ginger's thoughts

I’ve just tried to come to terms with the latest state of TTML, the Timed Text Markup Language.

TTML has been specified by the W3C Timed Text Working Group and released as a RECommendation v1.0 in November 2010. Since then, several organisations have tried to adopt it as their caption file format. This includes the SMPTE, the EBU (European Broadcasting Union), and Microsoft.

Both, Microsoft and the EBU actually looked at TTML in detail and decided that in order to make it usable for their use cases, a restriction of its functionalities is needed.

EBU-TT

The EBU released EBU-TT, which restricts the set of valid attributes and feature. “The EBU-TT format is intended to constrain the features provided by TTML, especially to make EBU-TT more suitable for the use with broadcast video and web video applications.” (see EBU-TT).

In addition, EBU-specific namespaces were introduce to extend TTML with EBU-specific data types, e.g. ebuttdt:frameRateMultiplierType or ebuttdt:smpteTimingType. Similarly, a bunch of metadata elements were introduced, e.g. ebuttm:documentMetadata, ebuttm:documentEbuttVersion, or ebuttm:documentIdentifier.

The use of namespaces as an extensibility mechanism will ascertain that EBU-TT files continue to be valid TTML files. However, any vanilla TTML parser will not know what to do with these custom extensions and will drop them on the floor.

Simple Delivery Profile

With the intention to make TTML ready for “internet delivery of Captions originated in the United States”, Microsoft proposed a “Simple Delivery Profile for Closed Captions (US)” (see Simple Profile). The Simple Profile is also a restriction of TTML.

Unfortunately, the Microsoft profile is not the same as the EBU-TT profile: for example, it contains the “set” element, which is not conformant in EBU-TT. Similarly, the supported style features are different, e.g. Simple Profile supports “display-region”, while EBU-TT does not. On the other hand, EBU-TT supports monospace, sans-serif and serif fonts, while the Simple profile does not.

Thus files created for the Simple Delivery Profile will not work on players that expect EBU-TT and the reverse.

Fortunately, the Simple Delivery Profile does not introduce any new namespaces and new features, so at least it is an explicit subpart of TTML and not both a restriction and extension like EBU-TT.

SMPTE-TT

SMPTE also created a version of the TTML standard called SMPTE-TT. SMPTE did not decide on a subset of TTML for their purposes – it was simply adopted as a complete set. “This Standard provides a framework for timed text to be supported for content delivered via broadband means,…” (see SMPTE-TT).

However, SMPTE extended TTML in SMPTE-TT with an ability to store a binary blob with captions in another format. This allows using SMPTE-TT as a transport format for any caption format and is deemed to help with “backwards compatibility”.

Now, instead of specifying a profile, SMPTE decided to define how to convert CEA-608 captions to SMPTE-TT. Even if it’s not called a “profile”, that’s actually what it is. It even has its own namespace: “m608:”.

Conclusion

With all these different versions of TTML, I ask myself what a video player that claims support for TTML will do to get something working. The only chance it has is to implement all the extensions defined in all the different profiles. I pity the player that has to deal with a SMPTE-TT file that has a binary blob in it and is expected to be able to decode this.

Now, what is a caption author supposed to do when creating TTML? They obviously cannot expect all players to be able to play back all TTML versions. Should they create different files depending on what platform they are targeting, i.e. a EBU-TT version, a SMPTE-TT version, a vanilla TTML version, and a Simple Delivery Profile version? Should they by throwing all the features of all the versions into one TTML file and hope that the players will pick out the right things that they require and drop the rest on the floor?

Maybe the best way to progress would be to make a list of the “safe” features: those features that every TTML profile supports. That may be the best way to get an “interoperable TTML” file. Here’s me hoping that this minimal set of features doesn’t just end up being the usual (starttime, endtime, text) triple.

UPDATE:

I just found out that UltraViolet have their own profile of SMPTE-TT called CFF-TT (see UltraViolet FAQ and spec). They are making some SMPTE-TT fields optional, but introduce a new @forcedDisplayMode attribute under their own namespace “cff:”.

Silvia Pfeiffer, Jan Gerber, Michael Dale “HTML5 video: how to process and publish video in an open format”, LCA 2010, Linux.conf.au, January 2010, Wellington NZ.

S. Pfeiffer, E. Carlson, M. Dale, V. Gal, J. Gerber, P. Jagenstedt,

D. van Deursen, S. Pfeiffer, R. Troncy, Y. Lafon, E. Mannens, R. van der Walle, “Implementing the Media Fragments URI Specification”, 19th International World Wide Web Conference (WWW’10), Developer’s Track, pages 1361-1364, Raleigh, North Calorina, USA, April 28-30, 2010.

Download it from the ACM Digital Library:

Implementing the media fragments URI specification

Davy Van Deursen, Raphaël Troncy, Erik Mannens, Silvia Pfeiffer, Yves Lafon, Rik Van de Walle
WWW ’10 Proceedings of the 19th international conference on World wide web,

S. Pfeiffer “Patents and their effect on Standards: Open video codecs for HTML5“, International Free and Open Source Software Law Review, Vol 1, No 2 (2009)

Download: http://www.ifosslr.org/ifosslr/article/view/21

Abstract

HTML5 is an updated version of the hypertext markup language that has been empowering the World Wide Web for the last 20 years. One of the things that HTML5 introduces is a element, which make video content as simple to include into Web pages as images. Similar to the issues that had to be overcome with the introduction of the tag in 1993, we are now facing the issue of a common baseline codec for the element

In this podcast, Mark Jones interviews Pia Waugh, ICT Policy Advisor for Senator Lundy; Senator Kate Lundy; Matt Barrie, CEO and founder, Freelance.com; and Silvia Pfeiffer, CEO and co-founder, Vquence about the ICT skills shortage and ways of addressing it. Education and tax incentives are two topics under discussion.

Today I am starting a new collection – recordings of interviews, talks I have given, and slides of the talks. There are many that I’ve missed, sorry. You may find some slides also on Slideshare.

Silvia Pfeiffer, “Taking HTML5 <video> a step further”, Web Directions South Conference 2009, W3C Standards Track, Sydney Convention Centre, October 2009.

C. Schremmer, S. Pfeiffer, A. Krumm-Heller, F. Mueller,