• the speech may sound "choppy" (see noise gate) and difficult to understand.
• the sudden change in sound level can be jarring to the listener.

To counteract these effects, comfort noise is added, usually on the receiving end in wireless or VoIP systems, to fill in the silent portions of transmissions with artificial noise. The noise generated is at a low but audible volume level, and can vary based on the average volume level of received signals to minimize jarring transitions.

In modern VoIP products, users may control whether they want comfort noise enabled or disabled.

During the siege of Leningrad, the beat of a metronome was used as comfort noise on the Leningrad radio network, indicating that the network was still functioning.^[1]

See also

• background noise
• ambient noise
• GSM
• Digital signal processing
• RTP audio video profile

References

1. ^ Encyclopaedia of St. Petersburg

• Gao Research - VAD/CNG
• Newton, Harry. Newton's Telecom Dictionary. 20th ed. 2004.