In-Band Captions
Captions come in two varieties, based on their relationship to the
video. Typically on the web, captions are delivered as a separate file
and associated with a video through the <track> element. This type
of captions are sometimes referred to as out-of-band.
The alternative method involves embedding the caption data directly into the video content and is sometimes called in-band captions. In-band captions exist in many videos today that were originally encoded for broadcast and they are also a standard method used to provide captions for live events. In-band HLS captions follow the CEA-708 standard.
In this project, in-band captions are parsed using a CaptionStream. For MPEG2-TS sources, the CaptionStream is used as part of the Transmuxer TS Pipeline. For ISOBMFF sources, the CaptionStream is used as part of the MP4 CaptionParser.
Is my stream CEA-608/CEA-708 compatible?
If you are having difficulties getting caption data as you expect out of Mux.js, take a look at our Troubleshooting Guide to ensure your content is compatible.
Useful Tools
References
- Rec. ITU-T H.264: H.264 video data specification. CEA-708 captions are encapsulated in supplemental enhancement information (SEI) network abstraction layer (NAL) units within the video stream.
- ANSI/SCTE 128-1: the binary encapsulation of caption data within an SEI user_data_registered_itu_t_t35 payload.
- CEA-708-E: describes the framing and interpretation of caption data reassembled out of the picture user data blobs.
- CEA-608-E: specifies the hex to character mapping for extended language characters.
- Closed Captioning Intro by Technology Connections