RTP Payload Format for Elementary Streams with MPEG Surround multi-channel audio
draft-schmidt-avt-mps-00.txt

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Abstract

This memo describes extensions for the RTP payload format defined in RFC3640 for the transport of MPEG Surround multi-channel audio. Additional MIME Type parameters are defined to signal backwards compatible transmission inside an MPEG-4 audio elementary stream. In addition a layered transmission scheme without using the MPEG-4 systems framework is presented to transport an MPEG Surround elementary stream via RTP in parallel with an RTP stream containing the downmixed audio data.

Table of Contents

1.  Introduction
2.  Conventions
3.  Definitions and Abbreviations
    3.1.  Definitions
    3.2.  Abbreviations
4.  Transport of MPEG Surround
    4.1.  Embedded spatial audio data in AAC payloads
    4.2.  MPEG Surround Elementary Stream
        4.2.1.  Low Bit-rate MPEG Surround
        4.2.2.  High Bit-rate MPEG Surround
5.  IANA Considerations
    5.1.  MIME Type registration
    5.2.  Registration of Mode Definitions with IANA
    5.3.  Usage of SDP
6.  Security Considerations
7.  Normative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

MPEG Surround (Spatial Audio Coding, SAC) (MPEG, “ISO/IEC International Standard 23003-1 - MPEG Surround (MPEG D),” 2007.) [23003‑1] is an International Standard that was finalized by MPEG in January 2007. It is capable of re-creating N channels based on M<N transmitted channels and additional control data. In the preferred modes of operating the spatial audio coding system, the M channels can either be a single mono channel or a stereo channel pair. The control data represents a significant lower data rate than the data rate required for transmitting all N channels, making the coding very efficient while at the same time ensuring compatibility with M channel devices.

The MPEG Surround standard incorporates a number of tools enabling features that allow for broad application of the standard. A key feature is the ability to scale the spatial image quality gradually from very low spatial overhead towards transparency. Another key feature is that the decoder input can be made compatible to existing matrixed surround technologies.

As an example, for 5.1 multi-channel audio, the MPEG Surround encoder creates a stereo (or mono) downmix signal and spatial information describing the full 5.1 material in a highly efficient parameterised format. The spatial information is transmitted alongside the downmix.

By using MPEG Surround, existing services can easily be upgraded to provide surround sound in a backward compatible fashion. While a stereo decoder in an existing legacy consumer device ignores the MPEG Surround data and plays back the stereo signal without any quality degradation, an MPEG Surround enabled decoder will deliver high quality multi-channel audio.

The MPEG Surround decoder can operate in modes that render the multi-channel signal to multi-channel output, stereo output or operate in a two-channel headphone mode to produce a virtual surround output signal.

2.  Conventions

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 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  Definitions and Abbreviations

3.1.  Definitions

This memo makes use of the definitions specified in [14496‑1] (MPEG, “ISO/IEC International Standard 14496-1 - Coding of audio-visual objects, Part 1 Systems,” 2004.), [14496‑3] (MPEG, “ISO/IEC International Standard 14496-3 - Coding of audio-visual objects, Part 3 Audio,” 2005.), [23003‑1] (MPEG, “ISO/IEC International Standard 23003-1 - MPEG Surround (MPEG D),” 2007.) and [RFC3640] (van der Meer, J., Mackie, D., Swaminathan, V., Singer, D., and P. Gentric, “RTP Payload Format for Transport of MPEG-4 Elementary Streams,” November 2003.). Frequently used terms are summed up for convenience:

Access Unit: An MPEG Access Unit is the smallest data entity to which timing information is attributed. In the case of audio, an Access Unit is the smallest individually accessible portion of coded audio data within an elementary stream.

AudioSpecificConfig(): Extends the class DecoderSpecificInfo(), as defined in [14496‑1] (MPEG, “ISO/IEC International Standard 14496-1 - Coding of audio-visual objects, Part 1 Systems,” 2004.) when the objectType indication refers to a stream complying with [14496‑3] (MPEG, “ISO/IEC International Standard 14496-3 - Coding of audio-visual objects, Part 3 Audio,” 2005.). AudioSpecificConfig() is used as the configuration structure for MPEG-4 Audio as specified in [14496‑3] (MPEG, “ISO/IEC International Standard 14496-3 - Coding of audio-visual objects, Part 3 Audio,” 2005.). It contains the field audioObjectType that distincts between the different audio codecs defined in [14496‑3] (MPEG, “ISO/IEC International Standard 14496-3 - Coding of audio-visual objects, Part 3 Audio,” 2005.), general audio information (e.g. the sampling frequency and number of channels) and further codec-dependent information structures.

SpatialSpecificConfig(): Configuration structure for MPEG Surround audio coding as specified in [23003‑1] (MPEG, “ISO/IEC International Standard 23003-1 - MPEG Surround (MPEG D),” 2007.). An AudioSpecificConfig() with an audioObjectType of value 30 contains a SpatialSpecificConfig() structure.

3.2.  Abbreviations

  AOT:  Audio Object Type
  ASC:  AudioSpecificConfig() structure
  AU:   Access Unit
  PLI:  Profile and Level Indication
  SSC:  SpatialSpecificConfig() structure

4.  Transport of MPEG Surround

From a top-level perspective MPEG Surround data can be subdivided into configuration data contained in the SpatialSpecificConfig() (SSC) and the SpatialFrame() that contains the MPEG Surround payload. The configuration data can be signaled in-band or out-of-band. In the case of in-band signaling the SSC is conveyed in an SacDataFrame() jointly with a SpatialFrame(). In the case of out-of-band signaling the SSC is transmitted to the decoder separately, e.g. by SDP means.

SpatialFrame()s may be transmitted either embedded into the downmix stream (Section 4.1 (Embedded spatial audio data in AAC payloads)) or as an individual elementary stream besides the downmix audio stream (Section 4.2 (MPEG Surround Elementary Stream)).

The buffer definition for AAC decoders limits the size of an AU as specified in [14496‑3] (MPEG, “ISO/IEC International Standard 14496-3 - Coding of audio-visual objects, Part 3 Audio,” 2005.). For high-bitrate applications that exceed this limit all MPEG Surround data MUST be put in a separate stream as defined in Section 4.2 (MPEG Surround Elementary Stream).

4.1.  Embedded spatial audio data in AAC payloads

[14496‑3] (MPEG, “ISO/IEC International Standard 14496-3 - Coding of audio-visual objects, Part 3 Audio,” 2005.) define the extension_payload() as a mechanism for transport of extension data inside AAC payloads. Typical extension data include SBR data and MPEG Surround data, i.e. a SacDataFrame() in extension_payload()s of type EXT_SAC_DATA. extension_payload()s reside inside the downmix AAC elementary stream. The resulting single elementary stream is transported as specified in [RFC3640] (van der Meer, J., Mackie, D., Swaminathan, V., Singer, D., and P. Gentric, “RTP Payload Format for Transport of MPEG-4 Elementary Streams,” November 2003.). As AAC decoders are required to skip unknown extension data, MPEG Surround data can be embedded backwards compatible and be transported with the mechanism already described in [RFC3640] (van der Meer, J., Mackie, D., Swaminathan, V., Singer, D., and P. Gentric, “RTP Payload Format for Transport of MPEG-4 Elementary Streams,” November 2003.).

The SacDataFrame() includes a SpatialFrame() and an optional header that contains an SSC. Any SSC in a SacDataFrame() MUST be identical to the SSC conveyed via SDP for that stream.

No new mode is introduced for SpatialFrame()s being embedded into AAC payloads. Either the modes AAC-lbr or AAC-hbr SHOULD be used. The additional MIME Type parameters as defined in Section 5.1 (MIME Type registration) MUST be present when SpatialFrame()s are embedded into AAC payloads.

For example:

  m=audio 5000 RTP/AVP 96
  a=rtpmap:96 mpeg4-generic/48000/2
  a=fmtp:96 streamType=5; profile-level-id=44; mode=AAC-hbr; config=131
    056E598; sizeLength=13; indexLength=3; indexDeltaLength=3; constant
    Duration=2048; MPS-profile-level-id=55; MPS-config=F1B4CF920442029B
    501185B6DA00;

In this example the stream specifies the HE-AAC Profile at Level 2 [Profile and Level Indication (PLI) 44] and the config string contains the hexadecimal representation of the HE-AAC ASC [audioObjectType=2 (AAC LC); extensionAudioObjectType=5 (SBR); samplingFrequencyIndex=0x6 (24kHz); extensionSamplingFrequencyIndex=0x3 (48kHz); channelConfiguration=2 (2.0 channels)] of the downmix AAC elementary stream using explicit backward compatible signaling.

Furthermore, the stream specifies the MPEG Surround Baseline Profile at Level 3 (PLI55) and the MPS-config string contains the hexadecimal representation of the MPEG Surround ASC [audioObjectType=30 (MPEG Surround); samplingFrequencyIndex=0x3 (48kHz); channelConfiguration=6 (5.1 channels); sacPayloadEmbedding=1; SSC=(48 kHz; 32 slots; 525 tree; ResCoding=1; ResBands=[0,13,13,13])].

Note that the a=fmtp lines of the example above have been wrapped to fit the page; they comprise each a single line in the SDP file.

4.2.  MPEG Surround Elementary Stream

MPEG Surround SpatialFrame()s can be present in an individual elementary stream. This stream complements the stream containing the downmix audio data, which may be coded by an arbitrary coding scheme. MPEG Surround elementary streams are packetized as specified in [RFC3640] (van der Meer, J., Mackie, D., Swaminathan, V., Singer, D., and P. Gentric, “RTP Payload Format for Transport of MPEG-4 Elementary Streams,” November 2003.). The mode signaled and used for an MPEG Surround elementary stream MUST be either MPS-hbr or MPS-lbr. The MPS-hbr mode SHALL be used when the frame size may exceed 63 bytes, e.g. when high-bitrate residual coding is in use.

The dependency relationships between the MPEG Surround elementary stream and the downmix stream are signaled as specified in [I‑D.ietf‑mmusic‑decoding‑dependency] (Schierl, T. and S. Wenger, “Signaling media decoding dependency in Session Description Protocol (SDP),” February 2008.).

The media clocks of the MPEG Surround elementary stream and the downmix stream SHALL operate in the same clock domain, i.e. the clocks MUST NOT drift. RTCP sender reports MUST indicate that the stream timestamps are not drifting, i.e. that a single sender report for each stream is sufficient to establish unambiguous timing. The sampling rate of the MPEG Surround signal and the decoded downmix signal MUST be identical.

If HE-AAC is used as the coding scheme for the downmix, the RTP clock-rate of the downmix MAY be the sampling rate of the AAC core, i.e. the clock-rate of the MPEG Surround elementary stream is an integer multiple of the clock-rate of the downmix stream.

Note that separate RTP streams have different random RTP timestamp offsets and therefore RTCP MUST be used to synchronize the coded downmix audio data and the MPEG surround elementary stream.

For example:

  a=group:DDP 1 2

  m=audio 5000 RTP/AVP 96
  a=rtpmap:96 mpeg4-generic/48000/2
  a=fmtp:96 streamType=5; profile-level-id=44; mode=AAC-hbr; config=2B1
    18800; sizeLength=13; indexLength=3; indexDeltaLength=3; constantDu
    ration=2048
  a=mid:1

  m=audio 5002 RTP/AVP 97
  a=rtpmap:97 mpeg4-generic/48000/6
  a=fmtp:97 streamType=5; profile-level-id=55; mode=MPS-hbr; config=F1B
    0CF920460029B601189E79E70; sizeLength=13; indexLength=3;  indexDelt
    aLength=3; constantDuration=2048
  a=mid:2
  a=depend:97 lay 1:96;

In this example the first stream specifies the High Efficiency AAC Profile at Level 2 (PLI44) and the config string contains the hexadecimal representation of the HE-AAC ASC [audioObjectType=2 (AAC LC); extensionAudioObjectType=5 (SBR); samplingFrequencyIndex=0x6 (24kHz); extensionSamplingFrequencyIndex=0x3 (48kHz); channelConfiguration=2 (2.0 channels)].

The second stream specifies Baseline MPEG Surround Profile at Level 3 (PLI55) and the config string contains the hexadecimal represenation of the ASC [AOT=30(MPEG Surround); 48 kHz; 5.1-ch; sacPayloadEmbedding=0; SSC=(48 kHz; 32 slots; 525 tree; ResCoding=1; ResBands=[7,7,7,7])]

Note that the a=fmtp lines of the example above have been wrapped to fit the page; they comprise each a single line in the SDP file.

4.2.1.  Low Bit-rate MPEG Surround

This mode is signaled by mode=MPS-lbr. This mode supports the transport of one or more complete Access Units, each consisting of a single MPEG Surround SpatialFrame(). The AUs can be variably sized and interleaved. The maximum size of a SpatialFrame() is 63 bytes. Fragmentation MUST NOT be used in this mode. Receivers MUST support de-interleaving.

The payload configuration is the same as in the AAC-lbr mode. It consists of the AU Header Section, followed by concatenated AUs. Note that Access Units are byte-aligned. The Auxiliary Section MUST be empty in the MPS-lbr mode. The one-octet AU-header MUST provide:

1. the size of each AAC frame encoded as 6 bits
2. 2 bits index information for computing the sequence (and hence timing) of each SpatialFrame().

The concatenated AU-header Section MUST be preceded by the 16-bit AU-header-length field.

In addition to the required MIME format parameters, the following parameters MUST be present with fixed values: sizeLength (fixed value 6), indexLength (fixed value 2) and indexDeltaLength (fixed value 2). The parameter maxDisplacement MUST be present when interleaving. SpatialFrame()s always have a fixed duration per AU; the fixed duration MUST be signaled by the MIME format parameter constantDuration.

The value of the "config" parameter is the hexadecimal representation of the ASC, as defined in [14496‑3] (MPEG, “ISO/IEC International Standard 14496-3 - Coding of audio-visual objects, Part 3 Audio,” 2005.) with an AOT of 30 and the sacPayloadEmbedding flag set to 0.

The "profile-level-id" parameter SHALL contain a valid PLI for MPEG Surround as specified in [14496‑3] (MPEG, “ISO/IEC International Standard 14496-3 - Coding of audio-visual objects, Part 3 Audio,” 2005.).

4.2.2.  High Bit-rate MPEG Surround

This mode is signaled by mode=MPS-hbr. This mode supports the transportation of either one fragment of an Access Unit or one complete AU or several complete AUs. Each AU consists of a single MPEG Surround SpatialFrame(). The AUs can be variably sized and interleaved. The maximum size of a SpatialFrame() is 8191 bytes. Receivers MUST support de-interleaving.

The payload configuration is the same as in the AAC-hbr mode. It consists of the AU Header Section, followed by either one SpatialFrame(), a fragment of a SpatialFrame() or several concatenated SpatialFrame()s. Note that Access Units are byte-aligned. The Auxiliary Section MUST be empty in the MPS-hbr mode. The two-octet AU-header MUST provide:

1. the size of each AAC frame encoded as 13 bits
2. 3 bits index information for computing the sequence (and hence timing) of each SpatialFrame(), i.e. the AU-Index or AU-Index-delta field.

Each AU-Index field MUST be coded with the value 0. The concatenated AU-header Section MUST be preceded by the 16-bit AU-header-length field.

In addition to the required MIME format parameters, the following parameters MUST be present with fixed values: sizeLength (fixed value 13), indexLength (fixed value 3) and indexDeltaLength (fixed value 3). The parameter maxDisplacement MUST be present when interleaving. SpatialFrame()s always have a fixed duration per AU; the fixed duration MUST be signaled by the MIME format parameter constantDuration.

The value of the "config" parameter is the hexadecimal representation of the ASC, as defined in [14496‑3] (MPEG, “ISO/IEC International Standard 14496-3 - Coding of audio-visual objects, Part 3 Audio,” 2005.) with an AOT of 30 and the sacPayloadEmbedding flag set to 0.

The "profile-level-id" parameter SHALL contain a valid PLI for MPEG Surround as specified in [14496‑3] (MPEG, “ISO/IEC International Standard 14496-3 - Coding of audio-visual objects, Part 3 Audio,” 2005.).

5.  IANA Considerations

This memo defines additional optional format parameters to the MIME subtype mpeg4-generic. These parameters SHALL only be used in combination with the AAC-lbr or AAC-hbr modes (cf. [RFC3640] (van der Meer, J., Mackie, D., Swaminathan, V., Singer, D., and P. Gentric, “RTP Payload Format for Transport of MPEG-4 Elementary Streams,” November 2003.) section 3.3).

5.1.  MIME Type registration

This memo defines the following additional optional parameters which SHALL be used if MPEG Surround data is present inside the payload of an AAC elementary stream.

MPS-profile-level-id: A decimal representation of the MPEG Surround Profile Level indication as defined in [14496‑3] (MPEG, “ISO/IEC International Standard 14496-3 - Coding of audio-visual objects, Part 3 Audio,” 2005.). This parameter MUST be used in the capability exchange or session set-up procedure to indicate the MPEG Surround Profile and Level that the decoder must be capable in order to decode the stream.

MPS-config: A hexadecimal representation of an octet string that expresses the AudioSpecificConfig (ASC) as defined in [14496‑3] (MPEG, “ISO/IEC International Standard 14496-3 - Coding of audio-visual objects, Part 3 Audio,” 2005.) for MPEG Surround. The ASC is mapped onto the hexadecimal octet string in an MSB-first basis. The AOT in this ASC SHALL have the value 30. The SSC inside the ASC MUST have the sacPayloadEmbedding flag set to 1.

5.2.  Registration of Mode Definitions with IANA

This memo defines the modes MPS-hbr and MPS-lbr.

5.3.  Usage of SDP

It is assumed that the MIME format parameters are conveyed via an SDP message as specified in [RFC3640] (van der Meer, J., Mackie, D., Swaminathan, V., Singer, D., and P. Gentric, “RTP Payload Format for Transport of MPEG-4 Elementary Streams,” November 2003.), section 4.4.

6.  Security Considerations

RTP packets using the payload format defined in this memo are subject to the security considerations of the RTP specification [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) and [RFC3640] (van der Meer, J., Mackie, D., Swaminathan, V., Singer, D., and P. Gentric, “RTP Payload Format for Transport of MPEG-4 Elementary Streams,” November 2003.) which is extended with this memo. This implies that confideniality of the media streams is achieved by encryption. Because the data compression used with this payload format is applied end-to-end, encryption may be performed on the compressed data so there is no conflict between the two operations.

7. Normative References

Authors' Addresses

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