Adaptive streaming is a technique to provide flexibility and scalability by offering variable bit-rate streams to the client. Designed to work over HTTP, it provides media content as separate streams with media type and various bit-rates, the client will be able to select according to its network bandwidth or its CPU power.

The most popular adaptive streaming systems are:

• HLS (Apple HTTP Live Streaming)
• MSS (Microsoft Smooth Streaming)
• ADS (Adobe HTTP Dynamic Streaming)
• MPEG-DASH (MPEG Dynamic Adaptive Streaming over HTTP)

MPEG-DASH is the most complete adaptive streaming technique. This format is based on an XML description file called an MPD (Media Presentation Description). This format describes a set of representations which has a media type (audio, video or subtitles) and provides various bit-rate or media format.

This solution is an open standard and is widely supported by the industry. For more information about it, you can visit the DASH-IF website.

In the example below, the MPD describes a static content with three media content type (adaptation sets). Each adaptations sets contains representations. The video has 5 different representations, which allows to switch to 5 different bit rates according to the playback constraints.

<MPD mediaPresentationDuration="PT634.566S" minBufferTime="PT2.00S" profiles="urn:hbbtv:dash:profile:isoff-live:2012,urn:mpeg:dash:profile:isoff-live:2011" type="static" xmlns="urn:mpeg:dash:schema:mpd:2011" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:mpeg:DASH:schema:MPD:2011 DASH-MPD.xsd">
 <BaseURL>./</BaseURL>
 <Period>
  <AdaptationSet id="1" mimeType="video/mp4" contentType="video" subsegmentAlignment="true" subsegmentStartsWithSAP="1" par="16:9">
   <SegmentTemplate duration="120" timescale="30" media="$RepresentationID$/$RepresentationID$_$Number$.m4v" startNumber="1" initialization="$RepresentationID$/$RepresentationID$_0.m4v"/>
   <Representation id="bbb_30fps_1280x720_4000k" codecs="avc1.64001f" bandwidth="4952892" width="1280" height="720" frameRate="30" sar="1:1" scanType="progressive"/>
   <Representation id="bbb_30fps_320x180_200k" codecs="avc1.64000d" bandwidth="254320" width="320" height="180" frameRate="30" sar="1:1" scanType="progressive"/>
   <Representation id="bbb_30fps_480x270_600k" codecs="avc1.640015" bandwidth="759798" width="480" height="270" frameRate="30" sar="1:1" scanType="progressive"/>
   <Representation id="bbb_30fps_640x360_800k" codecs="avc1.64001e" bandwidth="1013310" width="640" height="360" frameRate="30" sar="1:1" scanType="progressive"/>
   <Representation id="bbb_30fps_3840x2160_12000k" codecs="avc1.640033" bandwidth="14931538" width="3840" height="2160" frameRate="30" sar="1:1" scanType="progressive"/>
  </AdaptationSet>

  <AdaptationSet id="2" mimeType="audio/mp4" contentType="audio" subsegmentAlignment="true" subsegmentStartsWithSAP="1">
   <Accessibility schemeIdUri="urn:tva:metadata:cs:AudioPurposeCS:2007" value="6"/>
   <Role schemeIdUri="urn:mpeg:dash:role:2011" value="main"/>
   <SegmentTemplate duration="192512" timescale="48000" media="$RepresentationID$/$RepresentationID$_$Number$.m4a" startNumber="1" initialization="$RepresentationID$/$RepresentationID$_0.m4a"/>
   <Representation id="bbb_a64k" codecs="mp4a.40.5" bandwidth="67071" audioSamplingRate="48000">
    <AudioChannelConfiguration schemeIdUri="urn:mpeg:dash:23003:3:audio_channel_configuration:2011" value="2"/>
   </Representation>
  </AdaptationSet>

  <AdaptationSet id="3" mimeType="image/jpeg" contentType="image">
    <SegmentTemplate media="$RepresentationID$/tile_$Number$.jpg" duration="100" startNumber="1"/>
    <Representation bandwidth="12288" id="thumbnails_320x180" width="3200" height="180">
      <EssentialProperty schemeIdUri="http://dashif.org/thumbnail_tile" value="10x1"/>
    </Representation>
  </AdaptationSet>

 </Period>
</MPD>

DASH in GStreamer

Since 2012, GStreamer includes only a DASH client called dashdemux whereas, for HLS, it provides the both elements, demuxer and sink.

DASH Demuxer

This element landed in the GStreamer repository in 2012 and had evolved a lot since that time to support the various use-cases DASH offers in its specifications and its applications. Indeed it is able to support multiple streams (video/audio/subtitles) and allows the user to select from the available streams or automatically select the best representation according to the network.

DASH Sink

A first attempt to propose a DASH sink was also in 2012. The design was not mature enough to be landed and the author never had the time to complete it.

In 2014, a new element called splitmuxsink was introduced. It handles the most complex part of creating a fragmented stream, it cuts files with synchronized audio and video. Based on this element, a new HLS sink called hlssink2 was created in 2017. I decided to finally create a DASH sink based on this approach to fill the gap in GStreamer.

MPD Parser

In order to unify the MPD support, a first task has been to relocate and redesign the base classes to read, and write, an MPD file. Based on XML, the Media Presentation Description scheme is based on multiple nodes owning children and properties as described above. A first important work item was to split the code in 'objects' each identifying an XML node from the MPD schema, including the root node, periods, adaptation sets, etc. An object oriented approach has been selected to unify the work regarding the parsing, object property manipulation and the XML format generation.

The sink

Inspired from the work on hlssink2, the dash sink is a "super bin" and includes a splitmuxsink to provide the multiple media segments. Most of the challenge, here, was to write the MPD compliant with the DASHIF conformance test here with usable and suitable media segments.

This plugin is now capable of:

• Multiple input audio/video streams
• Multiple periods
• TS segment supported (MP4 support is not yet complete), need additional work for the segment transition in short segment scheme.
• Fragment segment with given duration
• Static/Dynamic MPD (Passing the DASH-IF conformance test)

An example is worth a thousand words

In the following pipeline a static MPD file is created in /tmp along with one single segment long for a single video stream during a period of 60s. The segment will be encoded as H.264 and encapsulated in MPEG transpor stream files.

$ gst-launch-1.0 -m dashsink name=dashsink mpd-root-path=/tmp target-duration=60 dynamic=false period-duration=60000 muxer=ts  v4l2src ! video/x-raw,framerate=30/1,width=320,height=240 ! videoconvert ! queue ! x264enc bitrate=400 ! dashsink.video_0

Future work includes implementing support for creating MP4 fragment files following the CMAF (MPEG-A Part 19) specification.

If you would like to learn more about dashsink or any other parts of GStreamer, please contact us!