DVB-MPEG-Dash：IP网络中ISO BMFF基DVB服务的传输标准

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DVB:MPEG-DASH 是一种结合了Digital Video Broadcasting (DVB)标准与MPEG-DASH（Moving Picture Experts Group Dynamic Adaptive Streaming over HTTP）技术的协议。它专为基于IP网络传输ISO Base Media File Format (ISOBMFF) 基础上的DVB服务而设计。DVB Document A168，发布于2016年7月，是DVB蓝皮书中的一份重要参考，旨在规范在互联网协议（IP）网络环境下，如何确保高质量、灵活的视频流传输。该文档的核心关注点在于定义和约束DVB对MPEG-DASH架构的适应性，特别是针对实时（Live）和点播（On-Demand）服务。以下是主要内容的详细解读： 1. **知识产权**：文档开头强调了知识产权问题，这在多媒体流传输中至关重要，以保护版权和合规性。 2. **前言**：简要介绍了DVB:MPEG-DASH的目的，即通过标准化MPEG-DASH技术，使之能够无缝融入DVB服务生态，提升服务质量。 3. **术语和定义**：这部分提供了关键术语的定义，例如“DVB Profile of MPEG-DASH”表示DVB对于MPEG-DASH规范的具体实现要求。定义和缩写列表有助于读者理解文中使用的专业术语。 4. **DASH约束、HTTP支持与同步**： - **DVB Profile of MPEG-DASH**：明确了DVB在MPEG-DASH中的角色，可能包括编码格式、内容分发策略以及数据包组织规则。 - **Media Presentation Description Constraints**：详细列出针对媒体播放描述的不同部分（如Period, Adaptation Set, Representation等）的具体限制，比如对于实时直播和点播服务的不同要求。 - **Segment Format Constraints**：规定了媒体片段的格式规范，确保不同设备间的兼容性和一致性。 - ** Presence of Attributes**：可能涉及元数据或自适应参数的必要性，如适应性级别、编码配置等。 4.2. **Period元素的约束**：这部分深入讨论了如何针对不同的服务类型（如直播和点播）对Period元素进行限制，包括共同约束和特定于实时或点播模式的额外条件。 DVB:MPEG-DASH文档不仅定义了一套用于DVB服务在IP网络上高效传输的标准，还提供了详细的实践指导，帮助内容提供商和接收端实现流畅、兼容的视频内容分发。通过遵循这些约束，DVB服务能够确保跨平台、跨终端的高质量观看体验。

DVB BlueBook A168

MPEG Moving Pictures Expert Group

NAL Network Abstraction Layer

NTP Network Time Protocol

PPS Picture Parameter Set

RFC Request For Comments

SD Standard Definition

SEI Supplemental Enhancement Information

SMPTE Society of Motion Picture and Television Engineers

SNTP Simple Network Time Protocol

SPS Sequence Parameter Set

TCP Transmission Control Protocol

TTML Timed Text Markup Language

TV TeleVision

TVA TV Anytime

URI Uniform Resource Identifier

URL Uniform Resource Locator

URN Uniform Resource Name

VoD Video on Demand

VUI Video Usability Information

XML eXtensible Markup Language

4 DASH Constraints, HTTP Support, and

Synchronization

4.1 DVB Profile of MPEG-DASH

The DVB Profile of MPEG-DASH, known as an "interoperability point" by MPEG, is based on the merging of the

ISO/IEC 23009-1 [1], ISO Base media file format live profile and ISO Base media file format On Demand profile. In

addition it includes "xlink" but only in combination with the actuate property set to "onLoad".

The DVB profile excludes multiplexed representations, segment list addressing and "xlink" with the actuate property set

to "onRequest".

The URN for the profile (MPEG Interoperability Point) shall be "urn:dvb:dash:profile:dvb-dash:2014".

4.2 Media Presentation Description Constraints

4.2.1 General

The MPD shall conform to the constraints in clause 4.2 in addition to the rules for the MPD as defined in

ISO/IEC 23001-9 [1], clause 7.3.

NOTE: MPDs, or their constituent parts, which do not conform to the constraints in clause 4.2 may be ignored by

the client.

4.2.2 Constraints on Period elements (common Period constraints)

 The Subset element may be ignored.

 The Period.SegmentList element shall not be present.

 If a Period element contains multiple Adaptation Sets with @contentType="video" then at least one

Adaptation Set shall contain a Role element with @schemeIdUri="urn:mpeg:dash:role:2011" and

@value="main".

 Remote elements with @xlink:actuate set to "onRequest" may be ignored.

 In a remote period @xlink:actuate shall not be present to avoid circular and repeated resolution

 Each Period element shall conform to either clause 4.2.3 or 4.2.6.

DVB BlueBook A168

NOTE: This means that each Period can be either based on ISO Base media file format live profile or ISO Base

media file format On Demand profile, but not any combination of both.

4.2.3 Constraints on Period elements conforming to Live profile

 AdaptationSet elements that do not conform to clause 4.2.4 may be ignored.

4.2.4 Constraints on AdaptationSet elements (for Live Periods)

 The @group attribute may be ignored.

 Adaptation Sets which contain BaseURL elements with Absolute BaseURLs may be ignored.

 Any AdaptationSet with a ContentComponent element may be ignored.

NOTE 1: This is so that Adaptation Sets with multiplexed media streams may be ignored.

 AdaptationSet element can be ignored unless AdaptationSet.SegmentTemplate is present and/or the

Representation.SegmentTemplate element is present for each Representation within this Adaptation Set.

NOTE 2: SegmentTimeline is supported according to ISO/IEC 23009-1 [1].

 AdaptationSet element that contains more than one Representation can be ignored unless all of the following

hold:

- AdaptationSet@segmentAlignment is present and has value of 'true' or '1'.

- AdaptationSet@startWithSAP is present and has value of 1 or 2.

- The MPD@maxSegmentDuration attribute is present or MPD@type="static"

 Representation elements that do not conform to 4.2.5 may be ignored.

NOTE 3: AdaptationSets may legitimately include one or more pairs of Representations which, under the

requirements of clause 10.4, conformant clients are not required to switch between. One example use case

for such an AdaptationSet can be found in clause 11.4.

4.2.5 Constraints on Representation elements (for Live Periods)

 Representations with a value of the @mimeType attribute starting with a string other than "xxx/mp4" where

xxx is one of "video", "audio", "application" or "text" may be ignored. Additional profile or codec specific

parameters may be added to the end of the value of the @mimeType attribute.

 Representations not inferred to have an @profiles attribute equal to "urn:dvb:dash:profile:dvb-dash:isoff-ext-

live:2014" may be ignored.

 Representations which contain BaseURL elements with Absolute BaseURLs may be ignored.

4.2.6 Constraints on Period elements conforming to On Demand profile

 The Period.SegmentTemplate element shall not be present.

 AdaptationSet elements that do not conform to clause 4.2.7 may be ignored.

4.2.7 Constraints on AdaptationSet element (On Demand)

 The @group attribute may be ignored.

 Any AdaptationSet with a ContentComponent element may be ignored.

NOTE 1: This is so that Adaptation Sets with multiplexed media streams may be ignored.

 AdaptationSet element can be ignored unless for each Representation within this Adaptation Set

Representation.BaseURL is present.

DVB BlueBook A168

 If either the AdaptationSet.SegmentList or the AdaptationSet.SegmentTemplate element is present in an

AdaptationSet element then this AdaptationSet element may be ignored.

 If an AdaptationSet element contains more than one Representation element, and this AdaptationSet element

can be ignored unless AdaptationSet@subsegmentAlignment is present and has value of 'true'; and

AdaptationSet@subsegmentStartsWithSAP is present and has value of 1 or 2.

 Representation elements that do not conform to clause 4.2.8 may be ignored.

NOTE 2: AdaptationSets may legitimately include one or more pairs of Representations which, under the

requirements of clause 10.4, conformant clients are not required to switch between. One example use case

for such an AdaptationSet can be found in clause 11.4.

4.2.8 Constraints on Representation element (On Demand)

 Representations with a value of the @mimeType attribute starting with a string other than "xxx/mp4" where

xxx is one of "video", "audio", "application" or "text" may be ignored. Additional profile or codec specific

parameters may be added to the end of the value of the @mimeType attribute.

 Representations not inferred to have an @profiles attribute equal to "urn:dvb:dash:profile:dvb-dash:isoff-ext-

on-demand:2014" may be ignored.

 If either the Representation.SegmentList or the Representation.SegmentTemplate element is present in a

Representation element then this Representation element may be ignored.

 If the Representation element does not contain a BaseURL element then this Representation element may be

ignored.

 If Representation consists of a single Segment that complies with Indexed Media Segment or Indexed Self-

Initializing Media Segment, this Representation element can be ignored unless SegmentBase@indexRange is

present.

4.3 Segment Format Constraints

For Representations and Segments referred to by the Representations in the profile-specific MPD for this profile, the

following constraints shall be met:

 For Periods conforming to clause 4.2.3 or 4.2.6:

- Representations and segments shall comply with the formats defined in ISO/IEC 23009-1 [1], clause 7.3.

- In Media Segments, all Segment Index ('sidx') and Subsegment Index ('ssix') boxes, if present, shall be

placed before any Movie Fragment ('moof') boxes.

- Subtitle segments shall be available at or before the time at which other media segments with which they

are presented become available.

- All the initialization segments for Representations within an Adaptation Set shall have the same sample

entry type (see ISO/IEC 14496-12 [6]). For example the inclusion of 'avc1' and 'avc3' based

Representations or 'avc3' and 'hev1' based Representations within an Adaptation Set is not permitted.

- The movie fragment box ('moof') shall contain only one track fragment box ('traf').

- All Representations within an Adaptation Set shall have the same track_ID. The track_ID is located in

the track header box and track fragment header box.

 For Periods conforming to clause 4.2.6:

- Each Representation shall have only one Segment. This segment shall comply with the Self-Initializing

Media Segment as defined in clause 6.3.5.2 of ISO/IEC 23009-1 [1].

- The segment shall contain only one single Segment Index box ('sidx') for the entire segment.

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