[theora] Closed Captioning
josephs at agames.com
Wed Dec 4 17:46:28 PST 2002
Also, I think the NTSC standard was that 7 frames of delay was "acceptable."
Although if you consider every device in the chain (encoder, muxer,
transmitter, decoder) it adds up to a HUGE possible delay. This is just
taken as unavoidable... However, there is some mechanism for delaying the
display of on-screen test to take advantage of empty space (and "catch-up")
when using pop-on or paint-on.
This same type of "render it NOW" mentality is in the DTVCC (708) spec, even
though they define a 128 char buffer. I wish they would have used the clock
signal in the mpeg2 stream and had a sync primitive.
> -----Original Message-----
> From: owner-theora at xiph.org [mailto:owner-theora at xiph.org]On Behalf Of
> Ralph Giles
> Sent: Wednesday, December 04, 2002 5:37 PM
> To: theora at xiph.org
> Subject: Re: [theora] Closed Captioning
> On Wednesday, December 4, 2002, at 08:23 pm, David Siebert wrote:
> > Accumulating lines is not a great option a delay could be a killer.
> > Suppose that you are doing a newscast and the delay some how has
> > captions talking about a child molester being arrested when there is a
> > picture of the Pope on the screen? Or the captions for a tampon ad
> > showing up with the video of a cookie ad? It has to sync with the
> > video.
> Advertisements will (and are currently) already be transcribed and just
> spliced into the stream, no?
> For live transcription I'm not convinced the delay would be unworkable.
> How much does the steganographer introduce? There will be some encoding
> delay too, a clever encoder could take advantage of that by inserting
> the text at the very last minute.
> From the point of view of theora, the issue is mostly designing
> something that can decode parts of a line at a time but be robust
> against packet loss. Complicates things somewhat.
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