To be useful, off-screen Timed Text (Closed Captions and Closed Subtitles) must be communicated by the Media Block to outboard transmitters and personal display systems. In practice, the transmitter and personal display devices comprise a self-contained commercial system. Such commercial systems receive the off-screen Closed Caption and Closed Subtitle data from the Media Block using a standardized set of protocols over Ethernet.
Ethernet communications are handled using the SMPTE ST 430-10 DCO – Auxilliary Content Synchronization Protocol. Using the protocol, the Media Block establishes communications with the outboard personal captioning system, and sends an list of XML timed text assets in the format defined by SMPTE ST 430-11 DCO – Auxilliary Resource Presentation List. Following that, the personal captioning system pulls the timed text data from the Media Block as needed, based on timing information supplied by the protocol, and displays the text on the personal display. Unlike Picture and Sound data, DCI security requirements allow decrypted Timed Text Track Files to be stored in the clear in local storage. The diagram below illustrates the process.
Figure OSTT-1. Off-Screen Timed Text Communications
