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COMMENT CLASS Where do captions & subtitles live? I t’s not uncommon to see “ability to insert/extract captions/subtitles” as part of a tender requirement or product specification. But in a file-based world, what does this mean and where are we inserting/ extracting to/from? by Bruce Devlin For this article, we’ll defi ne captions and subtitles as text overlay that the user turns on or off, where “captions” refers primarily to US-style closed captions and “subtitles” to the European/Australasian equivalents. Let’s start by looking at the very end of the signal chain – the consumer display. In the analogue SD world this was fairly simple because captions came in data packets (as defi ned by EIA-608) and subtitles were sent as WST (World Standard Teletext). The receiver in the TV set would decode the words and render them on to the screen. This data took advantage of space in the VBI, Vertical Blanking Interval – a part of the signal used for timing in old CRT displays. The advent of digital television, digital video broadcasting (DVB) and HD, broadened the number of options signifi cantly, with more “space” in the signal for caption/subtitle data, bringing about Free TV Australia’s OP47 and EIA-708 in addition to DVB Bitmaps and other methods to transmit caption and subtitle data through a facility to the consumer’s display. Until fairly recently, the caption/subtitle chain was commonly kept separate from the audio/video up to Business Layer Media Asset Management SERVER SPECIFIC VANC ST-436 VANC (MXF) CARRY 608, 708, OP-47, TR-B36 SD playout CABLELABS SCTE 20 ATSC A-53 ECHOSTAR HD playout PROPRIETARY (OTHER) MXF VOD GXF STORAGE MOV Web Streaming CDN SOME MAGIC ONE-FORMAT TO RULE THEM ALL? PROMOTIONAL INTERNATIONAL VERSIONING .SCC .CAP .STL .DFXP 40 | KITPLUS - THE TV-BAY MAGAZINE: ISSUE 104 AUGUST 2015 SMPTE-TT (W3C) SMIL SAMI