Use Closed Captioning

Closed Captioning allows live streams to have text versions of the audio rendered in-line to the video. This assists with foreign-language support, accessibility for the hearing impaired, and other scenarios.

For details on how to set up Closed Captions during playback using the query strings, refer to Specify Closed Captions for HLS delivery.

Closed Captioning styles

The following are different styles of closed captions.

  • CEA-608 / EIA-608: This refers to the older style of closed captioning used by traditional analog television signals. They are inserted into the analog broadcast in line 21. These type of captions support limited languages such as English, Spanish, French, Portuguese, Italian, German, and Dutch. There is no formatting (captions are typically white all-caps text on a black background), and they are encoded in plain text.
  • CEA-708 / EIA-708: This refers to the newer, updated captioning style intended for use with digital television broadcasts. 708 captions support enhanced formatting (color, font, size, background) and additional languages through the use of Unicode character sets (for example, Japanese and Chinese). These captions are typically placed in the SEI NALU of h.264 video stream as picture user data.

Support for captioning

HLS compatible players interpret the captioning in the H.264 video stream data. This is represented as ATSC picture user data in the NALU header type 6 (SEI). This captioning data will be passed through to the HLS output.

Note: The iOS player has a built-in toggle to display the caption data based on a user choice. Other players might need varying levels of additional programming to display the data.