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RE: [tt-af-1-0-req] Some (late) comments on the requirements

From: <Johnb@screen.subtitling.com>
Date: Wed, 21 Jan 2004 13:36:37 -0000
Message-ID: <11E58A66B922D511AFB600A0244A722E9EE6F7@NTMAIL>
To: luke-jr@cox.net
Cc: public-tt@w3.org
Luke et al,

> > Basically you say "the video file that goes with the timed text will
> > be paused". In a broadcast environment this is not possible. You CANNOT
> > the video..... E.g. It is coming off a server, through an MPEG encoder
> > up to a satellite.

> Ah, I was assuming you were doing the subtitles at the same place the
> was being broadcast originally.

We are, the subtitles are inserted over the top of a video broadcast at the
point of broadcast.
(Termed 'playout' subtitling).

> Personally, I would suggest seperating parts 
> of a TT file into scenes and marking subtitles with (very accurate) 
> percentages through the video. Then it would be easy to  adjust to
> source files or media.

Hmmm...  There seems to be a misunderstanding here.

How would a subtitle inserter detect the start of a scene?

Basically the broadcaster has video and audio on a tape or video server.
In accordance with a schedule of programming, the tape gets played (or the
server is started).

As the program plays out, the timecode on the video signal is monitored.
At the correct points in the program, the subtitles are inserted.
This has to be a real time rendering process. (although we can and do
pre-render in 
anticipation of the next insertion event).

If the broadcaster chooses to interupt his program with an advert, or public
service announcement, then the station automation system switches across to
a different
video segment. When the adverts finish, the previous program resumes from
where it left

The only clues the subtling inserter has are the timecode of the video, and
an indication
from the automation system that the program has changed.

This is not like producing a media clip, where the subtitles are added
against a media clip
of known duration (in play time terms), since the time required to air an
entire program
will depend on how many advert breaks the broadcaster chooses to have. This
may differ
from channel to channel and possibly at different times of day.

> > > > Basic colours (for text - 16 colour model is sufficient)
> > > 16 colours per dialogue may be sufficient, but overall it is
> > > unlikely to be enough. Usually when I subtitle something, 
> > > I use different colour schemes to represent who is speaking. Timed
text (not just
> > > subtitles) also would involve displaying, for example, the title of a
> > > movie or similar which could very well need complex effects including
colour fading.

> > Broadcast subtitling also uses colour for dialogue. E.g UK Teletext.
> > Teletext is limited to a basic colour set.

> So you're looking at TT as a format which you convert when 
> streaming it. I'm looking at TT as being the format that is actually

TT requirements are for primarily an authoring format.
TT **may** be used as a distribution format, but that is not the main
I.e. it is not for example a primary goal to develop the most wire efficient
TT AF will (hopefully) be optimsed for editing/authoring.

> > Speakers are allocated a colour, but often colours are re-used
> > if a character no longer appears in the program.

> Even reusing previous characters' colours, there are still 
> many different characters which would go over using 16 colour shades.

8 foreground colours works fine for Teletext subtitling, in fact not all 8
are commonly used. In fact, a **lot** of subtitling is done with just white
on black.

> > Extra colours would be useful for more creative purposes, but
> > they are not IMHO essential.
> They would be essential for a do-everything-TT format, which 
> I hope is what the TTWG is meant to create...

Yes, format should support them, but they might not need to be 'core'

> > There is also an issue about how many colours 
> > could be easily distinguished from each other. Certain colours
> > do not show well on video, and it is more difficult to distinguish
> > quickly between colours that vary by intensity but not hue.

> One could also argue that 256 colors is plenty for images too.

There is a difference between the use of shades within an image, and the use
of colour
for text. It is primarily an issue of readability - especially for TV
subtitling where
the text is over changing video, and viewed on a 17 inch screen from the
other side of
the room.

> Convince the TV industry to move to using MPEG4/Vorbis/TT for 
> distribution content. ;)
> Simple enough to do it for a DVD-replacement format though...

I doubt you'd make much headway in getting consumers to move away from DVD
A new standard would have to offer significant advantages over DVD.

> > > > Finally - although a distinction is (rightly) made concerning
> > > > and subtitles, in terms of the system requirments for their display
> > > > there is very
> > > > little difference. Captions may use more features for display than
> > > > subtitles, as captions carry non speech information as text and this
> > > > may be rendered using colour or styles that would not normally be
> > > > for speech related text display.

> > > Though not perhaps the technical definition of subtitles, they are
> > > usually considered to include overlaying translations of visual items
> > > making them much more complex than captions.

> > Actually that is more of a definition of 'description'.

> > According to SMPTE:
> > subtitles are translation of dialogue
> > captions are dialogue and sound effects in the same language (no
> > translation) (i.e. for hearing challenged)
> > description is text substitute for the visual items (i.e. for visually
> > challenged)

> I am referring to translations of visual items.
'Text over video' i.e. that describes what the video is (...what can be seen
in the picture)
is called 'description'.
'Text over video' i.e. that is equivalent to what is said is either
'subtitles' or 'captions'.

For SMPTE the distinction between the two is that subtitles involve language
translation, but
captions include text to describe sound effects in addition to speech.

John Birch

The views and opinions expressed are the author's own and do not necessarily
reflect the views and opinions of Screen Subtitling Systems Limited.
Received on Wednesday, 21 January 2004 08:38:40 UTC

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