Term: Closed captions
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Term: Closed captions
- Definition:
- Timed text information intended for display over a timeline, in synchronization with image and sound essence. Closed captions, unlike open captions, can be turned on or off in a media viewer (if the viewer supports this functionality). Closed captions offer limited font, color and position choices, and are aligned to a fixed grid.
When present, the captions in NTSC video governed by the Consumer Electronics Association standard ANSI/CTA-608-E (CEA-608) are generally encoded into line 21, which exists at the cusp between VBI and active picture. Many authorities state that this line is part of the vertical blanking interval, while others note that it is also the first line of NTSC active video. The caption data appears as lines and dashes in the black area above the picture. The lines and dashes are not viewable on consumer TVs, but can be seen with an underscan display, video scope or broadcast measurement equipment. ANSI/CTA-708-E (CEA-708) is the standard for closed captioning for ATSC digital television (DTV) streams in the United States and Canada. ANSI/CTA-708-E (formerly known as CEA-708) captions consist of binary-format textual data but this data is not carried on line 21 and will be pre-rendered by the receiver. For backwards compatibility for set top boxes and receivers that output analog video signals the CTA-708 captions contain the CEA-608 captions as a portion of the data, and only enhancements beyond basic text are carried in the CTA-708 extension data.
In the analog NTSC standard, the closed captions are carried in line 21. In uncompressed digital video they are carried in the SMPTE ST436 defined closed caption space in the video frame header. In lossy compressed video it is carried (if captions are permitted by the compression scheme: some early compression types did not allow closed captions to be carried) in the video user bits in the header of the compressed video frames.
Closed caption data stored as ANSI/CTA-608-E (CEA-608) and ANSI/CTA-708-E can be extracted and stored as a sidecar Timed Text file such as SMPTE Timed Text or EBU Timed Text.
External sidecar closed captions can be formatted as plain text (such as WebVTT/.vtt and SubRip/.srt) or as XML (such as Timed Text Markup Language or TTML). However, external sidecar files are dependent on media player support.
Benefits of Closed Captions- Closed captions can be turned on or off by the user (if supported by media viewer)
- Because they are stored in a separate file (sometimes called a sidecar) and not embedded into the video stream like open captions, they are easily editable.
- Closed captions can be created in a range of file formats (such as WebVTT/.vtt and SubRip/.srt) so have flexibility for different media players.
Potential Drawbacks of Closed Captions- Closed captions are not compatible with some media players and streaming platforms. They will only work if the platform supports closed caption files.
- Closed captions have limited font, color and position choices, and are aligned to a fixed grid.
- Closed captions place the technological responsibility on the viewer to understand how to turn the captions on and off which may be challenging for some audiences.
- Category:
- Video
- See also:
- Open captions; Subtitles; Teletext