Emergency
Dolby AC-4 elementary streams might carry audio components that indicate
Associated service: emergency (E) (110) in the corresponding
content_classifier field.
When this classifier is used, two use cases have to be differentiated:
- Emergency alerting, which is required by the public authorities.
-
Emergency information, which offers accessibility service for a restricted audience and is typically controlled by the broadcaster application.
Emergency alerts
In this user case, the ongoing broadcast should be replaced by or overlaid with the
emergency alert. The only available audio component indicates Associated service:
emergency (E) by means of the value 110 of the
corresponding content_classifier field. In the corresponding
MPD, a Role descriptor
indicating emergency instead of main must be included. An
Accessibility descriptor is not needed in this case.
content_classifier in the Dolby AC-4 elementary
stream is not set accordingly. In this case, the signaling in the MPD might not indicate emergency either. The
Role descriptor might still indicate main.Emergency information
If the audio component indicating Associated service: emergency (E) just
contributes to a Dolby AC-4 presentation as an associated service, it
is considered to be emergency information. In this case, the appropriate signaling
for the Role and Accessibility descriptors is
necessary for the corresponding preselection. Based on the signaling, applications
can decide whether the emergency information is of interest to the user and whether
the Dolby AC-4 presentation should be selected.
The emergency information is usually presented to viewers visually, for example, a crawling text overlay at the bottom of the TV display, without any additional information in the main audio. This emergency signaling in the context of an accessibility service can be used to indicate and present audible warning messages to viewers who have visual impairments and thus have configured accessibility preferences on their receiver accordingly.