The media industry is migrating to IP-based infrastructures for live production. The most important standard for this is ST 2110, which specifies how to transport audio, video and data over IP. This also allows for the carriage of subtitling, but in a largely traditional way. The mechanism is effectively the same as carrying teletext subtitles over SDI. In IP the subtitles (still in teletext format) would be wrapped in SMPTE RD 008 / Free TV OP-47 and mapped to RTP. The traditional model has served the industry well over many decades, but in an IP-based world a more flexible and elegant approach may be preferred: using a non-teletext format and mapping the subtitles into RTP directly.
A replacement format for teletext subtitles is available as W3C's Timed Text format and the EBU's profiles of it, known as EBU-TT. For live subtitling, EBU-TT Live (EBU Tech 3370) is especially relevant. This format has been created to offer the key features needed for the creation and handling of both prepared and live subtitling in media production operations. It includes, for example, ways to hand over between different live subtitlers and the concept of 'improver' nodes to automatically improve subtitles before sending them on-air.
EBU-TT Live is arguably the most advanced way of producing (live) subtitles. The EBU has submitted EBU-TT Live to W3C for use in their TTML work, to maximize worldwide harmonization.
For the second part of the solution, mapping Timed Text directly into RTP, a solution is available too. Earlier this year the IETF ratified RFC 8759 "RTP Payload for Timed Text Markup Language (TTML)". This specification describes how to transport TTML in RTP, especially aiming to support streaming workflows, such as those found in IP-based production environments. The new RFC provides the basis for making sure TTML can be fully integrated into new ST 2110 workflows.
Open source code
To help gain experience with this new way of transporting subtitles, BBC and the EBU provide three open source tools:
- The EBU-TT Live Toolkit allows for the creation of Timed Text subtitles that adhere to EBU Tech 3370.
- BBC Python code to encode and decode RTP bitstreams and examine them with a Wireshark plugin (read more here).
- The EBU Live IP Software Toolkit (LIST) that includes analysis of RTP packets carrying TTML.
On 28 July 2020, the EBU TTML over RTP webinar will provide more details on the work to support Timed Text subtitles in IP-based production, including a brief demo of how to use some of the tools described above. You can join the event here.