Adaptation sets

An adaptation set describes the overall media presentation.

The media presentation typically consists of multiple instances of the same audio, video, or audio/video content, with each instance encoded at a different data rate. A representation describes the parameters of each individual encoding of an adaptation set:

  • The codecs attribute specifies the codecs used to encode all representations within the adaptation set.

    For all Dolby Digital Plus bitstreams, the codecs attribute is set to ec-3 1. For Dolby Digital Plus bitstreams that carry Dolby Atmos content, the corresponding representations include additional SupplementalProperty descriptors.

  • The mimeType attribute describes the encapsulation format used to store the Dolby Digital Plus bitstreams present in the adaptation set. For adaptation sets that conform to ISO/IEC 14496-12, the mimeType attribute is set to audio/mp4 (for ISO base media files that contain a Dolby Digital Plus audio track but no accompanying video track).

In some applications, multiple Dolby Digital Plus bitstreams may be used to simultaneously deliver different audio elements of the overall media presentation. For example, one bitstream carries a main audio service (the main audio), and a second bitstream carries an associated audio service (such as commentary) intended to be mixed with the main audio service before presentation to the listener.

If the content provider enables user selection of specific combinations of bitstreams in the playback device (allowing different renditions of the overall media presentation to be selected and delivered), separate adaptation sets must be defined for each bitstream. For example, one adaptation set is used to describe the main audio service on its own, and a second adaptation set describes the associated audio service that is simultaneously delivered with the main audio service to the playback device, where both adaptation sets need to be decoded and mixed together. Refer to the MPD with associated audio services using Dolby Digital Plus section for more details.

1 As specified in Annex E of ETSI TS 102 366 , Dolby Digital Plus decoders are required to decode Dolby Digital Plus bitstreams with bsid values from 11 to 16, inclusive. Bitstreams that have a bsid value outside of this range must not be identified using the ec-3 entry.