MPEG-DASH descriptors

Much of what a manifest says about content is carried in descriptors: elements with a schemeIdUri naming a scheme and a value whose meaning the scheme defines. New schemes can be added without changing the schema. DASH-IF keeps a public registry of them.

Descriptor elements

Every descriptor has schemeIdUri (required), value and id.

Element	On	Meaning
`EssentialProperty`	MPD, adaptation set, representation and others	A property the player must understand: if it does not know the scheme, it must ignore the element that carries it.
`SupplementalProperty`	as above, and period	Extra information a player may use or ignore.
`Role`	adaptation set, content component, preselection	What the content is for (below).
`Accessibility`	the same	Accessibility features, such as captions in the video stream.
`Rating`	the same	A content rating.
`Viewpoint`	the same	A camera angle or viewpoint.
`AudioChannelConfiguration`	common elements	The audio channel layout (below).
`FramePacking`	common elements	How stereoscopic 3D frames are packed.
`ContentProtection`	MPD, period, common elements	A protection system (below).
`OutputProtection`	common elements, at most one	Output protection needed, such as HDCP.
`AssetIdentifier`	period	Identifies the content of the period, so periods from one asset can be joined.
`UTCTiming`	MPD	Where to get the time; see MPD element.
`Reporting`	in `Metrics`	How metrics are reported.
`Scope`	in `ServiceDescription`	Which players a service description is for.

Roles

The scheme urn:mpeg:dash:role:2011 is the usual one, for Role and also for Accessibility. Its first values were:

Value	Meaning
`main`	The main content
`alternate`	An alternative to the main content, such as another camera angle
`supplementary`	Extra content to go with the main content
`commentary`	A commentary
`dub`	Dubbed audio in another language
`caption`	Captions for the deaf and hard of hearing
`subtitle`	Subtitles, a translation of the dialogue

Later editions add more. Industry specifications that follow them use, among others, description (audio description), enhanced-audio-intelligibility, emergency and karaoke; the full current list is in the standard itself. For CEA-608 captions carried inside the video, Accessibility uses urn:scte:dash:cc:cea-608:2015, with a value listing the caption channels and their languages. The scheme urn:mpeg:dash:stereoid:2011, used with Role, marks the left (l0…) and right (r0…) views of stereo pairs.

Audio and video schemes

Scheme	Value
`urn:mpeg:mpegB:cicp:ChannelConfiguration`	A CICP channel configuration number (ISO/IEC 23091-3), such as `2` for stereo or `6` for 5.1. Preferred for all codecs.
`urn:mpeg:dash:23003:3:audio_channel_configuration:2011`	A channel configuration index from ISO/IEC 23003-3
`tag:dolby.com,2014:dash:audio_channel_configuration:2011`	A 16-bit Dolby channel mask, four hexadecimal digits (legacy)
`tag:dolby.com,2015:dash:audio_channel_configuration:2015`	A 24-bit Dolby channel mask, six hexadecimal digits (legacy)
`tag:dts.com,2014:dash:audio_channel_configuration:2012`	The number of DTS output channels, 1 to 32 (legacy)
`tag:dts.com,2018:uhd:audio_channel_configuration`	A DTS-UHD channel mask in hexadecimal (legacy)
`urn:mpeg:mpegB:cicp:ProgramLoudness`, `…:AnchorLoudness`, `…:OutputChannelPosition`	Loudness in LKFS, and loudspeaker positions, as properties
`tag:dolby.com,2018:dash:EC3_ExtensionType:2018`	`JOC`: Enhanced AC-3 with joint object coding (Dolby Atmos)
`urn:mpeg:mpegB:cicp:ColourPrimaries`, `…:TransferCharacteristics`, `…:MatrixCoefficients`, `…:VideoFullRangeFlag`	CICP colour numbers, as properties; transfer characteristics 16 means PQ (HDR10) and 18 means HLG
`urn:mpeg:dash:14496:10:frame_packing_arrangement_type:2011`, `urn:mpeg:mpegB:cicp:VideoFramePackingType`	Frame packing of stereoscopic video, for `FramePacking`

Content protection

ContentProtection adds robustness, refId and ref (so one full descriptor can be referred to from elsewhere) to the descriptor attributes. It usually appears twice or more: once with urn:mpeg:dash:mp4protection:2011 and the value cenc or cbcs (the Common Encryption scheme) and a cenc:default_KID attribute, and once for each DRM system, as urn:uuid: and the system’s ID, often with a cenc:pssh child holding its initialisation data in Base64.

System ID	System
`edef8ba9-79d6-4ace-a3c8-27dcd51d21ed`	Widevine
`9a04f079-9840-4286-ab92-e65be0885f95`	Microsoft PlayReady
`94ce86fb-07ff-4f43-adb8-93d2fa968ca2`	Apple FairPlay
`e2719d58-a985-b3c9-781a-b030af78d30e`	Clear Key (DASH-IF)
`1077efec-c0b2-4d02-ace3-3c1e52e2fb4b`	W3C common PSSH box
`5e629af5-38da-4063-8977-97ffbd9902d4`	Marlin
`9a27dd82-fde2-4725-8cbc-4234aa06ec09`	Verimatrix VCAS
`80a6be7e-1448-4c37-9e70-d5aebe04c8d2`	Irdeto
`adb41c24-2dbf-4a6d-958b-4457c0d27b95`	Nagra MediaAccess PRM 3.0
`a68129d3-575b-4f1a-9cba-3223846cf7c3`	Synamedia VideoGuard
`3d5e6d35-9b9a-41e8-b843-dd3c6e72c42c`	ChinaDRM

The DASH-IF registry lists about thirty systems in all; the table gives the common ones. For MPEG-2 transport stream representations, the scheme urn:mpeg:dash:13818:1:CA_descriptor:2011 names a conditional access system.

Other common elements

Element	Attributes	Meaning
`Label`, `GroupLabel`	`id` (default 0), `lang`; text content	A name for menus. A group label names a group of sets.
`Switching`	`interval` (required), `type`: `media` (default) or `bitstream`	How often a player can switch into this representation, and how.
`RandomAccess`	`interval` (required), `type`: `closed` (default), `open` or `gradual`; `minBufferTime`, `bandwidth`	Where playback can start, and the kind of GOP at those points.
`Resync`	`type` (SAP type, default 0), `dT`, `dImax`, `dImin` (default 0), `marker` (default `false`)	Points inside segments where a player can resynchronise, for low latency.
`ProducerReferenceTime`	`id` (required), `inband` (default `false`), `type`: `encoder` (default), `captured` or `application`; `applicationScheme`, `wallClockTime` (required), `presentationTime` (required); a `UTCTiming` child	Pairs a media time with the wall-clock time it was encoded or captured, so a player can measure its latency.
`ContentPopularityRate`	`source` (`content`, `statistics` or `other`; required), `source_description`; `PR` children with `popularityRate` (1 to 100), `start`, `r`	How popular parts of the content are, so caches can prepare.
`SegmentSequenceProperties`	`sapType` (default 0), `cadence` (default 1), `event` (default `true`), `alignment`	Properties of segment sequences (sixth edition).
`InbandEventStream`	as `EventStream`	Events carried inside the segments; see Events.

Worked example

<AdaptationSet contentType="audio" lang="fr" mimeType="audio/mp4"
               xmlns:cenc="urn:mpeg:cenc:2013">
  <ContentProtection schemeIdUri="urn:mpeg:dash:mp4protection:2011" value="cenc"
                     cenc:default_KID="34e5db32-8625-47cd-ba06-68fca0655a72"/>
  <ContentProtection schemeIdUri="urn:uuid:edef8ba9-79d6-4ace-a3c8-27dcd51d21ed">
    <cenc:pssh>AAAAW3Bzc2gAAAAA7e+LqXnWSs6jyCfc1R0h7Q…</cenc:pssh>
  </ContentProtection>
  <Role schemeIdUri="urn:mpeg:dash:role:2011" value="dub"/>
  <AudioChannelConfiguration schemeIdUri="urn:mpeg:mpegB:cicp:ChannelConfiguration" value="6"/>
  …
</AdaptationSet>

French dubbed 5.1 audio, encrypted with the cenc scheme, with Widevine data. The Base64 of the pssh box shows its type, pssh, and then the Widevine system ID ED EF 8B A9 ….