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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Comfort noise injection is the addition or generation of quiet, background-like audio during pauses in a voice call. When speech stops, a system may suppress ordinary audio packets to save transmission capacity; comfort noise prevents the far end from hearing an unnaturally absolute silence. The sender can signal noise characteristics, while the receiving device generates the sound.
Why voice systems use comfort noise
Even when nobody is speaking, a microphone and transmission path may carry room ambience, line noise, or other low-level sound. If a voice system stops sending packets during that interval, the listener can hear a sudden change to complete silence. Comfort noise aims to make the pause sound more like the original environment, rather than reproducing the exact background audio.
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In RTP voice systems, RFC 3389 defines a compact payload for conveying comfort-noise parameters. It specifies how to communicate information, not one required method for deciding when speech has stopped or synthesizing the resulting sound.
How comfort noise injection works
The process commonly involves three related functions. The sending side detects speech activity and reduces ordinary speech-packet transmission during inactivity; the receiving side uses any noise description it receives to generate background-like audio.
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- Voice activity detection (VAD): The sender classifies audio as active speech or inactivity. The standard does not prescribe a particular detection algorithm or threshold.
- Discontinuous transmission (DTX): During inactivity, the sender reduces or stops ordinary speech packets. It may send comfort-noise information so the receiver can maintain a plausible background sound.
- Comfort-noise generation (CNG): The receiver uses the parameters to update a noise model and synthesize audio. The synthesis method is implementation-specific.
RFC 3389 describes the aim as: “The role of the CNG is to sufficiently describe the ambient noise while minimizing the transmission rate.” The sender need not continually transmit the original room sound for the receiver to produce a suitable approximation.
What the RFC 3389 payload contains
The RTP comfort-noise payload conveys a noise level and can also carry spectral information represented as reflection coefficients for an all-pole noise model. These parameters help describe the background sound; they are not a recording or a complete recipe for audio generation.
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The encoded noise level ranges from 0 to 127, representing 0 to −127 dBov. Here, dBov is measured relative to the system overload point, so the encoded figure is not a universal acoustic sound-pressure level.
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How RTP and SDP signal comfort noise
RTP payload types let a receiver distinguish comfort-noise packets from packets carrying the selected audio codec. In SDP, the media description identifies the CN payload type when that format is to be used.
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- RFC 3389 describes static payload type 13 for an 8,000 Hz RTP timestamp clock rate.
- For other clock rates, the payload type must be mapped dynamically.
- If the CN payload type is omitted, that means this comfort-noise payload format will not be used. It does not, by itself, establish that silence suppression is disabled.
RFC 3551 likewise describes comfort-noise frames as parameters for artificial noise that may be generated during silence to approximate background noise at the source. Whether a particular call uses the format depends on its signaling and implementation.
What the standards leave to implementations
RFC 3389 standardizes interoperability for conveying comfort-noise parameters, but leaves the analysis and audio-generation details open. Its guidance says the comfort-noise analysis and synthesis, as well as VAD and DTX algorithms, are implementation-specific.
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That means the standard does not establish a single VAD threshold, a universal update interval, or a guaranteed subjective sound quality. A sender may update the noise description periodically or after a significant change in background noise; the update policy is implementation-specific.
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RFC 7874, published in March 2016, lists RFC 3389 comfort noise among the audio capabilities required of WebRTC endpoints in that document. This is a statement about the requirements in that RFC; it should not be taken alone as proof that every current WebRTC product negotiates or handles comfort noise identically.
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What to check when diagnosing a call
If a call sounds unnaturally silent between utterances, or background noise changes abruptly, the behavior can involve several separate choices rather than one setting. For an implementation-level investigation, check:
- whether comfort noise is signaled in SDP and the RTP payload type is mapped appropriately;
- the codec and RTP clock-rate mapping used in the session;
- how VAD and DTX classify and handle inactive periods;
- when the sender refreshes its noise description, especially if the background changes; and
- how the receiver’s CNG implementation turns those parameters into audio.
These checks identify where behavior may differ, but RFC 3389 does not define a universal tuning value or guarantee that two implementations will sound the same.
Quick Recap
Standards references
- RFC 3389: Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN), September 2002.
- RFC 3551: RTP Profile for Audio and Video Conferences with Minimal Control, July 2003.
- RFC 7874: WebRTC Audio Codec and Processing Requirements, March 2016.
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