Comfort noise (or comfort tone) is synthetic

background noise Background noise or ambient noise is any sound other than the sound being monitored (primary sound). Background noise is a form of noise pollution or interference. Background noise is an important concept in setting noise levels. Background no ...

used in

radio Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...

and

wireless Wireless communication (or just wireless, when the context allows) is the transfer of information (''telecommunication'') between two or more points without the use of an electrical conductor, optical fiber or other continuous guided transm ...

communications to fill the artificial

silence Silence is the absence of ambient hearing, audible sound, the emission of sounds of such low sound intensity, intensity that they do not draw attention to themselves, or the state of having ceased to produce sounds; this latter sense can be exten ...

in a transmission resulting from voice activity detection or from the audio clarity of modern digital lines. Some modern telephone systems (such as

and

VoIP Voice over Internet Protocol (VoIP), also known as IP telephony, is a set of technologies used primarily for voice communication sessions over Internet Protocol (IP) networks, such as the Internet. VoIP enables voice calls to be transmitted as ...

) use voice activity detection (VAD), a form of

squelch In telecommunications, squelch is a circuit function that acts to suppress the audio (or video) output of a receiver in the absence of a strong input signal. Essentially, squelch is a specialized type of noise gate designed to suppress weak s ...

ing where low

volume Volume is a measure of regions in three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch) ...

levels are ignored by the transmitting device. In

digital audio Digital audio is a representation of sound recorded in, or converted into, digital signal (signal processing), digital form. In digital audio, the sound wave of the audio signal is typically encoded as numerical sampling (signal processing), ...

transmissions, this saves bandwidth of the communications channel by transmitting nothing when the source volume is under a certain threshold, leaving only louder

sound In physics, sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the br ...

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 The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists ...

system.

Issues with silence

The result of receiving total

, 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 A noise gate or simply gate is an electronic device or software that is used to control the amplitude, volume of an audio signal. Comparable to a limiter, which attenuates signals ''above'' a threshold, such as loud attacks from the start of mu ...

) 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.

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. In many VoIP products, users may control how VAD and comfort noise are configured, or disable the feature entirely. As part of the RTP audio video profile, RFC 3389 defines a standard for distributing comfort noise information in VoIP systems.

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 Remembrance Sunday is held in the United Kingdom as a day to commemorate the contribution of British and Commonwealth military and civilian servicemen and women in the two World Wars and later conflicts. It is held on the second Sunday in Nov ...

, 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). During the

siege of Leningrad The siege of Leningrad was a Siege, military blockade undertaken by the Axis powers against the city of Leningrad (present-day Saint Petersburg) in the Soviet Union on the Eastern Front (World War II), Eastern Front of World War II from 1941 t ...

, the beat of a

metronome A metronome () is a device that produces an audible click or other sound at a uniform interval that can be set by the user, typically in beats per minute (BPM). Metronomes may also include synchronized visual motion, such as a swinging pendulum ...

was used as comfort noise on the

Leningrad Saint Petersburg, formerly known as Petrograd and later Leningrad, is the List of cities and towns in Russia by population, second-largest city in Russia after Moscow. It is situated on the Neva, River Neva, at the head of the Gulf of Finland ...

radio network There are two types of radio network currently in use around the world: the one-to-many (simplex communication) broadcast network commonly used for public information and mass media, mass-media entertainment, and the two-way radio (Duplex (teleco ...

, indicating that the network was still functioning.{{cite encyclopedia , url=http://www.encspb.ru/en/article.php?kod=2804033930 , title=Radio , encyclopedia=Encyclopaedia of St. Petersburg

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.

References

Gao Research - VAD/CNG
*Newton, Harry. ''Newton's Telecom Dictionary.'' 20th ed. 2004. Noise Radio technology Voice over IP Mobile telecommunications

Issues with silence

Noise

Examples

Related concepts

See also

References