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

RE: TT and subtitling

From: <Johnb@screen.subtitling.com>
Date: Fri, 31 Jan 2003 18:07:03 -0000
Message-ID: <11E58A66B922D511AFB600A0244A722E6C5FDB@NTMAIL>
To: glenn@xfsi.com
Cc: public-tt@w3.org

I wrote:
> My current personal view is that TT should define a streamable file format
> consisting of self contained access units: 
> Each access unit should reference a preferably orthogonal timing element
> that supports at the minimum an on air time, optionally an off air time,
> where timing is either relative or absolute (relative timing would require
> the timing element to include a reference to the previous (and next access
> unit - to support trick play / reverse play)). The ability to group
> 'access units' together into a composite group is also desirable (e.g.
> words into lines, lines into subtitles). Display style should be external
> to the 'access unit' and the 'access unit' should allow the inclusion of a
> content definition (e.g. speaker, audio description...). A facility for
> defining additional supplementary information eg authors, creation dates
> etc should be provided. Guidelines for the streaming of the format should
> be developed. 
Glenn A. Adams wrote:

> 	Could you elaborate on how you see "on-air" vs. "off-air" time
> 	being expressed? 
Both On-air and Off-air timings would have the characteristic of being
**both** expressed as relative timings to previous and next 'presentation
units' (see later comment!) - this would be necessary if bi-directional
playout was required in the authoring context. Alternatively, both On-air
and Off-air timings could be expressed as absolute timings (i.e. wrt another
timebase within or external to the 'stream'. I see this as an attribute of
the timing for a presentation unit (absolute or relative timing). The
off-air timing could be optional - in which case the 'presentation unit'
remains until replaced. There is I feel a need to support multiple
compositions of 'presentation units' - since a TT display might consist of
several 'regions' (and I don't intend to imply location here) of text that
might be independently derived from the screen - some of which are using
over-writing and some are self timed etc. Sorry if this is a bit vague.....

> 	If I may draw from MPEG terminology, in that context
> 	there are two kinds of timestamps: DTS (decoding time stamp) and
> 	PTS (presentation time stamp). They are separated in MPEG because
> 	it is necessary to stage decoding prior to presentation, and also
> 	because order of delivery and decoding of access units may be
> 	different than order of presentation of presentation units.
I am not aware of any use of DTS within DVB subtitling.

> 	An "access unit" is defined by MPEG-2 Systems (ISO 13818-1) as:
> 	"A coded representation of a presentation unit. In the case of
> audio,
> 	an access unit is the coded representation of an audio frame. In the
> 	case of video, an access unit includes all the coded data for a
> picture,
> 	and any stuffing that follows it, up to but not including the start
> of
> 	the next access unit..."
> 	In contrast, a "presentation unit" is defined as:
> 	"A decoded audio access unit or a decoded picture."
> 	I find these terms to be very useful in discussing streaming media,
> 	and I would think they can be simply extended to describe timed text
> 	data as well.
So in effect an 'Access Unit' is a coded 'Presentation unit'. In my previous
paragraph - my intended meaning would perhaps be clearer if I had used the
term 'presentation unit'?

> regards 
> John Birch 
> 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 Friday, 31 January 2003 13:07:48 UTC

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