Adaptation sets and representations

An adaptation set typically consists of multiple representations of the same audio, video, or A/V content. Each representation is associated with a Representation element.

All representations in an adaptation set are perceptually identical, meaning that only one or more major parameters, such as the bit rate, may differ across the Dolby AC-4 bitstreams in the same adaption set.

It is strongly recommended not to include representations that differ by means of the encoded audio channels (for example, stereo configuration and 5.1 channel configuration) in the same adaptation set, but rather to provide those representations through different adaptation sets. Specifically, the compatibility level of any representation must not exceed the compatibility level of the Dolby AC-4 presentation indicated in the Preselection or AdaptationSet element.

In Dolby AC-4 bitstreams, audio experiences are made available to the user for personalization by means of presentations. An adaptation set can contain a single presentation, multiple presentations, or only a part of a presentation. When an adaptation set contains multiple presentations or only a part of a presentation, Preselection elements and additional descriptors are needed to signal different presentations.

If multiple adaptation sets are provided as part of a selection, the selectionPriority attribute should be added to each adapation set. For example, if a selection must be made between an adaptation set for a Dolby AC-4 stream and an adaptation set for an AAC stream, the selectionPriority for the Dolby AC-4 stream should be higher to indicate the preference.