Re: Closed caption fun

Evan Kirshenbaum wrote:

DVD subtitles are, I believe, kept in separate files and merged into
the video stream by the player.

They are actually merged (interleaved, multiplexed) into a packetized video/audio stream (properly called a "program stream") when the DVD is being authored.

When playing back the disc, the DVD player filters the relevant audio/video/subtitle packets (or the substreams they represent) from that main program stream, according to the language choices the viewer made, and redirects these extracted substreams to the appropriate decoders.

DVD players typically have at least two separate framebuffers in their video output hardware: one for the decoded MPEG-2 video, and another one - the so-called "OSD layer" - on top of it, for the superimposed subtitles and menu (subpicture/highlight) graphics. The latter of the framebuffers can use partially or fully transparent (indexed) colors, or - depending on the chip in question - it may even have full alpha channel support for defining partially or fully transparent areas. The video hardware handles the mixing and overlaying of these two framebuffers automatically; the programmer only needs to fill them with pretty pictures.

The idea is basically the same for the European digital tv broadcasts (adhering to the "DVB" family of standards) and the subtitling employed there, although the subtitling data is not in the same format.

Interesting bit of trivia is that both the DVD subtitles and the DVB subtitles are actually bitmap pictures with a low color depth, indexed colors and a palette definition. In other words, the subtitles are not textual data at all, in these standards, and they do not rely on a font built-in to the decoder. This means that the author of the disc (in case of DVDs) or the broadcaster (in case of DVB broadcasts) can use any font and script they wish. You could create DVD or DVB subtitles even in Klingon, or with IPA markup, if you wanted to.

--
znark

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