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CEA-708 is the standard for closed captioning for ATSC digital television (DTV) streams in the United States and Canada. It was developed by the Electronic Industries Alliance. Unlike RLE DVB and DVD subtitles, CEA-708 captions are low bandwidth textual like traditional EIA-608 captions and EBU Teletext/Ceefax subtitles. However, unlike EIA-608 byte pairs, CEA-708 captions are not able to be modulated on an ATSC receiver's NTSC VBI line 21 composite output and must be pre-rendered by the receiver with the video frames, they also include more of the Latin-1 character set, and include stubs to support full UTF-32 captions, and downloadable fonts. CEA-708 captions are injected into MPEG-2 video streams in the picture user data. The packets are in picture order, and must be rearranged just like picture frames are. This is known as the DTVCC Transport Stream. It is a fixed-bandwidth channel that has 960 bit/s allocated for backward compatible Line 21 captions, and 9,600 bit/s allocated for CEA-708 captions. The (ATSC A/53 Standard ) contains the encoding specifics. The main form of signalling is via a PSIP caption descriptor which indicates the language of each caption and if formatted for "easy reader" (3rd grade level for language learners) in the PSIP EIT on a per event basis and optionally in the H.222 PMT only if the video always sends caption data. CEA-708 caption decoders are required in the U.S. by FCC regulation in all 13" (33 cm) diagonal or larger digital televisions. Further, some broadcasters are required by FCC regulations to caption a percentage of their broadcasts. == Packets in CEA-708 == Caption streams are transmitted with many packet wrappers around them. These are the picture user data, which contains the caption data, which contains the cc_data, which contains the Caption Channel packets, which contains the Service Block, which contains the caption streams. These packets are described in detail in this section. But the streams themselves are described in the following sections. This layering is based on the OSI Protocol Reference Model: This section will describe the various packets, the Coding Layer and Presentation Layers are described in the remainder of this document. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「CEA-708」の詳細全文を読む スポンサード リンク
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