AVT WG                                                          R. Zopf
Internet Draft                                      Lucent Technologies
Document: draft-ietf-avt-rtp-cn-05.txt                     January 2002
Category: Standards Track


                     RTP Payload for Comfort Noise


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].

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   Abstract

   This document describes an RTP [2] 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 [3], G.726 [4], G.727
   [5], G.728 [6], and G.722 [7].


. 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 [8].


. Introduction

   This document describes an RTP payload format for transporting
   comfort noise.  The payload format is based on Appendix II of ITU-T
   Recommendation G.711 [9] which defines a comfort noise payload
   format (or bit-stream) for ITU-T G.711 use in packet-based
   multimedia communication systems.  The payload format is generic and
   may also be used with other audio codecs without built-in

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   Discontinuous Transmission (DTX) capability such as ITU-T
   Recommendations G.726 [4], G.727 [5], G.728 [6], and G.722 [7].  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.
   However, an example solution for G.711 has been tested and is
   described in the Appendix [9].  It uses the VAD and DTX of G.729
   Annex B [10] and a comfort noise generation algorithm (CNG) which is
   provided in the Appendix for information.

   The comfort noise payload 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.


. CN Payload Definition

   The comfort noise payload consists of a description of the noise
   level and spectral information in the form of reflection
   coefficients. The use of spectral information is optional and the
   all-pole model order is left unspecified.   The encoder can
   determine the appropriate model order based on such considerations
   as quality, complexity, expected environmental noise, and signal
   bandwidth.  The model order is not explicitly transmitted since it
   can be derived from the length of the payload at the receiver. For
   complexity or other reasons, the decoder may reduce the model order
   by setting higher order reflection coefficients to zero.

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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                        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 [9]. 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

. 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[11], 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.

   Each RTP packet containing comfort noise MUST contain exactly one CN
   payload per channel.  This is required since the CN payload has a


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   variable length.  If multiple audio channels are used, each channel
   MUST use the same spectral model order 'M'.

. 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 Silence Insertion Descriptor (SID) frame containing a description
   of the noise is packed into the CN payload and sent to the receiver
   to drive the CNG.  The DTX algorithm determines when a SID frame 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 SID frame packed into a
   CN packet is transmitted in the same RTP stream and indicated by the
   CN payload type.  The SID frame update rate is left implementation
   specific.  For example, the SID frame 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 SID 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 [9].  It uses
   the VAD and DTX of G.729 Annex B [10] 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) [12] 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 [11] 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

   When using CN with a codec that has a different RTP timestamp clock
   rate, a dynamic payload type mapping (rtpmap attribute) is required.




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   This example shows CN used with the G.722.1 codec (see RFC 3047
   [13]):
         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 comfort noise will not be used, but it does not imply
   that silence will not be suppressed.  Discontinuous transmission
   (silence suppression) is indicated by an increment in the RTP
   timestamp without a corresponding increment in the RTP sequence
   number.

. 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).

   [Note to IANA and the RFC Editor, to be deleted before publication
   of this draft as an RFC: The RTP Payload Types table referenced in
   the previous paragraph is "closed" per the policy change established
   in draft-ietf-avt-profile-new-12, but that draft also says "Payload
   type 13 is reserved for a comfort noise payload format to be
   specified in a separate RFC."  This document is intended to be that
   separate RFC.]

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.

   Encoding considerations:
   This type is only defined for transfer via RTP [RFC 1889].

   Security considerations: see Section 8 "Security Considerations".

   Interoperability considerations: none

   Published specification:
   This document and Appendix II of ITU-T Recommendation G.711

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   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

. Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [2]. 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.

. References


   1  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.

   2  H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: A
      Transport Protocol for Real-Time Applications", RFC 1889.

   3  ITU Recommendation G.711 (11/88) - Pulse code modulation (PCM) of
      voice frequencies.

   4  ITU Recommendation G.726 (12/90) - 40, 32, 24, 16 kbit/s Adaptive
      Differential Pulse Code Modulation (ADPCM).

   5  ITU Recommendation G.727 (12/90) - 5-, 4-, 3- and 2-bits sample
      embedded adaptive differential pulse code modulation (ADPCM).

   6  ITU Recommendation G.728 (09/92) - Coding of speech at 16 kbits/s
      using low-delay code excited linear prediction.

   7  ITU Recommendation G.722 (11/88) - 7 kHz audio-coding within 64
      kbit/s.



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   8  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.

   9  Appendix II to Recommendation G.711 (to be published) - A comfort
      noise payload definition for ITU-T G.711 use in packet-based
      multimedia communication systems.

   10 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.

   11 Schulzrinne, H., "RTP Profile for Audio and Video Conferences
      with Minimal Control", RFC 1890, January 1996.

   12 Handley, M. and V. Jacobson, "SDP: Session Description Protocol",
      RFC 2327, April 1998.

   13 Luthi, P., "RTP Payload Format for ITU-T Recommendation G.722.1",
      RFC 3047, January 2001.


. Author's Address

   Robert Zopf
   Lucent Technologies
   INS Access VoIP Networks
   480 Red Hill Road
   Middletown, NJ 07748
   USA

   e-mail: zopf@lucent.com
   Tel:    1-732-615-4157
   Fax:    1-732-615-4526




















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