RFC 3389 (rfc3389) - Page 2 of 8
Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN)
Alternative Format: Original Text Document
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