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NALs with CEA608 captions go in. Cues come out.

Lead Maintainers:

Maintenance Status: Beta

Usage

CaptionStream is the main class here, and you'll want to use it pretty much like a standard node.js stream, but with two optional arguments in the constructor:

// if not provided, timescale is 90000 and CueObject is a simple wrapper
var captionStream = new CaptionStream(timescale, CueObject);

Register to the data event to receive caption objects of type CueObject, whose interface must conform to the VTTCue interface, but can be any object.

Push NALs with CEA608 captions into the CaptionStream with CaptionStream.push. When you think it's appropriate to flush, call CaptionStream.flush.

When new streams are detected, a newStream event is dispatched with the stream as the data of the event (e.g. CC1, CC2, etc).

Building

If you're using this project in a node-like environment, just require() whatever you need. If you'd like to package up a distribution to include separately, run npm run build. See the package.json for other handy scripts if you're thinking about contributing.

References

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.

  • 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.