Comfort noise

<h2 id="issues-with-silence">Issues with silence</h2>
<p class="note">Main article: <a href="/facts/Dead_air/nTX758u0">Dead air</a></p>
<p>The result of receiving total <a href="/facts/Silence/dDQlMYN7">silence</a>, especially for a prolonged period, has a number of unwanted effects on the listener, including the following:
</p>
<ul><li>the listener may believe that the transmission has been lost, and therefore hang up prematurely</li>
<li>the speech may sound "choppy" (see <a href="/facts/Noise_gate/SMgb75l8">noise gate</a>) and difficult to understand</li>
<li>the sudden change in sound level can be jarring to the listener.</li></ul>
<p>To counteract these effects, comfort noise is added, usually on the <a href="/facts/Receiver_(radio)/5olDTxkg">receiving</a> end in wireless or VoIP systems, to fill in the silent portions of transmissions with artificial noise.
</p>
<h2 id="noise">Noise</h2>
<p>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.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a>
</p><p>In many VoIP products, users may control how VAD and comfort noise are configured, or disable the feature entirely.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a>
</p><p>As part of the <a href="/facts/RTP_audio_video_profile/RsPjoweL">RTP audio video profile</a>, RFC 3389 defines a standard for distributing comfort noise information in VoIP systems.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a>
</p>
<h3>Examples</h3>
<p>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  <a href="/facts/Remembrance_Sunday/uWFhd587">Remembrance Sunday</a>, 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).<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a>
</p><p>During the <a href="/facts/Siege_of_Leningrad/uCkWCzu0">siege of Leningrad</a>, the beat of a <a href="/facts/Metronome/UhRkhj2W">metronome</a> was used as comfort noise on the <a href="/facts/Leningrad/pNuCBxaM">Leningrad</a> <a href="/facts/Radio_network/rp6kCehQ">radio network</a>, indicating that the network was still functioning.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a>
</p>
<h2 id="related-concepts">Related concepts</h2>
<p>A similar concept is that of <a href="/facts/Sidetone/tVnJaEVN">sidetone</a>, 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.
</p>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Ambient_noise/Vw1Fd6Nx">Ambient noise</a></li>
<li><a href="/facts/Talkspurt/VI9IX6v4">Talkspurt</a></li>
<li><a href="/facts/Discontinuous_transmission/boTbwXJK">Discontinuous transmission</a> (DTX)</li>
<li><a href="/facts/Presence_(sound_recording)/HGkp339U">Presence (sound recording)</a></li>
<li><a href="/facts/Autonomous_sensory_meridian_response/KADvPeCv">Autonomous sensory meridian response</a></li>
<li><a href="/facts/Sound_masking/7uGdFfW2">Sound masking</a></li></ul>

<ul><li><a href="http://www.gaoresearch.com/products/telephony/other/vad_cng.php">Gao Research - VAD/CNG</a></li>
<li>Newton, Harry. <i>Newton's Telecom Dictionary.</i> 20th ed. 2004.</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>"Troubleshooting Hissing and Static: Comfort Noise and VAD". CISCO. Retrieved 18 July 2014. <a href="http://www.cisco.com/c/en/us/support/docs/voice/voice-quality/22388-hissing.html#topic1" target="_blank">http://www.cisco.com/c/en/us/support/docs/voice/voice-quality/22388-hissing.html#topic1</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>US patent 7649988, Suppapola, Seth; Ebenezer, Samuel Ponvara & Allen, Justin L., "Comfort noise generator using modified Doblinger noise estimate"
 <a href="https://worldwide.espacenet.com/textdoc?DB=EPODOC&IDX=US7649988" target="_blank">https://worldwide.espacenet.com/textdoc?DB=EPODOC&IDX=US7649988</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>"Troubleshooting Hissing and Static: Comfort Noise and VAD". CISCO. Retrieved 18 July 2014. <a href="http://www.cisco.com/c/en/us/support/docs/voice/voice-quality/22388-hissing.html#topic1" target="_blank">http://www.cisco.com/c/en/us/support/docs/voice/voice-quality/22388-hissing.html#topic1</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>R. Zopf (September 2002). Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN). Network Working Group. doi:10.17487/RFC3389. RFC 3389. Proposed Standard.
 <a href="https://datatracker.ietf.org/doc/html/rfc3389" target="_blank">https://datatracker.ietf.org/doc/html/rfc3389</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>"RB-SD1 Silence Detect Unit". Sonifex. Retrieved 2013-05-28. <a href="http://www.sonifex.co.uk/redbox/rbsd1_ld.shtml" target="_blank">http://www.sonifex.co.uk/redbox/rbsd1_ld.shtml</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
<li id="fn:6"><p>"Radio". Encyclopaedia of St. Petersburg. <a href="http://www.encspb.ru/en/article.php?kod=2804033930" target="_blank">http://www.encspb.ru/en/article.php?kod=2804033930</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li>
</ol>

Comfort noise open-in-new

Comfort noise