DASH相关

谁践踏了优雅 2022-08-24 15:30 20阅读 0赞

# DASH Streaming Support #

转自  [http://blog.csdn.net/kevin\_android\_123456/article/details/8122888][http_blog.csdn.net_kevin_android_123456_article_details_8122888]

Posted on  [1 February 2012][]  by  [Jean][]

For lunch today

*  [DASH Basics: MPD and Segments][DASH Basics_ MPD and Segments]
 *  [ISO Base Media File Format][]
 *  [MPEG-2 TS][]
 *  [ Multiple Representations][Multiple Representations]
 *  [Osmo4 Playback][]
 *  [Some guidelines][]

We have been updating the support for DASH Streaming (ISO/IEC 23009-1) in GPAC as of revision 3849. The update covers both content generation tools and player, and both DASH and HLS support.

For more details on what is DASH and HTTP streaming, please refer to [this post][].

## DASH Basics: MPD and Segments ##

Let’s quickly summarize how a DASH content is made of:

*  MPD: an XML document describing where the various media resources present in the content are located. The media resources can be single-media (for example, a video-only MP4 file) or a multiplexed set of streams (for example an AV MPEG-2 Transort Stream). Streams can be scalable (such as SVC) but we won’t go into such details as GPAC doesn’t support advanced description of scalable streams in DASH. Some media resources may exist in different versions, for example different bitrate or language or resolutions. In DASH, such a “version” of the stream is called a representation, and all representations are grouped together in an AdaptationSet.
 *  segment: a continuous part of a media resource. The segment is the smallest part of the media that can be located in the MPD. What a segment exactly contains depends on the underlying media format of the content.
 *  subsegment: a continuous part of a segment, or of a subsegment.
 *  sidx: short name for SegmentIndexBox, this is an ISOBMF (MP4/3GP container) structure describing a segment by giving its earliest presentation time, how the segment is further divided into subsegments, random access points locations (byte offset) and timing in the segment payload. The goal of the SIDX is to build an index of the segment at a given granularity to simplify trick modes (seeking, fast-forward, fast-rewind, …).

There are several ways to refer to a segment in an MPD. If the file is made of a single segment (-single-segment option for ISOBMF), one will likely use SegmentBase element. If a file is made of several segments, each segment will be identified by the SegmentList syntax in the MPD, using byte ranges. For other cases, we need to instruct MP4Box how to refer to segments (and how to store them as well). The following switches are defined:

*  \-segment-ext EXT: tells MP4Box to generate segments with EXT extension (by defaultm4s for ISOBMF and or ts for MPEG-2)
 *  \-segment-name NAME: tells MP4Box to generate each segment in a dedicated file, called NAME%d.EXT. NAME can also have %s in it, in which case %s will be replaced by the name of the file being dashed without folder indication and extension. By default, such segments will be stored using the SegmentList syntax in the MPD.
 *  \-url-template: if set when generating segements in different files, the segments will be refered to using the SegmentTemplate syntax in the MPD.

## ISO Base Media File Format ##

For content based on ISOBMF (ISO/IEC 14496-12), MP4Box can be used to cut files into DASH segments. Before going any further, some definitions will be needed:

*  segment: for ISOBMF, a segment is a consecutive set of movie fragments. Each movie fragment is composed of a moof box followed by mdat box(es), and all data adressing in the mdat(s) are done using relative offsets in the moof.
 *  subsegment: a part of a segment, made of a consecutive set of movie fragments. A subsegment can be further divided in subsegments, until only a single movie fragment per subsegment is present.

With that in mind, we can generate DASH content by playing with the following MP4Box parameters:

*  \-dash X: produce segments of roughly X milliseconds.
 *  \-frag Y: use movie fragments of roughly Y milliseconds. By default, fragments duration is 500 milliseconds.
 *  \-rap:  attempts to cut segments so that they all start with an access point (IDR, I-frame or beginning of a gradual decoding refresh for example).
 *  \-subsegs-per-sidx N: specifies how many subsegments per sidx we would like. This only covers the first level of segment spliting (MP4Box doesn’t handle subsegments subdivision into subsegments). Noticable values are:

>  *  <0: disable: sidx will not be produced
>  *  0: a single sidx box is used for the entire segment, and each subsegment is made of a single movie fragment (i.e., there will be X/Y subsegments in sidx). This is the default value.  
>     
>  *  >0: produces X/Y/N subsegments referenced in the first sidx.

*  \-daisy-chain: this is only used when producing multiple subsegments per segment (-subsegs-per-sidx). If specified, subsegments will be described in SIDX in which the last entry (subsegment) points to the next SIDX. Otherwise, multiple SIDXs will be stored in a hierarchical way, with the first SIDX pointing to each SIDX of the subsegments.
 *   -single-segment: special mode indicating that the file should be segmented as one single segment. In that case, the dash duration X becomes the subsegment duration, and a single sidx is produced before any movie fragment.

Now let’s see an example.

Dashing a file with 10 seconds, rap-aligned segments with a fragment duration (i.e. subsegment duration since we don’t subdivide the SIDX)  of 1 sec:

MP4Box -dash 10000 -frag 1000 -rap test.mp4

The same with a separated segment using template addressing, and 5 subsegments per segments:

MP4Box -dash 10000 -frag 1000 -rap -segment-name myDash
              -subsegs-per-sidx 5 -url-template test.mp4

Generating an onDemand profile DASH file (single segment) is just as simple:

MP4Box -dash 10000 -frag 1000 -rap -single-segment test.mp4

## MPEG-2 TS ##

MP4Box can also be used to segment MPEG-2 TS files. The same options as the ISOBMF case are used, with the following restrictions:

*  \-single-segment, -frag, -subsegs-per-sidx and -daisy-chain are ignored
 *  \-rap splits at the PAT preceeding the RAP found, but does not repacketize the TS to make sure it begins with the RAP

For example, spliting a TS in 10 seconds segments can be done with

MP4Box -dash 10000 -url-template -segment-name segments test.ts

Also note that it is possible to use MP4Box to translate an existing m3u8 (Apple HLS) to a conformant MPD, using the -mpd switch:

MP4Box -mpd test.mpd [-url-template] [http://...]myfile.m3u8

##  Multiple Representations ##

You now know how to create a conformant DASH content from a given file, but what about the ‘A for Adaptive’ in DASH ? At first thought it would just be enough to let you with a bunch of MPD and a good XSLT, to produce your final MPD (which you will have to do anyway I believe). However, there are some tricks in the segment generation itself that cannot be easily done.

The most problematic thing is that, when building ISOBMF files designed for bitstream switching, the initial bootstrap of the DASH session (i.e. moov and co) must contain all sample descriptions used in all representations of the media. Therefore, we will need MP4Box here to generate files with correct sample descriptions, and segments with correct sampleDescriptionIndex

Although this might look a bit complex, the process is itself quite simple; assuming you have encoded file1, .., fileN version of your movie, you can generate a nice adaptive MPD as follows:

MP4Box -dash 10000 \[other options as seen above\] -out final.mpd file1.mp4 … fileN.mp4

This works for both ISOBMF and TS files. Additionnaly, you don’t want segments to be overriden when generating segments in dedicated files for each representation. MP4Box gives you the possibility to format the name of the segment using -segment-name:

MP4Box -dash 10000 -segment-name mysegs_%s -url-template
             -out final.mpd file1.mp4 ... fileN.mp

## Osmo4 Playback ##

You can test DASH playback in GPAC using Osmo4/MP4Client. The player supports a good part of the technology:

*  playback from an HTTP server or from local storage for test purposes.
 *  Media segments based on TS and ISOBMF
 *  independent component download (one adaptation set for audio, one for video)
 *  most MPD syntax is supported
 *  (some) bitstream switching. ISOBMF is not complete, multiple SampleDescription are not supported yet
 *  manual quality switching by using ctrl+h and ctrl+l
 *  basic automatic quality switching when playing HTTP urls

## Some guidelines ##

DASH is still quite new, and few players support it. The main problematic, especially with ISOBMF, is to address the bitstream switching scenario while not breaking existing implementations. We would therefore recommend:

*  do NOT mix codecs in your various representations of the media streams (whether multiplexed or not). This is not supported by existing players and will break seemless switching in most cases. And there is no point doing so, so don’t do it
 *  for AVC video, use dedicated SPS and PPS IDs for your different streams. This can even be done with x264 using –sps-id option. This will ensure that the produced files can still be played by most implementations supporting movie fragments.
 *  Obey the specification: use the same codec for each media in an adaptation set.

=================================================================================

# DASH #

转自  [http://blog.csdn.net/kevin\_android\_123456/article/details/8122879][http_blog.csdn.net_kevin_android_123456_article_details_8122879]

MP4Box can be used to generate content conformant to the MPEG-DASH specification, aka ISO/IEC 23009-1 available in [ISO Publicly Available Standards][].

For more details on what is DASH and HTTP streaming, please refer to [this post][]. For more help, type MP4Box -h dash

\-dash Duration : enables DASH segmentation of input files with the given segment duration. For onDemand profile, where each media presentation is a single segment, this option sets the duration of a subsegment.

\-frag dur\_in\_ms : specifies the duration of subsegments in ms. This duration is always less than the segment duration. By default (when not set), the subsegment duration is the DASH duration, i.e. there is only one subsegment per segment. For onDemand profile, where each media presentation is a single segment, this option sets the duration of a subsegment’s subsegment.

\-out filename specifies output file name for MPD. May use relative path. All segments will be produced in the same directory as the MPD.

\-tmp dirname specifies a directory for temporary file creation (the default temporary directory is OS-dependent).

\-dash-profile NAME specifies the target DASH profile: onDemand, live, main, simple,full. This will set default option values to ensure conformance to the desired profile.

\-rap forces segments to begin with random access points. Segment duration may not be exactly what asked by \-dash switch since encoded video data is not modified.

===============================================================================================

# 行动影音串流标准MPEG-DASH #

转自 [http://laoyaoba.com/ss6/html/74/n-350374.html][http_laoyaoba.com_ss6_html_74_n-350374.html]

今日电视、手机、平板、PC迈向多屏应用整合的时代，其中线上行动影音串流技术是相关影视服务能否成功的关键。资策会资深分析师朱南勋在第一届智慧媒体论坛《Define New TV》会议中指出，MPEG-DASH (Dynamic Adaptive Streaming over HTTP)是一项值得关注的线上影音串流标准。

MPEG-DASH是在2011年底由MPEG和ISO共同制定的标准，透过HTTP共同影音档案通讯协定，可使高品质影音内容透过网路传送到联网电视、机上盒及行动终端设备，提供Live、VoD 、Time-shift等线上影音应用，并适用于现存网路骨干，同时支援五大DRM系统。

MPEG-DASH标准的特色包括支援不同网路基础设施和多样化终端设备、与标准网路伺服器和视讯缓冲技术相容、支援窍门模式（trick mode）/多语言字幕及声道/广告插播、支援保护内容的DRM数位许可权管理技术。

朱南勋表示，此标准除对OTT服务业者有利外，也提供传统付费电视业者发展跨平台装置的OTT影视服务。包括IPTV Middleware大厂微软、OTT服务商Netflex与Youview、晶片大厂Qualcomm与ST、多媒体播放软体Flash业者Adobe与品牌设备商Ericsson、Samsung等皆支持MPEG-DASH标准，这对IP影音串流到各类终端装置市场的成长，将有很大的帮助。

除了MPEG-DASH外，市场上相类似的行动影音串流技术还有Apple 的HLS (HTTP Live Streaming)、Microsoft Smooth Streaming 和Adobe Dynamic Streaming，这些技术规格的相似度很多，但因竞争定位，彼此并不相容。相较于其他技术，MPEG-DASH是由两标准组织所制定，自然更有公信力，而且它开放了CODEC和广泛支援DRM，这是它的优势所在。

![20140211175058187][]

Source: Cash's Blog

下载白皮书：[MPEG's Dynamic Adaptive Streaming over HTTP (DASH)‐An Enabling Standard for Internet TV][MPEG_s Dynamic Adaptive Streaming over HTTP _DASH_An Enabling Standard for Internet TV]

[MPEG-DASH: The Standard for Multimedia Streaming Over Internet][MPEG-DASH_ The Standard for Multimedia Streaming Over Internet]

[http_blog.csdn.net_kevin_android_123456_article_details_8122888]: http://blog.csdn.net/kevin_android_123456/article/details/8122888
[1 February 2012]: http://gpac.wp.mines-telecom.fr/2012/02/01/dash-support/
[Jean]: http://gpac.wp.mines-telecom.fr/author/lefeuvre/
[DASH Basics_ MPD and Segments]: http://gpac.wp.mines-telecom.fr/2012/02/01/dash-support/#DASH%20Basics:%20MPD%20and%20Segments
[ISO Base Media File Format]: http://gpac.wp.mines-telecom.fr/2012/02/01/dash-support/#ISO%20Base%20Media%20File%20Format
[MPEG-2 TS]: http://gpac.wp.mines-telecom.fr/2012/02/01/dash-support/#MPEG-2%20TS
[Multiple Representations]: http://gpac.wp.mines-telecom.fr/2012/02/01/dash-support/# Multiple%20Representations
[Osmo4 Playback]: http://gpac.wp.mines-telecom.fr/2012/02/01/dash-support/#Osmo4%20Playback
[Some guidelines]: http://gpac.wp.mines-telecom.fr/2012/02/01/dash-support/#Some%20guidelines
[this post]: http://gpac.wp.mines-telecom.fr/2011/02/02/mp4box-fragmentation-segmentation-splitting-and-interleaving/
[http_blog.csdn.net_kevin_android_123456_article_details_8122879]: http://blog.csdn.net/kevin_android_123456/article/details/8122879
[ISO Publicly Available Standards]: http://standards.iso.org/ittf/PubliclyAvailableStandards/
[http_laoyaoba.com_ss6_html_74_n-350374.html]: http://laoyaoba.com/ss6/html/74/n-350374.html
[20140211175058187]: /images/20220824/7c7a26f957474e718390eb6adb7b12b2.png
[MPEG_s Dynamic Adaptive Streaming over HTTP _DASH_An Enabling Standard for Internet TV]: http://www.w3.org/2011/09/webtv/slides/W3C-Workshop.pdf
[MPEG-DASH_ The Standard for Multimedia Streaming Over Internet]: http://mpeg.chiariglione.org/working_documents/mpeg-dash/DASH-IEEE-multimedia-preprint.pdf