Role descriptor

As defined in the MPEG-DASH role scheme (urn:mpeg:dash:role:2011), the value attribute of the role descriptor must be set to describe the purpose of each adaptation set in the overall presentation, as listed here:

• If the adaptation set is delivering a full audio service intended for direct presentation to the listener, the value attribute must be main or alternate.
• 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 must be set to commentary.
• 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 must be set to alternate.