Digital Closed Captioning
For ATSC (digital television) programming, three streams are encoded in the video: two are backward compatible Line 21 captions (CEA-608), and the third is a set of additional caption streams encoded in EIA-708 format. The captioning data is carried in the video user bits of the MPEG-2 bitstream (as specified in ATAC A/53B) and is applicable to both HD and SD. The CEA-708 specification provides for improved captioning compared with CEA-608:
- Up to 63 services per program (16 announced in PSIP)
- 8 independently controlled display windows
- Extended range of characters (i.e. more accented letters and non-Latin letters) and fonts (monospaced and proportional spaced, serif and sans-serif)
- Additional font sizes (viewer-adjustable), backgrounds (including both transparent and translucent backgrounds to optionally replace the big black block), colors and edges
- More text styles, including edged or drop-shadowed text rather than the letters on a solid background
- Higher bandwidth, to allow more data per minute of video
- More language channels, to allow the encoding of more independent caption streams
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