RFC 3389 (rfc3389) - Page 2 of 8

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

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
   reasons.

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
   6.18dBm0.




Zopf                        Standards Track