Service Discovery & Metadata
DVB services encompass a wide variety of programmes carried via a diverse range of transmission channels. For the receiving device to be able to tune to such channels, and for the end user to be able to navigate through the available programmes, powerful navigational aids are provided as part of the DVB streams.
For broadcast digital television, DVB-SI provides the required functionality, while the recent creation of DVB-I provides an internet-centric framework for service discovery and programme information. DVB also provides fully featured solutions for the delivery of subtitles.
DVB-SI specifies the information carried in a DVB multiplex describing the contents of different multiplexes. This information enables receivers to automatically tune to particular services and allows services to be grouped into categories with relevant schedule information.
The DVB-SI data carried in DVB bitstreams helps receivers – or Integrated Receiver Decoders, IRDs – to automatically configure itself for the selected services. The standard doesn’t specify how the programme information should be presented to the user – manufacturers can choose how to do this. The Service Information data can be used as the basis for Electronic Programme Guides (EPGs).
To aid the unique identification of DVB broadcasts and networks, DVB Services Sàrl acts as a registration authority for all DVB identifiers, including those defined in DVB-SI. TS 101 162 defines the allocation of identifiers pertaining to different DVB specifications, while a complete listing of all allocated identifiers is available at www.dvbservices.com.
To retain their competitive position and access new markets and demographics, traditional television broadcasters are increasingly looking to exploit the internet as a delivery mechanism for their services.
The DVB-I specification is designed to ensure that linear television over the internet is as user-friendly and robust as traditional broadcast television. DVB-I will support any device with a suitable internet connection and media player, including TV sets, smartphones, tablets and media streaming devices.
The specification defines DVB-I Service Lists, a means for internet-connected devices to find curated sets of linear television services that may be delivered through broadband or broadcast mechanisms. It also defines the methods to retrieve electronic programme data for those services, which can be integrated into a single coherent offering that is accessed through a consistent user interface.
Where both broadband and broadcast connections are available, devices can present an integrated list of services and content, combining both streamed and broadcast services – users don’t have to know or care whether a service arrives via broadband or broadcast.
Subtitles are an important aspect of both linear and on-demand video delivery, increasing the accessibility of content for the deaf or hard of hearing (sometimes incorporating additional commentary), for foreign-language content, or in situations where audio playback is not possible.
DVB has three specifications for subtitling, two of which were most recently updated in 2018. The original DVB subtitling system, which was based on Teletext, was first published in 1994 and is still widely used. The image (or bitmap) based DVB Subtitling specification brought more character sets and colours along with higher quality. Later updates added support for HDTV and UHD services.
A new text-based system was published as an ETSI European Standard in May 2018. The DVB-TTML subtitling systems specification offers more flexibility and further options to improve the user experience.
DVB-TTML is based on W3C’s Timed Text (TTML). First published in 2010, TTML separates the text, styling and region elements of subtitles. It is expressed in XML, which means it can be easily extended with new features. It is a complex and powerful specification that is the basis of several other subtitling specifications, including those developed by the EBU and DVB. TTML enables a range of new styling features such as bold, italic or underlined text, background colours and line padding to improve readability.