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Re: Liaison response - template on MIME type parameter for TimedText

From: Nigel Megitt <nigel.megitt@bbc.co.uk>
Date: Thu, 1 May 2014 10:23:13 +0000
To: Michael Dolan <mdolan@newtbt.com>, "'Timed Text Working Group'" <public-tt@w3.org>
Message-ID: <CF87D07D.1CF97%nigel.megitt@bbc.co.uk>
The proposal for TTWG is to host a register of short labels against TTML profiles and variants, without in doing so endorsing those variants. Glenn took ACTION-284 to create this registry page according to the resolution agreed, minuted in [1].

[1] http://www.w3.org/2014/04/24-tt-minutes.html


In my view namespace location is an orthogonal issue to specification conformance. Namespaces are signalled within the document, and they are poor identifiers of specification conformance, especially when they are mutable, which for example TTML's namespace is.

What's needed here is a mechanism to allow a processor to choose appropriate behaviour before parsing the document. This behaviour could include choosing not to attempt to parse, selecting an appropriate parser and renderer, or even directing a parser and renderer to adopt specification-related semantics (initial values could be an example). (plus other options I guess)

It may be wise for organisations requesting registration of short labels for a specification to consider specification versioning.

Kind regards,

Nigel


On 01/05/2014 00:40, "Michael Dolan" <mdolan@newtbt.com<mailto:mdolan@newtbt.com>> wrote:

Nigel, Dave and all-

Is there a TTWG Proposal?

Would “stpp” be registered somewhere where it would be unambiguous from other Codecs strings unrelated to TTML?  I don’t mean the sample entry 4C code, but in the Codecs string namespace.  Wouldn’t it have to be “application/mp4+stpp….”?  Or why not use “application/ttml+xml” (and similar for WebVTT)?

I understand how one could signal profiles of TTML that a document conformed to concurrently, as in the example – all of TTML and EBU-TT.  But the signaling requirements go beyond that – there is often multiple namespaces in use in one document that are not, as an aggregate, a single “profile”. So, these must be explicitly signaled as well since nearly all profiles permit foreign namespaces.  To accommodate this, the “short names” have to be defined as “profiles of namespaces” I think.

For example, if a document uses the CFF-TT text profile of the TTML Full profile, plus SMPTE-TT #608 (US captions), plus CFF-TT metadata, and it was compatible with IMSC, SDP-US and EBU-TT, then it might look like: Codecs=xxxxx.TTMLFULL+IMSC+SDPUS+EBUTT+CFFT+CFFM+SMPTE608.

Regards,
                Mike

p.s. I would give the SC29 Secretary a hint about the target of the liaison (MPEG v JPEG).  And, you understand you will not receive a reply until mid-to-late July, right?


From: Nigel Megitt [mailto:nigel.megitt@bbc.co.uk]
Sent: Wednesday, April 30, 2014 11:48 AM
To: watanabe@itscj.ipsj.or.jp<mailto:watanabe@itscj.ipsj.or.jp>
Cc: Timed Text Working Group
Subject: Liaison response - template on MIME type parameter for TimedText

Dear Mr. Watanabe,

Thank you for your liaison N14444 of April 2014.

We think that we can indeed find a solution together.  We are looking into creating a table of formal "short names" for the profiles of W3C TTML and
the profiles of formats derived from it (such as SMPTE-TT, EBU-TT, and so on).  If MPEG were to propose how to step from the four-character-code of
the sample entry (XMLSubtitleSampleEntry and XMLMetaDataSampleEntry) to something that identifies "a document compatible with one or more profiles of TTML", then we could propose a string composed of a set of one or more these short names as the next parameter.

For example, say W3C defines two profile short names "W3CTTML" and "EBUTTML", and MPEG defines the name "TTML" as referring to the overall
family, one might see

codecs=stpp.TTML.W3CTTML+EBUTTML,avc1

as a codecs string of a file carrying AVC (H.264) and TTML subtitles that
are additionally EBU-TT conformant.

We would check with those deriving from TTML (e.g. at SMPTE, EBU and DECE) if this approach and design are acceptable, before we formalise this.

Kind regards,

Nigel Megitt, David Singer (chairs, Timed Text Working Group, W3C)

--

Nigel Megitt
Lead Technologist, BBC Technology, Distribution & Archives
Telephone: +44 (0)3030807996
Internal (Lync): 0807996
BC4 A3 Broadcast Centre, Media Village, 201 Wood Lane, London W12 7TP




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Received on Thursday, 1 May 2014 10:22:59 UTC

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