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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