role descriptor
As defined in the MPEG-DASH role scheme (urn:mpeg:dash:role:2011
), the
value
attribute of the role
descriptor is set to describe the purpose of each adaptation set in the
overall presentation, as follows:
- If the adaptation set is delivering a full audio service intended for
direct presentation to the listener, the
value
attribute ismain
oralternate
. - If the adaptation set is delivering an audio service that must be decoded
and mixed with a full audio service delivered in a different adaptation set before
presentation to the listener (sometimes referred to as a receiver-mix service), the
value
attribute is set tocommentary
. - If the adaptation set is delivering a full audio service intended for
direct presentation to the listener, but this audio service is intended as an alternative
presentation to the main audio service (for example, when delivering a service that contains
premixed main audio and audio elements for visually impaired listeners, sometimes referred
to as a broadcast-mix service), the
value
attribute isalternate
.