EBU-TT stands for EBU Timed Text, which is the follow-up to the widely used EBU STL format (EBU Tech 3264). The EBU-TT family consists of the following specifications:
1. Base spec
The base specification is EBU-TT part 1 (EBU Tech 3350 v1.1). It defines an easy-to-use XML structure for the interchange and archiving of subtitles. It builds on the W3C Timed text Markup Language (TTML) 1.0.
3. Live contribution
EBU-TT Part 3 (EBU Tech 3370) was published in December 2015 as draft v0.9 for prototype implementation. It provides a System Model and Content Profile for authoring and contribution of live subtitling. It also introduces the concept of processing 'nodes' that can help improve subtitling quality in production.
Part 3 feedback is welcome via: firstname.lastname@example.org.
EBU-TT part 'D' (EBU Tech 3380) is the format for the distribution of subtitles over IP. There also is a document for the use of EBU-TT-D in ISO BMFF / MPEG-DASH (EBU Tech 3381). Other specs reference EBU-TT-D, such as HbbTV 2.0, and Freeview Play.
The EBU has developed a recommendation on the transport of subtitles inside and outside of MXF files (EBU R 133).
Open source toolkit for EBU-TT Live
The EBU has set up an open source software project to develop an Interoperability Toolkit for live subtitles, which you are invited to contribute to (there is no requirement to be a member of the EBU or the Subtitles in XML Group).
Also check out this list of existing open source subtitling software and samples.
The EBU Subtitling Group (MIM-XMLSubs) is currently focussed on providing a maintenance update of part 2, and finalising the part 3 work. Join the group if you want to contribute and stay up-to-date.