Comfort tone

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 clarity of modern digital lines.

Some modern telephone systems (such as wireless and VoIP) use voice activity detection (VAD, or VOX), a form of squelching where low volume levels are ignored by the transmitting device. In digital 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.

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

• the listener may believe that the transmission has been lost, and therefore hang up prematurely
• the speech may sound "choppy" and even be hard 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.

See also

• GSM
• Digital signal processing

References

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