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{{Short description|Synthetic background noise}}
{{More citations needed|date=April 2012}}

'''Comfort noise''' (or '''comfort tone''') is synthetic [[background noise]] used in [[radio]] and [[wireless]] communications to fill the artificial [[silence]] in a transmission resulting from [[voice activity detection]] or from the audio clarity of modern digital lines.<ref name="cisco">{{cite web|title=Troubleshooting Hissing and Static: Comfort Noise and VAD|url=http://www.cisco.com/c/en/us/support/docs/voice/voice-quality/22388-hissing.html#topic1|website=CISCO|accessdate=18 July 2014}}</ref>

Some modern telephone systems (such as [[wireless telephony|wireless]] and [[VoIP]]) use voice activity detection (VAD), a form of [[squelch]]ing where low [[loudness|volume]] levels are ignored by the [[transmitter (radio)|transmitting]] device.  In [[digital audio]] transmissions, this saves [[bandwidth (computing)|bandwidth]] of the communications channel by transmitting nothing when the source volume is under a certain threshold, leaving only louder [[sound]]s (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 of primary importance, exhaustive removal of noise may not properly simulate the typical behavior of terminals on the [[PSTN]] system.

==Issues with silence==
{{seemain|Dead air}}
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 [[receiver (radio)|receiving]] end in wireless or VoIP systems, to fill in the silent portions of transmissions with artificial noise.

==Noise==
Generated comfort noise is at a low but audible volume level, and can vary based on the average volume level of received signals to minimize jarring transitions.<ref>{{cite patent|inventor-last1=Suppapola|inventor-first1=Seth|inventor-last2=Ebenezer|inventor-first2=Samuel Ponvara|inventor-last3=Allen|inventor-first3=Justin L.|title=Comfort noise generator using modified Doblinger noise estimate|status=patent|country=US|number=7649988}}</ref>

In many VoIP products, users may control how VAD and comfort noise are configured, or disable the feature entirely.<ref name="cisco"/>

As part of the [[RTP audio video profile]], RFC 3389 defines a standard for distributing comfort noise information in VoIP systems.{{Ref RFC|3389}}

===Examples===

Many radio stations broadcast birdsong, city-traffic or other atmospheric comfort noise during periods of deliberate silence.  For example, in the UK, silence is observed on  [[Remembrance Sunday]], and London's quiet city ambiance is used. This is to reassure the listener that the station is on-air, but primarily to prevent silence detection systems at transmitters from automatically starting backup tapes of music (designed to be broadcast in the case of transmission link failure).<ref>{{cite web |url=http://www.sonifex.co.uk/redbox/rbsd1_ld.shtml |title=RB-SD1 Silence Detect Unit |publisher=Sonifex |accessdate=2013-05-28}}</ref>

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.<ref>{{cite encyclopedia |url=http://www.encspb.ru/en/article.php?kod=2804033930 |title=Radio |encyclopedia=Encyclopaedia of St. Petersburg}}</ref>

==Related concepts==
A similar concept is that of [[sidetone]], the effect of sound that is picked up by a telephone's mouthpiece and introduced (at low level) into the earpiece of the same handset, acting as feedback.

==See also==
* [[Ambient noise]]
* [[Talkspurt]]
* [[Discontinuous transmission]] (DTX)
* [[Presence (sound recording)]]
* [[Autonomous sensory meridian response]]
* [[Sound masking]]

==References==
<references/>
*[http://www.gaoresearch.com/products/telephony/other/vad_cng.php Gao Research - VAD/CNG]
*Newton, Harry. ''Newton's Telecom Dictionary.'' 20th ed. 2004.

[[Category:Noise]]
[[Category:Radio technology]]
[[Category:Voice over IP]]
[[Category:Mobile telecommunications]]