DVB-TTML spec published as ETSI standard
DVB's new specification for text-based subtitles has been published by ETSI as EN 303 560 V1.1.1. As an EN it is a European Standard approved by the national standards organizations of Europe. Complementing DVB's existing bitmap-based specification, the new specification uses TTML (Timed Text Markup Language) to offer more flexibility and better options to improve the user experience.
A TTML-based approach to subtitling enables a very wide range of text styles, made possible by advances in processing and rendering. TTML is already established as a common solution for IP-based platforms. DVB's specification enables the distribution of TTML subtitles together with audio/video content via broadcast. It is closely aligned with other TTML-based systems including the EBU-TT-D specification, which was adopted for both DVB-DASH and HbbTV.
The DVB bitmap subtitling specification (EN 300 743) has also been updated to ensure compatibility with UHD services, with a new version set to be published by ETSI in due course. Broadcasters and service providers may choose between the bitmap-based and TTML approaches depending on their current and future subtitling needs on both broadcast and IP services.
TTML or bitmap?
While bitmap subtitles are already well established and give the broadcaster more control over the look of the subtitles, a higher bitrate is required – and it increases with resolution. The new TTML delivery specification will offer a path to transition to a common TTML subtitle format for both broadcast and internet-delivered services. Subtitles used for broadcast can be reused later for video-on-demand services over DASH/CMAF.
TTML is certainly ideal for new greenfield deployments, but even broadcasters already using bitmapped subtitles could migrate to TTML and convert the subtitles for legacy platforms. This may even improve legacy bitmap subtitles if they are currently being generated from Teletext.