Preselections

An audio preselection defines an audio experience created by one or multiple audio components that are decoded and rendered simultaneously.

For MPEG‑DASH, the Preselection element is defined in ISO/IEC 23009-1, and further refined in the DASH-IF IOP for ATSC 3.0 or ETSI TS 103 285. The usage of the Preselection element is allowed only within the ISO base media file format broadcast TV profile or within profiles derived therefrom, such as the 2017 edition of the DVB-DASH profile.

The MPEG‑DASH Preselection element describes the overall experience of a Dolby AC-4 presentation. The Preselection elements are used in the following situations:

  • There is more than one Dolby AC-4 presentation within one stream ("fat streams"). In this case, one adaptation set can be referenced by multiple preselections.
  • Audio components from separate streams are needed for a Dolby AC-4 presentation. In this case, multiple adaptation sets can be referenced by multiple preselections.

Dolby AC-4 content that contains one single presentation per stream does not require Preselection elements in the MPD to sufficiently describe the presentation. Signaling of single-presentation Dolby AC-4 streams ("slim streams") can be sufficiently handled by the AdaptationSet and Representation elements.

If at least one Dolby AC-4 stream from a period requires the use of Preselection elements, for consistency and ease of the playback selection process, Preselection elements should be used for all Dolby AC-4 presentations available in that period, even for single-presentation Dolby AC-4 streams.

When referenced by at least one Preselection element, the adaptation set that contains the main audio component should be tagged by a supplemental property preselection descriptor. The value attribute of this descriptor must be empty or absent.

<SupplementalProperty schemeIdUri="urn:mpeg:dash:preselection:2016" value="" />

If more than one adaptation set contributes to a preselection, those adaptation sets that cannot be played on their own must be tagged with an essential property preselection descriptor. The value attribute of this descriptor must be empty or absent.

<EssentialProperty schemeIdUri="urn:mpeg:dash:preselection:2016" value="" />