Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN)
RFC 3389

Document Type RFC - Proposed Standard (October 2002; Errata)
Author Robert Zopf 
Last updated 2020-01-21
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Network Working Group                                            R. Zopf
Request for Comments: 3389                           Lucent Technologies
Category: Standards Track                                 September 2002

   Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   This document describes a Real-time Transport Protocol (RTP) payload
   format for transporting comfort noise (CN).  The CN payload type is
   primarily for use with audio codecs that do not support comfort noise
   as part of the codec itself such as ITU-T Recommendations G.711,
   G.726, G.727, G.728, and G.722.

1. Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [7].

2. Introduction

   This document describes a RTP [1] payload format for transporting
   comfort noise.  The payload format is based on Appendix II of ITU-T
   Recommendation G.711 [8] which defines a comfort noise payload format
   (or bit-stream) for ITU-T G.711 [2] use in packet-based multimedia
   communication systems.  The payload format is generic and may also be
   used with other audio codecs without built-in Discontinuous
   Transmission (DTX) capability such as ITU-T Recommendations G.726
   [3], G.727 [4], G.728 [5], and G.722 [6].  The payload format
   provides a minimum interoperability specification for communication
   of comfort noise parameters.  The comfort noise analysis and
   synthesis as well as the Voice Activity Detection (VAD) and DTX
   algorithms are unspecified and left implementation-specific.

Zopf                        Standards Track                     [Page 1]
RFC 3389             RTP Payload for Comfort Noise        September 2002

   However, an example solution for G.711 has been tested and is
   described in the Appendix [8].  It uses the VAD and DTX of G.729
   Annex B [9] and a comfort noise generation algorithm (CNG) which is
   provided in the Appendix for information.

   The comfort noise payload, which is also known as a Silence Insertion
   Descriptor (SID) frame, consists of a single octet description of the
   noise level and MAY contain spectral information in subsequent
   octets.  An earlier version of the CN payload format consisting only
   of the noise level byte was defined in draft revisions of the RFC
   1890.  The extended payload format defined in this document should be
   backward compatible with implementations of the earlier version
   assuming that only the first byte is interpreted and any additional
   spectral information bytes are ignored.

3. CN Payload Definition

   The comfort noise payload consists of a description of the noise
   level and spectral information in the form of reflection coefficients
   for an all-pole model of the noise.  The inclusion of spectral
   information is OPTIONAL and the model order (number of coefficients)
   is left unspecified.  The encoder may choose an appropriate model
   order based on such considerations as quality, complexity, expected
   environmental noise, and signal bandwidth.  The model order is not
   explicitly transmitted since the number of coefficients can be
   derived from the length of the payload at the receiver.  The decoder
   may reduce the model order by setting higher order reflection
   coefficients to zero if desired to reduce complexity or for other

3.1 Noise Level

   The magnitude of the noise level is packed into the least significant
   bits of the noise-level byte with the most significant bit unused and
   always set to 0 as shown below in Figure 1.  The least significant
   bit of the noise level magnitude is packed into the least significant
   bit of the byte.

   The noise level is expressed in -dBov, with values from 0 to 127
   representing 0 to -127 dBov.  dBov is the level relative to the
   overload of the system.  (Note: Representation relative to the
   overload point of a system is particularly useful for digital
   implementations, since one does not need to know the relative
   calibration of the analog circuitry.)  For example, in the case of a
   u-law system, the reference would be a square wave with values +/-
   8031, and this square wave represents 0dBov.  This translates into

Zopf                        Standards Track                     [Page 2]
RFC 3389             RTP Payload for Comfort Noise        September 2002
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