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.

The attributes of an adaptation set for Dolby AC-4 must be set as follows:

  • The codecs attribute specifies the codecs used to encode all representations within the adaptation set. For Dolby AC-4 bitstreams, the value element of the codecs attribute is created according to the syntax described in IETF RFC 6381. The value consists of a dot-separated list of these four parts, of which the latter three are represented by two-digit hexadecimal numbers:
    • The fourCC ac-4
    • The bitstream_version as indicated in the ac4_dsi()
    • The presentation_version as indicated for the first or default presentation in the ac4_dsi()
    • The mdcompat parameter as indicated for the first or default presentation in the ac4_dsi()
  • The mimeType attribute describes the encapsulation format used to store the Dolby AC-4 bitstreams present in the adaptation set. For adaptation sets that conform to ISO/IEC 14496-12, the mimeType attribute is set to audio/mp4.
  • The lang attribute corresponds to the language of the first or default presentation conveyed in the language_tag_bytes of the ac4_substream_group_dsi structure (within the ac4_dsi_v1 structure), which is tagged as dialogue or complete main in the corresponding content_classifier.