MPEG MMT标准详解：高效媒体传输在广播直播中的应用

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"MPEG MMT standard" MPEG MMT（MPEG Media Transport）标准是针对直播，特别是广播领域的一种高效媒体传输技术。该标准由国际标准化组织（ISO）和国际电工委员会（IEC）联合制定，由ISO/IEC JTC1/SC29/WG11工作组负责，旨在提供在异构环境中高效编码和媒体分发的方法。MPEG MMT的核心目标是优化媒体数据在不同网络条件下的传输效率，确保高质量的流媒体服务。文档内容包括标准的前言、介绍、范围、规范性参考、术语定义、约定、MMT数据模型、基于ISOBMFF的MPU（Media Processing Unit）、MMT提示轨道、包化传输等关键部分。 1. 范围：MPEG MMT标准规定了如何在实时流媒体服务，尤其是广播应用中，有效地传输和分发多媒体内容。它涵盖了从内容创建到用户接收的整个过程。 2. 术语与定义：标准中列出了相关的术语和缩写，如“包”（Package）、“资产”（Asset）、“媒体处理单元”（MPU）、“资产交付特性”（Asset delivery characteristics）等，以便于理解和应用。 3. MMT数据模型：数据模型包括了包、资产、MPU等概念。包是媒体内容的容器，资产是具有特定属性的媒体数据集合，MPU则包含了实际的媒体样本和元数据。 4. ISOBMFF-based MPU：基于ISOBMFF（ISO基础媒体文件格式）的MPU用于结构化媒体数据，ISOBMFF是一种广泛使用的媒体容器格式，能够容纳音频、视频和其他相关数据。 5. MMT提示轨道：这部分描述了MMT提示轨道的结构和功能，它提供了关于媒体数据包化和传输的附加信息，帮助接收端正确解码和播放。 6. 包化传输：标准详细阐述了如何通过网络将包化的媒体内容分发给用户，包括MMT协议、MMTP（MPEG Media Transport Protocol）有效载荷和操作流程。 MPEG MMT标准对于实现高效、可靠的广播级流媒体服务至关重要，它综合考虑了网络条件、带宽限制和延迟问题，确保即使在网络不稳定的情况下也能提供连续的媒体体验。这一标准的实施，对于广播公司、内容提供商和网络运营商来说，意味着更高的服务质量、更广泛的覆盖范围以及更好的用户体验。

ISO 23008-1:2016(E)

Each MPU constitutes a non-overlapping piece of an Asset, i.e. two consecutive MPUs of the same

Asset shall not contain the same media samples. Each MPU may be consumed independently by the

presentation engine of the MMT receiving entity.

6.2 Package

As shown in Figure 4, a Package is a logical entity. A Package shall contain one or more presentation

information documents such as one specified in ISO/IEC 23008-11, one or more Assets and for each

Asset an associated asset delivery characteristics (ADC). In other words, as processing of a Package

is applied per MPU basis and an Asset is a collection of one or more MPUs that share the same Asset

ID. It can be also considered that one Package is composed of one Presentation Information, one or

more MPUs and associated ADC for each Asset.

An Asset is a collection of one or more media processing units (MPUs) that share the same Asset ID.

An Asset contains encoded media data such as audio or video or a web page. Media data can be

either timed or non-timed.

Figure 4 — Overview of Package

Presentation information (PI) documents specify the spatial and temporal relationship among the

Assets for consumption. The combination of HTML5 and Composition Information (CI) documents

specified in ISO/IEC 23008-11 are an example of PI documents. In addition, Media Presentation

Description (MPD) specified in ISO/IEC 23009-1 can also be used as PI document. A PI document

may also be used to determine the delivery order of Assets in a Package. A PI document shall be

delivered either as one or more signalling messages defined in this part of ISO/IEC 23008 (see 9.3.3)

or as a complete document by some means that is not specified in this part of ISO/IEC 23008. In the

case of broadcast delivery, service providers may decide to carousel presentation information

documents and determine the frequency at which carouseling is to be performed.

Asset delivery characteristics (ADC) shall provide the required QoS information for transmission of

Assets. Multiple Assets can be associated with a single ADC. However, a single Asset shall not be

associated with multiple ADCs. This information can be used by the entity packetizing the Package

ISO 23008-1:2016(E)

to configure the fields of the MMTP payload header and MMTP packet header for efficient delivery

of the Assets.

ADC may provide information about an Asset that are relevant for the transport of that Asset.

6.3 Asset

An Asset is any multimedia data to be used for building a multimedia presentation. An Asset is a

logical grouping of MPUs that share the same Asset ID for carrying encoded media data. Encoded

media data of an Asset can be either timed data or non-timed data. Timed data are encoded media

data that has an inherent timeline and may require synchronized decoding and presentation of the

data units at a designated time. Non-timed data are any other type of data that does not have an

inherent timeline for decoding and presenting of its media content. The decoding time and the

presentation time of each item of non-timed data are not necessary related to that of other items of

the same non-timed data. For example, it can be determined by user interaction or presentation

information.

Two MPUs of the same Asset carrying timed media data shall have no overlap in their presentation

time.

Any type of data which is referenced by the presentation information is an Asset. Examples of

media data types which can be considered as an individual Asset are audio, video, or a Web page.

6.4 Media processing unit (MPU)

A media processing unit (MPU) is a media data item that may be processed by an MMT entity and

consumed by the presentation engine independently from other MPUs.

Processing of an MPU by an MMT entity includes encapsulation/de-capsulation and

packetization/de-packetization. An MPU may include the MMT hint track indicating the boundaries

of MFUs for media-aware packetization.

Consumption of an MPU includes media processing (e.g. encoding/decoding) and presentation.

For packetization purposes, an MPU may be fragmented into data units that may be smaller than an

Access Unit (AU). The syntax and semantics of MPU is not dependent on the type of media data

carried in the MPU.

MPUs of a single Asset shall have either timed or non-timed media.

An MPU may contain a portion of data formatted according to other standards, e.g. MPEG-4 AVC or

MPEG-2 TS.

For any Asset with asset_id X that depends on Asset with asset_id Y, the m-th MPU of the

Asset with asset_id X and the n-th MPU of the Asset with asset_id Y shall be non-

overlapping whenever m is not equal to n, i.e. no sample in the m-th MPU of Asset with asset_id

X is inside the time interval defined by the sample boundaries of the n-th MPU of Asset with

asset_id Y. Additionally, if the “sidx” box is present, the media intervals defined by the “sidx”

box shall be non-overlapping, i.e. no media sample in the k-th media interval (defined by the

“sidx” box) in an MPU is inside the time interval defined by the sample boundaries of the j-th

media time interval (defined by the “sidx” box) for j different from k. In the absence of an “sidx”

box, the concatenation of the j-th MPU of Asset with asset_id Y with the j-th MPU of the

Asset with asset_id X without its MPU metadata results in a valid MPU. When an “sidx” box is

present, the concatenation of the k-th media interval (defined by the “sidx” box) of the j-th

ISO 23008-1:2016(E)

MPU of Asset with asset_id Y with the k-th media interval (defined by the “sidx” box) of the

j-th MPU of the Asset with asset_id X following the metadata of the MPU with asset_id Y

results in a valid MPU.

A single MPU shall contain an integral number of AUs or non-timed data. In other words, for timed

data, a single AU shall not be fragmented into multiple MPUs. For non-timed data, a single MPU

contains one or more non-timed data items to be consumed by presentation engine.

An MPU shall be identified by an associated Asset identification (asset_id) and a sequence

number.

An MPU that contains timed media shall have at least one Stream Access Point (SAP) as defined in

ISO/IEC 14496-12:2015, Annex I. The first access unit of an MPU shall be a SAP (of SAP type 1, 2, or

3) for processing by an MMT entity. For timed media, this implies that the first AU in the MPU

payload is always the first in decoding order. For the MPU containing the data formatted according

to other standards, the MPU payload starts with the information necessary for the processing of

such a format. For example, if an MPU contains video data, the MPU payload contains one or more

groups of pictures and the decoder configuration information required to process them. For timed

media data, the presentation duration and the decoding order, and the presentation order of each

AU are signalled as part of the fragment metadata. The MPU does not have its initial presentation

time. The presentation time of the first AU in an MPU is described by the presentation information

document. The PI document specifies the initial presentation time of each MPU. Figure 5 depicts an

example of the timing of the presentation of MPUs from different Assets that is provided by the PI

document. In this example, the PI document specifies that the MMT receiving entity shall present

MPU #1 of Asset #1 and of Asset #2 simultaneously. At a later point, MPU #1 from Asset #3 is

scheduled to be presented. Finally, MPU #2 of Asset #1 and Asset #2 are to be presented in

synchronization. The specified presentation time for an MPU defines the presentation time of the

first AU in presentation order of that MPU. If any “elst” box is available, the indicated offset shall

be applied to the composition time of the first sample in presentation order of the MPU in addition

to the presentation time provided by any presentation information. The presentation time of every

MPU shall be provided as part of the presentation information.

Figure 5 — Example of mapping MPUs to the presentation timeline

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