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제목 | 자막(caption, subtitles) 2021-08-20 627
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(발췌:https://www.drastic.tv/solutions/170-closed-captioning)
* CEA-608: 608 was developed during the 1970s to carry caption and data services on NTSC standard definition video. It was designed to survive poor transmission, recording and re-recording while maintaining the caption information. It was encoded as a black and white series of pulses on line 21 of the first field of video and line 284 for the second field. Each field allowed two possible caption channels, allowing a total or four, known as CC1, CC2, CC3 and CC4. The total amount of data that could be transmitted was 960 bits per second.

* OP-42: OP-42 is a standard definition specification for transmitting captions on PAL (25/50 Hz) systems. It is based on a part of the Teletext specification that has since become the WST described below.

* CEA-708: 708 was developed to replace 608 for HD television transmissions. 708 actually includes the 608 bytes as a compatibility layer for older decoding equipment, but also provides a completely new, enhanced captioning system. 708 is transmitted digitally as part of the compressed HD DTV stream. It is allocated ten times as much space, 9600 bits per second, as 608. Like 608, the meaning of the captions is described in the Consumer Electronics Association docs. The transmission of the packets is described in SMPTE 334-1 (VANC video packet) and as an ancillary packet in SMPTE 334-2.

* WST: The World System Teletext is the formalized version of the teletext transmission system used to transmit information, subtitles, and news to televisions before the internet. This specification is still important, though no longer used directly, as it is the basis of OP-42, and OP-47 / RDD-08.

* OP-47 (RDD-08): OP-47 (known by SMPTE as RDD-08) is the European standard for subtitles and closed captions in HD. It has more capabilities than OP-42 and CEA-608, but not as many as CEA-708, as it is based on a limited spec from the larger WST specification.

* Subtitles: Subtitles are now also used as closed captions, especially for web distribution, where it is easier to distribute a series of text or XML files for each language for a show, rather than pre-encode them for automatic transmission.
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