Subtitling

DVB-SUB TTML (Subtitling) - Timed Text Mark-up Language

DVB TTML Subtitling Systems


Introduction

Today’s broadcast content can be distributed in many forms and via many different paths. Whilst 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. TTML (Timed Text Markup Language) is established as a common solution for IP-based platforms, but multiple profiles exist.

In Europe, the EBU Group ‘Subtitles in XML’ published the EBU-TT-D TTML profile which was adopted by the DVB DASH specification as defined in ETSI TS 103 285 and by HbbTV® as defined in ETSI TS 102 79. The W3C later published the IMSC1 TTML profiles, which were adopted by ATSC.

There are only minor differences between EBU-TT-D and IMSC1 Text Profile.

The present specification builds on the existing widespread device support for EBU-TT-D, in particular through support for HbbTV®, to enable the distribution of TTML subtitles together with audio/video content via broadcast.

Clause 4 specifies TTML subtitle constraints for a default conformance point, to be supported by both EBU-TT-D and IMSC1 Text Profile compatible processors.

Clause 5 specifies subtitle delivery, including PSI/SI signalling, TS packetisation, TTML segmentation and synchronization requirements.
Finally clause 6 specifies IRD requirements.

Scope

The present document specifies the transport of TTML subtitle streams in DVB MPEG-2 transport streams, based on the MPEG-2 system described in ISO/IEC 13818-1. TTML is an XML-based representation. The present document provides syntax for delivery of TTML subtitle streams over MPEG-2 transport stream, and is based on EBU-TT-D compatible with the IMSC1 Text Profile of W3C TTML.


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