Signaling and transmitting

HbbTV applications are only allowed to run in the context of a DVB service (as an overlay on top of the service or using the scaled TV image on top of full screen HTML pages) if they are “broadcast-related”. Such a relation is defined by the individual broadcaster by doing specific signaling in the respective broadcast services. HbbTV 1.0^[1] defines in section 6.2 a “lifecycle” for HbbTV application – this is e.g. how applications are started on DVB services, how one application can start another and what the behavior is when the TV program is changed. The DVB signaling^[2] specification specifies the required elements to do that signaling in the DVB streams. All detailed information about the application itself (URL …) as well as the information to control the “lifecycle” is contained in the “Application Information Table” (AIT).

PSI/SI Signaling

MPEG Systems defines the “Program specific information” (PSI), a basic signaling mechanism within the MPEG2-Transportstream (MPEG2-TS) which indicates on which elementary streams of the MPEG2-TS all the video and audio streams can be found and to which services they belong. This mechanism is used by the DVB standards^[2]. The drawing shows the standard signaling for DVB in blue color: “The Program Association Table” (PAT) which can be found in MPEG2-TS packets with the “Packet Identifier” (PID) with the number “0” links to one “Program Map Table” (PMT) per DVB service in this multiplex. In our example we have two services and the two corresponding PMTs which are linked in the PAT can be found on PID and PID f. For service 1 the PMT links to the related video stream on PID b and the audio on PID c. The AIT is linked in the PMT like a additional stream associated with this service. In this example, the AIT is transmitted on PID d and the link is provided in the PMT. In the PMT the minimum required is an additional elementary stream entry plus the “Application signaling descriptor” as defined in section 5.3.5.1 of DVB signaling^[2]. This descriptor does not have to carry more specific information, it just specifies the link to the AIT which then provides all relevant information. So the signaling in the PMT is very lightweight and can be kept static even if application parameters like URL or status change. The DVB defined Service information (SI) does not have to be modified for HbbTV, so no changes are needed in NIT, SDT, BAT, EIT to start or control HbbTV applications.

Transmitting via DSMCC carousel

References

1. ETSI TS 102 796 v1.1.1 (Juni 2010)
2. ETSI TS 102 809 (V1.1.1): "Digital Video Broadcasting (DVB); Signaling and carriage of interactive applications and services in Hybrid Broadcast/Broadband environments" (Jan. 2010)

from : http://www.hbbtv-developer.com/site/wiki/index.php/Signaling_and_transmitting