Accessibility and Role descriptors

The Accessibility descriptor indicates the presence of accessibility features such as AD, visual description service, or dialogue enhancement. The Role descriptor for audio describes a whole audio user experience represented by either an adaptation set or a preselection.

Note: The content_classifier field in the ac4_substream_group_dsi structure describes the content type of an individual audio component. The field usually does not describe the overall content experience provided by a Dolby AC-4 presentation, which might consist of several audio components.

For ATSC 3.0, both the Accessibility and Role descriptors use the role schema, which is defined in ISO/IEC 23009-1. The schema is referenced in the descriptors by the schemeIdUri attribute.

schemeIdUri="urn:mpeg:dash:role:2011"

For Dolby AC-4 audio in ATSC 3.0, the values of the descriptors should be restricted to the following strings:

Descriptor	Value
`Accessibility`	`description`
`Accessibility`	`enhanced-audio-intelligibility`
`Role`	`main`
	`alternate`
	`supplementary`
	`commentary`
	`dub`
	`emergency`

For DVB-DASH, the role scheme urn:mpeg:dash:role:2011 is used for the Role descriptor. However, for the Accessibility descriptor, the audio purpose classification scheme urn:tva:metadata:cs:AudioPurposeCS:2007 must be used.

To support a broader range of playback clients, both Accessibility descriptors should be used on an adaptation set or a preselection to signal the same accessibility feature.