[Technologist]

Back a few …this is how subtitles are handled

At the moment the interchange format is EBU 3264
This dates from 1989 tech.ebu.ch 

It is worth noting that this was all subtitles not specifically teletext ….
So the code table used is not teletext (prat trap 1) and there is not always a link
between the authoring language and national character set (prat trap 2)
and the setting of diacritical marks by packet x/26 can be inconstant (prat trap 3)

The Teletext is now carried in HD using OP47 www.freetv.com.au 
And then at coding and mux is turned into DVB ST for Freeview and Freesat
and DVB Teletext for sky (and often European broadcasters.)

But the future is timed text and there is a convergence on this
With EBU TT tech.ebu.ch 
SMPTE TT which is “safe harbour for ip delivered tv channels”
Under FCC regulation …it’s the law ! ST 2052
ieeexplore.ieee.org 

www.dwt.com 

This (a bit dated ) from w3C describes where things were
www.w3.org 
But IMSC2 is the way forward ..

Note that these techniques work for both broadcast and internet based
Closed Captioning .. as well as theatric projection ……
And being Unicode based handles ideographic character sets …… and those with diacriticals

But there is so much EBU 3264 and DVB ST investment in Europe
And a range of proprietary authoring systems in USA … and EiA608
That its rollout is taking some time ….