W3C home > Mailing lists > Public > public-tt@w3.org > February 2003

RE: TT and subtitling

From: <Johnb@screen.subtitling.com>
Date: Mon, 3 Feb 2003 10:05:38 -0000
Message-ID: <11E58A66B922D511AFB600A0244A722E6C5FF2@NTMAIL>
To: glenn@xfsi.com
Cc: public-tt@w3.org

> Let me back up a bit, I guess I am having trouble understanding
> exactly what you mean by "on-air" vs "off-air" times. So, perhaps
> you can educate me a bit about this distinction.
Sorry, in subtitling the start of presentation of a subtitle is referred to
as its on-air (or in-cue), the time at which the subtitle is removed from
display is the off-air (or out-cue). On-air and off-air are probably more
correctly used when talking about Open subtitling (where the subtitle is
burnt in to the video image prior to transmission) - although 'we' also tend
to use on-air and off-air when discussing DVB rendering of subtitles
(conversion to bitmap form and encapsulation in PES packets). In the
authoring environment the terms in-cue and out-cue are the norm - and refer
to the timecode at which the subtitle is desired to first appear and the
timecode at which it must disappear.
> As for access unit vs presentation unit, I would tend to use the
> former when talking about the coded representation and its delivery
> and buffering modes, and use the latter term for talking about its
> decoded to-be or currently presented modes.
I must admit I find this a sublte distinction - do you anticipate there
being a major difference between the format of an access unit and a
presentation unit for TT? (encryption and compression issues aside).
> Regarding DTS vs PTS, I would probably expect the DTS to be implied
> in the context of TT, and that only an equivalent of PTS be specified
> or otherwise computable. I can see an implementation of a streaming
> TT decoder might want to dynamically compute a DTS based upon its
> ability to decode and compute a presentation unit from an access unit.
> For example, some access unit might embed font outline data that
> requires rasterization. Different decoders may have different
> performance profiles with regard to their ability to perform this
> rasterization.
I wish this was the case in DVB decoders. Recalling previous emails about
hanging subtitles - this is part of the issue. The decoders we have
encountered (and I doubt there are many we haven't) - all generally decode
subtitle data packets strictly in order of reception - regardless of whether
the presentation time has passed even - then when they get behind they drop
packets! A classic case of implementing only the bare essentials.... 

regards John B

The views and opinions expressed are the author's own and do not necessarily
reflect the views and opinions of the Screen Subtitling Systems Limited.
Received on Monday, 3 February 2003 04:59:53 UTC

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