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.
Abstract
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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.
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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.
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RFC 3389 RTP Payload for Comfort Noise September 2002
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0| level |
+-+-+-+-+-+-+-+-+
Figure 1: Noise Level Packing
3.2 Spectral Information
The spectral information is transmitted using reflection coefficients
[8]. Each reflection coefficient can have values between -1 and 1
and is quantized uniformly using 8 bits. The quantized value is
represented by the 8 bit index N, where N=0..,254, and index N=255 is
reserved for future use. Each index N is packed into a separate byte
with the MSB first. The quantized value of each reflection
coefficient, k_i, can be obtained from its corresponding index using:
k_i(N_i) = 258*(N_i-127) for N_i = 0...254; -1 < k_i < 1
-------------
32768
3.3 Payload Packing
The first byte of the payload MUST contain the noise level as shown
in Figure 1. Quantized reflection coefficients are packed in
subsequent bytes in ascending order as in Figure 2 where M is the
model order. The total length of the payload is M+1 bytes. Note
that a 0th order model (i.e., no spectral envelope information)
reduces to transmitting only the energy level.
Byte 1 2 3 ... M+1
+-----+-----+-----+-----+-----+
|level| N1 | N2 | ... | NM |
+-----+-----+-----+-----+-----+
Figure 2: CN Payload Packing Format
4. Usage of RTP
The RTP header for the comfort noise packet SHOULD be constructed as
if the comfort noise were an independent codec. Thus, the RTP
timestamp designates the beginning of the comfort noise period. When
this payload format is used under the RTP profile specified in RFC
1890 [10], a static payload type of 13 is assigned for RTP timestamp
clock rate of 8,000 Hz; if other rates are needed, they MUST be
defined through dynamic payload types. The RTP packet SHOULD NOT
have the marker bit set.
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Each RTP packet containing comfort noise MUST contain exactly one CN
payload per channel. This is required since the CN payload has a
variable length. If multiple audio channels are used, each channel
MUST use the same spectral model order 'M'.
5. Guidelines for Use
An audio codec with DTX capabilities generally includes VAD, DTX, and
CNG algorithms. The job of the VAD is to discriminate between active
and inactive voice segments in the input signal. During inactive
voice segments, the role of the CNG is to sufficiently describe the
ambient noise while minimizing the transmission rate. A CN payload
(or SID frame) containing a description of the noise is sent to the
receiver to drive the CNG. The DTX algorithm determines when a CN
payload is transmitted. During active voice segments, packets of the
voice codec are transmitted and indicated in the RTP header by the
static or dynamic payload type for that codec. At the beginning of
an inactive voice segment (silence period), a CN packet is
transmitted in the same RTP stream and indicated by the CN payload
type. The CN packet update rate is left implementation specific. For
example, the CN packet may be sent periodically or only when there is
a significant change in the background noise characteristics. The
CNG algorithm at the receiver uses the information in the CN payload
to update its noise generation model and then produce an appropriate
amount of comfort noise.
The CN payload format provides a minimum interoperability
specification for communication of comfort noise parameters. The
comfort noise analysis and synthesis as well as the VAD and DTX
algorithms are unspecified and left implementation-specific.
However, an example solution for G.711 has been tested and is
described in Appendix II of ITU-T Recommendation G.711 [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.
Additional guidelines for use such as the factors affecting system
performance in the design of the VAD/DTX/CNG algorithms are described
in the Appendix.
5.1 Usage of SDP
When using the Session Description Protocol (SDP) [11] to specify RTP
payload information, the use of comfort noise is indicated by the
inclusion of a payload type for CN on the media description line.
When using CN with the RTP/AVP profile [10] and a codec whose RTP
timestamp clock rate is 8000 Hz, such as G.711 (PCMU, static payload
type 0), the static payload type 13 for CN can be used:
m=audio 49230 RTP/AVP 0 13
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When using CN with a codec that has a different RTP timestamp clock
rate, a dynamic payload type mapping (rtpmap attribute) is required.
This example shows CN used with the G.722.1 codec (see RFC 3047
[12]):
m=audio 49230 RTP/AVP 101 102
a=rtpmap:101 G7221/16000
a=fmtp:121 bitrate=24000
a=rtpmap:102 CN/16000
Omission of a payload type for CN on the media description line
implies that this comfort noise payload format will not be used, but
it does not imply that silence will not be suppressed. RTP allows
discontinuous transmission (silence suppression) on any audio payload
format. The receiver can detect silence suppression on the first
packet received after the silence by observing that the RTP timestamp
is not contiguous with the end of the interval covered by the
previous packet even though the RTP sequence number has incremented
only by one. The RTP marker bit is also normally set on such a
packet.
6. IANA Considerations
This section defines a new RTP payload name and associated MIME type,
CN (audio/CN). The payload format specified in this document is also
assigned payload type 13 in the RTP Payload Types table of the RTP
Parameters registry maintained by the Internet Assigned Numbers
Authority (IANA).
6.1 Registration of MIME media type audio/CN
MIME media type name: audio
MIME subtype name: CN
Required parameters: None
Optional parameters:
rate: specifies the RTP timestamp clock rate, which is usually (but
not always) equal to the sampling rate. This parameter should have
the same value as the codec used in conjunction with comfort noise.
The default value is 8000.
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Encoding considerations:
This type is only defined for transfer via RTP [RFC 1889].
Security considerations: see Section 7 "Security Considerations".
Interoperability considerations: none
Published specification:
This document and Appendix II of ITU-T Recommendation G.711
Applications which use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Robert Zopf
zopf@lucent.com
Intended usage: COMMON
Author/Change controller:
Author: Robert Zopf
Change controller: IETF AVT Working Group
7. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [1]. This implies that confidentiality of the media
streams is achieved by encryption. Because the payload format is
arranged end-to-end, encryption MAY be performed after encapsulation
so there is no conflict between the two operations.
As this format transports background noise, there are no significant
security, confidentiality, or authentication concerns.
8. References
[1] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
1889, January 1996.
[2] ITU Recommendation G.711 (11/88) - Pulse code modulation (PCM)
of voice frequencies.
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[3] ITU Recommendation G.726 (12/90) - 40, 32, 24, 16 kbit/s
Adaptive Differential Pulse Code Modulation (ADPCM).
[4] ITU Recommendation G.727 (12/90) - 5-, 4-, 3- and 2-bits sample
embedded adaptive differential pulse code modulation (ADPCM).
[5] ITU Recommendation G.728 (09/92) - Coding of speech at 16
kbits/s using low-delay code excited linear prediction.
[6] ITU Recommendation G.722 (11/88) - 7 kHz audio-coding within 64
kbit/s.
[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[8] Appendix II to Recommendation G.711 (02/2000) - A comfort noise
payload definition for ITU-T G.711 use in packet-based
multimedia communication systems.
[9] Annex B (08/97) to Recommendation G.729 - C source code and test
vectors for implementation verification of the algorithm of the
G.729 silence compression scheme.
[10] Schulzrinne, H., "RTP Profile for Audio and Video Conferences
with Minimal Control", RFC 1890, January 1996.
[11] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[12] Luthi, P., "RTP Payload Format for ITU-T Recommendation
G.722.1", RFC 3047, January 2001.
9. Author's Address
Robert Zopf
Lucent Technologies
INS Access VoIP Networks
2G-234A
101 Crawfords Corner Rd
Holmdel, NJ 07733-3030 US
Phone: 1-732-949-1667
Fax: 1-732-949-7016
EMail: zopf@lucent.com
Zopf Standards Track [Page 7]RFC 3389 RTP Payload for Comfort Noise September 2002
10. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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