[theora] Choosing subtitle language

Ralph Giles giles at xiph.org
Sat Jun 28 05:14:26 PDT 2003

On Saturday, June 28, 2003, at 12:31 pm, Hakon G wrote:

> I am wondering how negotiating about text subtitles is supposed to 
> work in Theora?

Ooh, my favorite topic. :)

> (1) Will it be like the server first transmit a list of available 
> languages, and then it's up to the client to possibly pick a language, 
> either automatically or by user interaction, and then ask the server 
> for that particular subtitle stream?

You're asking about how they'll be streamed really, which is less well 
understood. In the source ogg file, all the various options will be 
stored together, so if you're playing off disk (or disc) the player can 
use whatever method it wants to select among the options, and like dvd, 
it's easy to switch between them. The plan is to specify a labeling 
scheme which allows automatic language choice in the player based on 
user preferences.

> (2) Or will all subtitle streams be sent simultaneously and 
> regardless, thus fully leaving the decision to the client? That would 
> be a waste of bandwidth though.

Since everything's in the source ogg file, this is the easy thing to do 
with http streaming and probably what icecast will do, at least for the 
first release. It does use extra bandwidth, but we need to start 

> (3) Or will the client send a list of prefered languages, similar to 
> the http_accept_languages header? And then the server tries from that 
> list and sees what it can possible provide?

Actually, using http content negotiation for this is a really good 
idea. I'd not thought of that. I suspect it would require some 
architectural changes in icecast though. (i.e. the best way to get this 
is to do the work yourself.)

> I'd say the fist option is better. of course, it should be possible to 
> switch subtitle track anytime though.

That would require a reconnect with an http stream that didn't already 
include all the options.

With RTP streams, the practice is to send every substream over a 
parallel connection, so flexibility is much easier to implement, and 
avoiding wasted bandwidth is one of the primary design goals. So 
everything will be different there. I've no idea how to chose among the 
available streams in that case. Does RTSP have a established mechanism 
for this?

> One problem though, would be how to behave in e.g. newscasts, where 
> there may be different spoken languages as the clip rolls.
> I'm not sure if the LANGUAGE comment for audio could be changed as the 
> stream is running. Nevertheless, it would probably be too much to ask, 
> expecting content providers to bother and dynamically set such flags 
> throughout a clip.

Probably, yes. This is also a problem with dramatic works (esp. movies) 
which often have multiple languages, if only in short sections. That's 
actually a case where inserting flags is more reasonable, but it so 
complicates things I suspect it's not worth implementing.

DVD has this problem too and I've seen discs that have both a subtitle 
track to cover non-english dialog for english speaks, and another 
"hearing empaired" track that covers all the dialog regardless of 

> Anyway, it would be imporant that the subtitles feature is promoted 
> good, otherwise content providers would stick to the conventional 
> methods, and just include all subtitles as part of the video image.

I agree. We also need good tool support for producing and transcoding 
the titles, which is something we really need volunteers to write. 
(hint hint :)


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