SDHcaption

SDHcaption is a proposed framework for embedding and transporting caption data within Synchronous Digital Hierarchy (SDH) networks to support accessible video delivery.

It defines the data model for captions, including timing information, language metadata, and display attributes, and specifies how this data is carried alongside standard SDH payloads without disrupting existing traffic.

In operation, caption data is mapped into a dedicated portion of the SDH payload using a transport-agnostic encoding so that captions remain synchronized with audio-visual content across network elements and path diversions.

Standardization and deployment: As of now, SDHcaption exists as a research and industry initiative rather than a final ITU-T recommendation. It has seen pilots among some broadcasters and telecom operators, with ongoing discussions on interoperability, encoding formats (such as TTML-based cues), and compatibility with legacy caption systems.

Impact and relation: The approach aims to simplify caption workflows, improve resilience, and enable multilingual captioning in SDH-based delivery. Limitations include added complexity to network equipment and potential vendor lock-in; it complements, rather than replaces, source-stage captioning in video production. Related concepts include SDH, TTML/Timed Text, and broadcast captioning standards.