


AVT Working Group                                                  M. Xu
Internet-Draft                                                 CS. Zheng
Intended status: Informational                                  T. Jiang
Expires: February 25, 2010                                     WX. Zhang
                                                               JP. Zhang
                                                   Digital Wave Co., Ltd
                                                         August 24, 2009


                    RTP Payload Format for DRA Audio
                          draft-xu-avt-dra-00

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Abstract

   The present document describes a RTP packaging scheme for DRA
   compressed audio data transmission, as well as the corresponding RTP
   payload format.  According to the properties of DRA compressed audio
   frame and the Maximum Transmission Unit (MTU) of network, the scheme
   provides 3 packaging modes for different coding and transmission
   requirements.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  3
   3.  Overview of DRA  . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  RTP Payload Format . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  RTP Header Usage . . . . . . . . . . . . . . . . . . . . .  4
     4.2.  Structure of the RTP DRA Payload Format  . . . . . . . . .  5
   5.  SDP related information for DRA Audio  . . . . . . . . . . . .  6
     5.1.  Media Type Registration  . . . . . . . . . . . . . . . . .  7
     5.2.  SDP Format for DRA audio . . . . . . . . . . . . . . . . .  8
       5.2.1.  Attributes (a=)  . . . . . . . . . . . . . . . . . . .  8
       5.2.2.  Media Description(m=)  . . . . . . . . . . . . . . . .  9
       5.2.3.  Examples . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10





















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1.  Introduction

   DRA multi-channel digital audio coding technology is capable of
   preserving high fidelity sounds over limited storage and limited
   transmission bandwidth, suitable for a wide variety of application
   areas including digital audio broadcasting, digital TV accompany
   sound, home theater, Internet streaming media, and personal media
   player.  In the fields of IP network transmission and real-time
   broadcasting, Real-Time Transport Protocol, as described in RFC 3550
   [RFC3550], can be employed for transmission and synchronization of
   DRA audio streams.
   This document describes an RTP payload format for transmitting audio
   data compressed by DRA technology .


2.  Requirements Language

   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 [RFC2119].


3.  Overview of DRA

   DRA digital audio coding technology keeps 24-bit precision (except
   deliberating quantization loss)for all channels.  Its supported
   channel configurations include commonly known stereo, 5.1 , 6.1, and
   7.1 surround sound, besides a large space reserved for future audio
   development (maximum 64.3 surround sound).  DRA can handle standard
   sampling frequencies spanning from 8kHz to 192kHz, including 44.1kHz
   and 48kHz.  There is no explicit restriction on DRA encoding bitrate,
   which is determined by the channel bandwidth and audio quality
   requirement.

   DRA has a fixed input frame length of 1024 samples for encoding, thus
   its time duration can be calculated according to sampling rate.  Bit
   count for encoded frame is controlled by encoding configuration
   parameters, which are recorded in the frame header.  Figure 1
   illustrates the structure of a DRA encoded frame described in
   "Specification for multichannel digital audio coding technology"
   (China national standard) [GB/T 22726-2008].

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Sync Word|Header|normal ch data|LFE ch data|error check|ancillary|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 1: DRA Frame Structure




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   Every audio frame starts with the sync word 0x7fff, then frame
   header.  Figure 2 illustrates parts of the header structure.

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |frame type |audio data frame length| other header information  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 2: DRA Frame Header's Structure

   Frame type: 1 bit, '0' for normal frame header, '1' for extension
   header.

   Audio data frame length: 10 bits (normal frame header) or 13 bits
   (extension header), representing the length from the sync word to the
   error check word, counting in unit of 32-bit.


4.  RTP Payload Format

   DRA encoded frame length varies with sampling rate and bitrate, and
   possibly overruns the Maximum Transmission Unit (MTU) in high bitrate
   modes.  In this case, a single audio frame is parted to several
   blocks.  On the other hand, DRA supports Variable Bit-Rate (VBR)
   encoding, allocating bit resources according to perceptible
   information in each frame.  Therefore, within the limit of MTU,
   several short audio frames are packaged into one RTP packet for
   higher transmission efficiency.

   3 RTP DRA payload formats are employed:

   o  Single-Frame packet: RTP payload comprising a single complete DRA
      compressed frame;

   o  Multi-Frame Packet: RTP payload comprising multiple complete DRA
      compressed frame;

   o  Frag-Frame Packet: RTP payload comprising a fragment of one DRA
      compressed frame.

   Subsection 4.2 has detailed description of the above 3 modes.

4.1.  RTP Header Usage

   The usage of RTP header information specified in this document
   observes the RTP header information defined inRFC 3550 [RFC3550].
   Figure 3 gives the RTP header format for the reader's convenience.





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           0                  1               2                3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X| CC  |M|     PT      |        sequence number        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         timestamp                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier          |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers           |
      |                          ....                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                Figure 3: RTP Header according to RFC 3550

   Marker bit (M): 1 bit

        Set to '1' only if at least one complete audio frame or the last
        block of an audio frame contained in the current RTP packet.

   Payload type (PT): 7 bits

        DRA RTP packet payload type designation is not discussed in the
        present document.  Its is set by the RTP profile of DRA RTP
        payload format, or dynamically set.

   TimeStamp (timestamp): 32 bits

        The timestamp of RTP is used to record the sampling time of the
        first audio frame in the current RTP packet.  For DRA RTP
        payload, the clock frequency corresponding to the time stamp is
        equal to the audio sampling frequency.  If the current RTP
        packet comprises more than one DRA audio frame, the sampling
        time for each audio frame can be derived from the time stamp
        plus the accumulated audio frame duration time.  In Frag-Frame
        Packet mode, all RTP packets belong to the same audio frame
        should have the same time stamp.

4.2.  Structure of the RTP DRA Payload Format

   RTP DRA payload comprises a 16-bit payload header and audio frame
   data.  Only the latter vary among the 3 RTP packaging modes.  Figure
   4 illustrates the payload structure of the Multi-Frame Packet mode.







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    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Payload Header | DRA frame  1 | DRA frame 2 |  ...   DRA frame n|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 4: Multi-Frame Packet RTP payload structure

   The payload header structure is given in figure 5.

                      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      |PM |   MBZ   |        N      |
                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 5: RTP payload header structure

   Packaging Mode (PM): 2 bits

        Indicating the current RTP packaging modes, as is shown in table
        1:

                    +----------+---------------------+
                    | PM value |         Mode        |
                    +----------+---------------------+
                    |     0    | Single-Frame Packet |
                    |     1    |  Multi-Frame Packet |
                    |     2    |  Frag-Frame Packet  |
                    |     3    |      Forbidden      |
                    +----------+---------------------+

                          Table 1: Packaging Mode

   Must Be Zero (MBZ, reserved): 6 bits

        Reserved for future use, all 6 bits must be set to '0'

   Number (N, block serial number or frame number): 8 bits

        If PM = 1, or Multi-Frame Packet mode, 'Number' is set to the
        number of DRA frames in the current RTP packet; if PM = 2,
        'Number' is set to the serial number of the current block in a
        DRA audio frame; if PM = 0, 'Number' is set to '1'.


5.  SDP related information for DRA Audio







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5.1.  Media Type Registration

   This section uses the template provided in RFC 4288 [RFC4288] and
   follows RFC 4855 [RFC4855].

   o  MIME media type name: audio

   o  MIME subtype name: vnd.dra

   o  Required parameters:

           rate: The RTP timestamp clock rate is equal to one of the
           audio sampling rates.  The sampling rates consist of 13
           possible values as specified in Table 10 of GB/T 22726-2008
           [GB/T 22726-2008]: 8000 Hz, 11025 Hz, 12000 Hz, 16000 Hz,
           22050 Hz, 24000 Hz, 32000 Hz, 44100 Hz, 48000 Hz, 88200 Hz,
           96000 Hz, 176400 Hz, 192000 Hz.

   o  Optional parameters: none

   o  Encoding considerations: Transport_provided

           This media type is framed and contains binary data.

   o  Security considerations:

           See section 7 of this document.

   o  Interoperability considerations: None

   o  Published specification:

           This specification and see GB/T 22726-2008 [GB/T 22726-2008].

   o  Applications which use this media type:

           Audio and audio for video can be multi-channel audio
           compressed with this media type.

   o  Additional information:

           Magic number(s): The first 16 bits of an DRA frame are fixed
           as synchronization word, which is 0x7FFF in hex expression.

   o  Person & email address to contact for further information:

           Jiang Tian <jiangt@digitalwave.cn>




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   o  Intended usage: COMMON

   o  Author/Change controller: XU Mao, et al.

5.2.  SDP Format for DRA audio

   Only the channel number and the sampling rate information are needed
   besides raw DRA bitstream defined in GB/T 22726-2008 [GB/T
   22726-2008] for decoding.  Both will be present in SDP defined in
   RFC 4566 [RFC4566]to assist decoding.  But there is no explicit
   configuration information embedded.

5.2.1.  Attributes (a=)

   a.  First, a short item.a=maxptime:<maximum packet time>

       This is an optional parameter, found by:

       <maximum packet time>

       = <maximum packet size in byte>*8*1000 /<minimum bitrate in
       kbps>.

       The maximum packet size is confined by Maximum Transmission Unit
       (MTU).

   b.  a=rtpmap:<payload type> <encoding name> / <clock rate>
       [/<encoding parameters>]

       This is a required attribute, where

       <payload type> = 97, defined in RFC 3551 [RFC3551], dynamically
       assigned

       <encoding name> = "dra"

       <clock rate> = DRA audio sampling rate

       <encoding parameters> = channel number

   c.  a=fmtp:<format> <format specific parameters>

       This is a optional attribute used if and only if the < format
       specific parameters> is not empty, where

       <format> = <payload type> = 97,

       <format specific parameters> currently includes only bitrate=#



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       where "#" is an integer bitrate in bps.

5.2.2.  Media Description(m=)

   m=<media> <port>/<number of ports> <proto> <fmt>

   This is a required field, where

   <media> = "audio"

   <port> = 58636 or any available even numbered port

   <number of ports>

   = number of RTP sessions, only used for multiple sessions

   <proto> = "RTP/AVP"

   <fmt> = <payload type> = 97.

5.2.3.  Examples

   This section provides two examples of the SDP data for DRA as
   follows:

   a.  48000Hz, stereo DRA bitstream:

       m=audio 58636 RTP/AVP 97

       a=rtpmap:97 dra/48000/2

   b.  320kbps, 44100Hz, 5.1 channel DRA bitstream, two sessions:

       m=audio 58800/2 RTP/AVP 97

       a=rtpmap:97 dra/44100/6

       a=fmtp:97 bitrate=320000


6.  IANA Considerations

   A new media subtype has been assigned for DRA by IANA; see Section
   5.1.







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7.  Security Considerations

   The payload format described in this document is subject to the
   security considerations defined in RFC 3550 [RFC3550].
   Confidentiality protection would have to be applied, so as to protect
   the users' provacy and contents with copyright.


8.  References

8.1.  Normative References

   [GB/T 22726-2008]
              You et al., "Specification for multichannel digital audio
              coding technology", Dec, 2008.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              July 2003.

   [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", BCP 13, RFC 4288, December 2005.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, February 2007.

8.2.  Informative References

   [RFC2736]  Handley, M. and C. Perkins, "Guidelines for Writers of RTP
              Payload Format Specifications", BCP 36, RFC 2736,
              December 1999.

   [RFC4184]  Link, B., Hager, T., and J. Flaks, "RTP Payload Format for
              AC-3 Audio", RFC 4184, October 2005.







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Authors' Addresses

   Mao Xu
   Digital Wave Co., Ltd
   Room 2002A, Building B, Cyber Tower, No.2,  ZhongGuanCun South Avenue
   Beijing,   100031
   China

   Phone: +86-10-5278-6263
   Email: xumao@digitalwave.cn


   ChuSheng Zheng
   Digital Wave Co., Ltd
   Room 2002A, Building B, Cyber Tower, No.2,  ZhongGuanCun South Avenue
   Beijing,   100031
   China

   Phone: +86-10-5278-6263
   Email: zhengcs@digitalwave.cn


   Tian Jiang
   Digital Wave Co., Ltd
   Room 2002A, Building B, Cyber Tower, No.2,  ZhongGuanCun South Avenue
   Beijing,   100031
   China

   Phone: +86-10-5278-6263
   Email: jiangt@digitalwave.cn


   WeiXiong Zhang
   Digital Wave Co., Ltd
   Room 2002A, Building B, Cyber Tower, No.2,  ZhongGuanCun South Avenue
   Beijing,   100031
   China

   Phone: +86-10-5278-6263
   Email: zhangwx@digitalwave.cn











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   JingPing Zhang
   Digital Wave Co., Ltd
   Room 2002A, Building B, Cyber Tower, No.2,  ZhongGuanCun South Avenue
   Beijing,   100031
   China

   Phone: +86-10-5278-6263
   Email: zhangjp@digitalwave.cn











































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