Comments to draft for TTML CodecsRegistry

Dear all,

Unfortunately I can't join today's TPAC meeting but I would like to make 
following proposals regarding the current draft of the Codecs Registry [1]

I suggest to initiate a liaison with other standard bodies that will 
most probably use this registry (e.g. EBU or DVB) and invite them to 
review the proposal.

It would be good to check if there is a possibility to use the registry 
before TTML 2 is published.

I propose further checks on the restriction of the registry to processor 
profiles. Some TTML specs to not state explicitly if they are a 
processor profile or content profile. They may be categorized by the 
definition as content profiles but nevertheless would use the mechanisms 
provided by the registry. So it should be considered to drop the 
categorisation or to drop the restriction to processor profiles.

Furthermore I have concerns that the definition will lead to more 
rejection of documents by processors than needed. If you take this text 
(from section purpose):

 >---

For example:

|codecs="stpp.ttml.A+B|C+D|E"|

states that a TTML processor that implements any one of A+B or C+D or E 
processor profiles satisfies, at first order, the requirements to fetch 
and begin decode/processing of a TTML document, *where /X/+/Y/ means 
that both /X/ and /Y/ processor profiles must be supported*, and /X/|/Y/ 
means that either /X/ or /Y/ processor profile must be supported.

 >--

The text implies a recommended behaviour that a processor should reject 
a document when it does no support a profile. But a document may not use 
all of the listed "features" in a profile. From my view a processor 
should always make best effort to present a document (possibly even if 
it's not valid). Missing error recovery for XHTML documents is an 
example from the past how it should not be done.

Best regards,

Andreas

[1] https://www.w3.org/wiki/TTML/CodecsRegistry

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Andreas Tai
Production Systems Television IRT - Institut fuer Rundfunktechnik GmbH
R&D Institute of ARD, ZDF, DRadio, ORF and SRG/SSR
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Phone: +49 89 32399-389 | Fax: +49 89 32399-200
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