Steering Board approves TTML subtitling spec

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At the most recent meeting of the DVB Steering Board, a new specification for DVB TTML subtitling systems was approved. The specification enables the delivery of next generation subtitles in transport streams for broadcast television services, based on W3C Timed Text Markup Language (TTML). The specification complements the bitmap based subtitle solutions, which have been in use for over 20 years.

Today's broadcast content can be distributed in many forms and via many different paths. While broadcast services traditionally relied on bitmap-based subtitles because of processing simplicity, advanced processing and text rendering have made text-based approaches feasible. These offer more flexibility and better options to improve the user experience.

The main purpose of the new subtitling system was as a subtitling system for UHDTV and to some extent it completes the tools available for UHDTV broadcasting. DVB now has all the main things needed for the age of UHDTV – video, audio, and subtitling.

Wide adoption

TTML subtitles are increasingly being adopted, including by HbbTV, ARIB, ATSC, the EBU, SMPTE, ISO/MPEG (in the Common Media Application Format specification), and many others. TTML subtitles were initially chosen by DVB for internet delivery of services using the DVB profile of MPEG DASH (TS 103 285). TTML subtitles provide a data-rate-efficient solution for SD, HD and even UHD resolutions.

This new TTML delivery specification will allow service providers to transition over time to a common TTML subtitle format for both broadcast and internet delivered services.

DVB Technical Module Chair, Kevin Murray, remarking on the new specification said "The approval of the transport stream carriage of the TTML specification adds a very important component to the DVB specifications, most notably providing support for UHD subtitles. This also provides a common subtitling mechanism between transport stream and DASH delivered content and, being a textual based format, opens up a range of new possibilities for enhancing accessibility of services."

Commenting on the approval by the Steering Board, DVB Chairman, Peter MacAvock said "Access Services facilitated by DVB's new specification are a cornerstone of media services. DVB's existing bitmapped subtitling specification now has a complement that facilitates subtitling across different platforms and helps DVB addressing the ever increasing demand for accessible content across all platforms."

BlueBook A174

The specification has been published as BlueBook A174. It has been forwarded to ETSI for standardization as an EN (European Norm). The elevated EN status is designed to ensure the TTML specification has a similar status to its bitmapped cousin (EN 300 743).

(Image courtesy of BBC)