RTP Payload Format for Elementary Streams with MPEG Surround Multi-Channel Audio
How to send surround-sound audio over RTP so that old stereo players still work and new ones get all the channels.
de Bont, F., Doehla, S., Schmidt, M., and R. Sperschneider, "RTP Payload Format for Elementary Streams with MPEG Surround Multi-Channel Audio", RFC 5691, DOI 10.17487/RFC5691, October 2009, <https://www.rfc-editor.org/info/rfc5691>.
@misc{rfc5691,
series = {Request for Comments},
number = 5691,
howpublished = {RFC 5691},
publisher = {RFC Editor},
doi = {10.17487/RFC5691},
url = {https://www.rfc-editor.org/info/rfc5691},
author = {F. de Bont and S. Doehla and M. Schmidt and R. Sperschneider},
title = {{RTP Payload Format for Elementary Streams with MPEG Surround Multi-Channel Audio}},
pagetotal = 12,
year = 2009,
month = oct,
} RFC text
Contents
Network Working Group F. de Bont
Request for Comments: 5691 Philips Electronics
Updates: 3640 S. Doehla
Category: Standards Track Fraunhofer IIS
M. Schmidt
Dolby Laboratories
R. Sperschneider
Fraunhofer IIS
October 2009
RTP Payload Format for Elementary Streams
with MPEG Surround Multi-Channel Audio
Abstract
This memo describes extensions for the RTP payload format defined in
RFC 3640 for the transport of MPEG Surround multi-channel audio.
Additional Media Type parameters are defined to signal backwards-
compatible transmission inside an MPEG-4 Audio elementary stream. In
addition, a layered transmission scheme that doesn't use 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.
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) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
Table of Contents
1. Introduction ....................................................2
2. Conventions .....................................................3
3. Definitions and Abbreviations ...................................3
3.1. Definitions ................................................3
3.2. Abbreviations ..............................................4
4. Transport of MPEG Surround ......................................4
4.1. Embedded Spatial Audio Data in AAC Payloads ................4
4.2. MPEG Surround Elementary Stream ............................5
4.2.1. Low Bitrate MPEG Surround ...........................7
4.2.2. High Bitrate MPEG Surround ..........................8
5. IANA Considerations .............................................8
5.1. Media Type Registration ....................................9
5.2. Registration of Mode Definitions with IANA .................9
5.3. Usage of SDP ..............................................10
6. Security Considerations ........................................10
7. References .....................................................11
7.1. Normative References ......................................11
7.2. Informative References ....................................11
1. Introduction
MPEG Surround (Spatial Audio Coding, SAC) [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 significantly 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 that enable
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 backwards-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 or stereo output, or it can 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].
3. Definitions and Abbreviations
3.1. Definitions
This memo makes use of the definitions specified in [14496-1],
[14496-3], [23003-1], and [RFC3640]. 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], when the objectType indication refers to a
stream complying with [14496-3]. AudioSpecificConfig() is used as
the configuration structure for MPEG-4 audio as specified in
[14496-3]. It contains the field audioObjectType, which
distinguishes between the different audio codecs defined in
[14496-3], 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]. An AudioSpecificConfig()
with an audioObjectType of value 30 contains a
SpatialSpecificConfig() structure.
3.2. Abbreviations
AOT: Audio Object Type
AAC: Advanced Audio Coding
ASC: AudioSpecificConfig() structure
AU: Access Unit
HE AAC: High Efficiency AAC
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(), which 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 a
SacDataFrame() jointly with a SpatialFrame(). In the case of out-of-
band signaling, the SSC is transmitted to the decoder separately,
e.g., by Session Description Protocol (SDP) [RFC4566] means.
SpatialFrame()s may be transmitted either embedded into the downmix
stream (Section 4.1) or as individual elementary streams besides the
downmix audio stream (Section 4.2).
The buffer definition for AAC decoders limits the size of an AU, as
specified in [14496-3]. 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.
4.1. Embedded Spatial Audio Data in AAC Payloads
[14496-3] defines the extension_payload() as a mechanism for
transport of extension data inside AAC payloads. Typical extension
data include Spectral Band Replication (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]. As AAC decoders are required
to skip unknown extension data, MPEG Surround data can be embedded in
backwards-compatible fashion and be transported with the mechanism
already described in [RFC3640].
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 mode AAC-lbr or the mode AAC-hbr SHOULD be
used. The additional Media Type parameters, as defined in
Section 5.1, SHOULD 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 that is using
explicit backwards-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 line of the example above has been wrapped to
fit the page; it would comprise 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]. 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 [RFC5583].
The media clocks of the MPEG Surround elementary stream and the
downmix stream SHALL operate in the same clock domain, i.e., the
clocks are derived from a common clock and 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 L1 L2
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:L1
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:L2
a=depend:97 lay L1:96
In this example, the first stream specifies the HE 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 representation
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 would each comprise a single line in the SDP file.
4.2.1. Low Bitrate 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 1-octet AU-header MUST provide:
1. the size of each AAC frame, encoded as 6 bits.
2. 2 bits of 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-
headers-length field.
In addition to the required Media 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 Media format parameter
constantDuration.
The value of the "config" parameter is the hexadecimal representation
of the ASC, as defined in [14496-3], 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].
4.2.2. High Bitrate 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 2-octet AU-header MUST provide:
1. the size of each AAC frame, encoded as 13 bits.
2. 3 bits of 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-headers-length
field.
In addition to the required Media 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 Media format parameter
constantDuration.
The value of the "config" parameter is the hexadecimal representation
of the ASC, as defined in [14496-3], 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].
5. IANA Considerations
This memo defines additional optional format parameters to the Media
type "audio" and its subtype "mpeg4-generic". These parameters SHALL
only be used in combination with the AAC-lbr or AAC-hbr modes (cf.
Section 3.3 of [RFC3640]) of "mpeg4-generic".
5.1. Media 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 and Level indication as defined in [14496-3]. 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 of 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],
for MPEG Surround. The ASC is mapped onto the hexadecimal octet
string in a most significant bit (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 section of this memo requests the registration of the "MPS-hbr"
value and the "MPS-lbr" value for the "mode" parameter of the "mpeg4-
generic" media subtype within the media type "audio". The "mpeg4-
generic" media subtype is defined in [RFC3640], and [RFC3640] defines
a repository for the "mode" parameter. This memo registers the modes
"MPS-hbr" and "MPS-lbr" to support MPEG Surround elementary streams.
Media type name:
audio
Subtype name:
mpeg4-generic
Required parameters:
The "mode" parameter is required by [RFC3640]. This memo
specifies the additional modes "MPS-hbr" and "MPS-lbr", in
accordance with [RFC3640].
Optional parameters:
For the modes "AAC-hbr" and "AAC-lbr", this memo specifies the
additional optional parameters "MPS-profile-level-id" and "MPS-
config". See Section 4.1 for usage details.
Optional parameters for the modes "MPS-hbr" and "MPS-lbr" may be
used as specified in [RFC3640]. The optional parameters "MPS-
profile-level-id" and "MPS-config" SHALL NOT be used for the modes
"MPS-hbr" and "MPS-lbr".
5.3. Usage of SDP
It is assumed that the Media format parameters are conveyed via an
SDP message, as specified in Section 4.4 of [RFC3640].
6. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [RFC3550], in the RTP payload format specification for
MPEG-4 elementary streams [RFC3640] (which is extended with this
memo), and in any applicable RTP profile. The main security
considerations for the RTP packet carrying the RTP payload format
defined within this memo are confidentiality, integrity, and source
authenticity. Confidentiality is achieved by encryption of the RTP
payload. Integrity of the RTP packets is achieved through a suitable
cryptographic integrity-protection mechanism. Such a cryptographic
system may also allow the authentication of the source of the
payload. A suitable security mechanism for this RTP payload format
should provide confidentiality, integrity protection, and source
authentication capable of at least determining if an RTP packet is
from a member of the RTP session.
The AAC audio codec includes an extension mechanism to transmit extra
data within a stream that is gracefully skipped by decoders that do
not support this extra data. This covert channel may be used to
transmit unauthorized data in an otherwise valid stream.
Note that the appropriate mechanism to provide security to RTP and
payloads following this memo may vary. It is dependent on the
application, the transport, and the signaling protocol employed.
Therefore, a single mechanism is not sufficient; although, if
suitable, usage of the Secure Real-time Transport Protocol (SRTP)
[RFC3711] is recommended. Other mechanisms that may be used are
IPsec [RFC4301] and Transport Layer Security (TLS) [RFC5246] (RTP
over TCP); other alternatives may exist.
7. References
7.1. Normative References
[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", 2009.
[23003-1] MPEG, "ISO/IEC International Standard 23003-1 - MPEG
Surround (MPEG D)", 2007.
[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.
[RFC3640] van der Meer, J., Mackie, D., Swaminathan, V., Singer, D.,
and P. Gentric, "RTP Payload Format for Transport of
MPEG-4 Elementary Streams", RFC 3640, November 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5583] Schierl, T. and S. Wenger, "Signaling Media Decoding
Dependency in the Session Description Protocol (SDP)",
RFC 5583, July 2009.
7.2. Informative References
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
Authors' Addresses
Frans de Bont
Philips Electronics
High Tech Campus 5
5656 AE Eindhoven,
NL
Phone: ++31 40 2740234
EMail: frans.de.bont@philips.com
Stefan Doehla
Fraunhofer IIS
Am Wolfmantel 33
91058 Erlangen,
DE
Phone: +49 9131 776 6042
EMail: stefan.doehla@iis.fraunhofer.de
Malte Schmidt
Dolby Laboratories
Deutschherrnstr. 15-19
90537 Nuernberg,
DE
Phone: +49 911 928 91 42
EMail: malte.schmidt@dolby.com
Ralph Sperschneider
Fraunhofer IIS
Am Wolfmantel 33
91058 Erlangen,
DE
Phone: +49 9131 776 6167
EMail: ralph.sperschneider@iis.fraunhofer.de
AI ELI5
On this page
§1Why MPEG Surround Needs a Payload Format
MPEG Surround (Spatial Audio Coding) is an international standard finalized by MPEG in January 2007. It can re-create N channels from M < N transmitted channels plus additional control data. The M channels are usually a single mono channel or a stereo pair.
For 5.1 audio, the encoder makes a stereo or mono downmix and a compact description of the full 5.1 material. The control data takes far less bandwidth than sending every channel. A legacy stereo decoder ignores it and plays the stereo signal with no quality loss, while a surround-enabled decoder produces multi-channel audio.
This RFC extends the RTP payload format of RFC 3640 to carry this data and adds media type parameters to signal it.
Imagine a song recorded for many speakers, plus a small note saying how to spread it out. Plain players use the simple version and ignore the note. Fancy players use the note to fill the room.
§3Configuration and Frames
The RFC splits MPEG Surround data into two parts. The SpatialSpecificConfig() (SSC) holds configuration. The SpatialFrame() holds the actual surround payload.
The configuration can be sent in-band, where the SSC travels in a SacDataFrame() together with a SpatialFrame(). It can also be sent out-of-band, for example with the Session Description Protocol (RFC 4566). An AudioSpecificConfig() with an audioObjectType of 30 contains an SSC.
Spatial frames can be embedded in the downmix stream or sent as separate elementary streams next to it. The AAC decoder buffer limits the size of an Access Unit, so for high-bitrate uses that exceed the limit, all MPEG Surround data MUST go in a separate stream.
There are two kinds of pieces: a setup sheet that says how things are arranged, and the sound pieces themselves. The setup sheet can ride along with the sound or be handed over separately.
§4.1Embedding Surround Data in AAC
MPEG-4 Audio lets extra data ride inside AAC payloads as an extension_payload(). MPEG Surround data uses the type EXT_SAC_DATA. Because AAC decoders are required to skip unknown extension data, the result is a single stream that older decoders handle unchanged. It is transported as specified in RFC 3640.
No new mode is introduced here. Either AAC-lbr or AAC-hbr SHOULD be used, and the new MPS-profile-level-id and MPS-config parameters SHOULD be present. Any SSC inside a SacDataFrame() MUST be identical to the SSC conveyed through SDP for that stream.
The RFC gives an SDP example with mode=AAC-hbr in which the downmix is HE AAC Profile at Level 2 (PLI 44) and the surround part is MPEG Surround Baseline Profile at Level 3 (PLI 55).
The surround note is tucked inside the normal audio like a small envelope inside a package. Old players do not open envelopes they do not know, so they just play the music.
§4.2A Separate Surround Stream
The spatial frames can instead form their own elementary stream that complements the downmix stream. The downmix can use any coding scheme. The mode for this stream MUST be MPS-hbr or MPS-lbr, and MPS-hbr SHALL be used when a frame may exceed 63 bytes.
The two streams are linked with the dependency signaling of RFC 5583. Their media clocks SHALL be in the same clock domain and MUST NOT drift, and the sampling rate of the surround signal and the decoded downmix MUST be identical. Because separate RTP streams have different random timestamp offsets, RTCP MUST be used to synchronize them. RTCP sender reports MUST indicate that the timestamps are not drifting.
If HE AAC is the downmix coder, the downmix RTP clock rate MAY be the sampling rate of the AAC core.
Here the surround note travels in its own envelope next to the music. Both must use the same clock so they line up, like two runners keeping exactly the same pace.
§4.2.1The Low and High Bitrate Modes
MPS-lbr carries one or more complete Access Units, each a single SpatialFrame() of at most 63 bytes. Fragmentation MUST NOT be used. Its 1-octet AU header gives the frame size in 6 bits and 2 bits of index information. The parameters sizeLength, indexLength and indexDeltaLength MUST be present with fixed values 6, 2 and 2.
MPS-hbr carries one fragment of an Access Unit, one complete one, or several complete ones, with a SpatialFrame() of at most 8191 bytes. Its 2-octet AU header gives the size in 13 bits and 3 bits of index information, and the fixed values are 13, 3 and 3. Each AU-Index field MUST be coded as 0.
In both modes the Auxiliary Section MUST be empty, receivers MUST support de-interleaving, maxDisplacement MUST be present when interleaving, and constantDuration MUST signal the fixed duration. The config value is the hexadecimal ASC with an object type of 30 and sacPayloadEmbedding set to 0.
There is a small-box mode and a big-box mode. Small boxes hold tiny pieces and can never be split. Big boxes hold bigger pieces and may be split across several packets.
| Property | MPS-lbr | MPS-hbr |
|---|---|---|
| Max SpatialFrame size | 63 bytes | 8191 bytes |
| AU header | 1 octet | 2 octets |
| sizeLength | 6 | 13 |
| indexLength and indexDeltaLength | 2 and 2 | 3 and 3 |
| Fragmentation | MUST NOT be used | One fragment of an AU allowed |
§5Media Type Parameters and Modes
The RFC adds optional parameters to the audio/mpeg4-generic media type. They SHALL be used when MPEG Surround data is inside an AAC elementary stream, and only with the AAC-lbr or AAC-hbr modes.
MPS-profile-level-id: a decimal MPEG Surround Profile and Level indication, used in capability exchange or session set-up to say what the decoder must be able to handle.MPS-config: a hexadecimal ASC for MPEG Surround. Its object type SHALL be 30 and its SSC MUST havesacPayloadEmbeddingset to 1.
It also registers the MPS-hbr and MPS-lbr values for the mode parameter. The two new parameters SHALL NOT be used with those two modes. The media format parameters are assumed to be conveyed in SDP as described in RFC 3640.
The document adds new labels to the official list so everyone writes the same words on the box. Some labels go with one box type and must never be mixed with the other.
§6Security Considerations
These packets inherit the security considerations of the RTP specification (RFC 3550) and of RFC 3640. The main concerns are confidentiality, integrity and source authenticity. The right mechanism depends on the application, transport and signaling, but SRTP (RFC 3711) is recommended if suitable. IPsec and TLS are also mentioned.
The RFC adds one specific warning. The AAC extension mechanism is gracefully skipped by decoders that do not understand it, and this covert channel may be used to transmit unauthorized data in an otherwise valid stream.
Because old players ignore the hidden envelope, someone could sneak a secret message in it. The document warns about that and suggests locking the whole package with encryption.