{Minutes} TTWG Teleconference 2022-04-14

Thanks all for joining today’s TTWG call. Minutes can be found in HTML format at https://www.w3.org/2022/04/14-tt-minutes.html


In text format:

   [1]W3C

      [1] https://www.w3.org/


                Timed Text Working Group Teleconference

14 April 2022

   [2]Previous meeting. [3]Agenda. [4]IRC log.

      [2] https://www.w3.org/2022/03/31-tt-minutes.html

      [3] https://github.com/w3c/ttwg/issues/215

      [4] https://www.w3.org/2022/04/14-tt-irc


Attendees

   Present
          Atsushi, Cyril, Gary, Mike, Nigel, Philippe, Pierre

   Regrets
          Andreas

   Chair
          Gary, Nigel

   Scribe
          gkatsev, nigel

Contents

    1. [5]This meeting
    2. [6]IMSC-HRM Wide Review update
    3. [7]DAPT-REQs
    4. [8]Timed Text in Low Latency Streaming applications
    5. [9]Rechartering status update

Meeting minutes

  This meeting

   Nigel: Today we have IMSC HRM Wide Review update,
   … DAPT-REQs update
   … Timed Text in Low Latency Streaming applications,
   … Behaviour with non-native controls (in case there's anything
   more to say about that today)

   Gary: Probably not this week

   Nigel: And Rechartering status update (for 2nd half of the hour
   only)
   … Any other business, or points to make sure we cover?

   Atsushi: I'd like to point to an error in the DAPT-REQs

   Nigel: OK, let's cover that in that agenda topic

   group: [no more AOB]

  IMSC-HRM Wide Review update

   Nigel: Quick update: I sent the comms out, and got some thank
   you acknowledgements back.
   … So I think we can tick that action as being done.
   … As is often the case with these occasional liaison messages,
   there are some updates to the
   … liaisons page that came to light.
   … I have not forwarded any of the thank you/acknowledgements to
   the member-tt list.
   … Anyone think that's worth doing?

   Pierre: I think you can safely not do it.

   Nigel: I'm hoping for that!
   … Ok, I won't then.
   … Any questions or points on this before moving on?

  DAPT-REQs

   Atsushi: For now we have SVG images integrated in the HTML.
   … That causes fatal errors in the HTML validator which doesn't
   support XML namespaces in SVG
   … That error comes from HTML Validator and will cause a fatal
   error during streamlined publication
   … during GitHub Action, so one way is to ask the HTML Validator
   to support integrated SVG images,
   … which I believe is quite hard.
   … Another is to separate that part as a distinct file, not
   integrated into HTML.
   … If possible I'd like to propose the latter way.

   Nigel: If I understand correctly that will break the page,

   <atsushi> [10]pubrules error

     [10] https://www.w3.org/pubrules/?url=https://www.w3.org/TR/2022/DNOTE-dapt-reqs-20220412/&profile=DNOTE&validation=simple-validation&informativeOnly=false&echidnaReady=false&patentPolicy=pp2020


   Nigel: because it will stop the click-through fragment links in
   the SVG from working.
   … I think!

   Cyril: Yes because inside the SVG you have href fragment ids
   that point to elements in the HTML,
   … for navigation.

   Atsushi: Ah!

   Nigel: Exactly

   Atsushi: For now I have no idea then.

  Cyril: Let's think offline. I don't know if it's a Must, it's a
   Nice to Have.

   Pierre: Have we asked how much trouble it would be to update
   the validator?

   Atsushi: No idea. I suppose there's quite a small activity on
   that now.

   Nigel: I'd note that the W3C Process doc includes an embedded
   SVG with some behaviour as well.

   Nigel: I think the action is to ask about updating the
   validator.

   Pierre: At least.

   Atsushi: Yes. In any case please note that there is a
   possibility of some errors being shown due to this.

   Nigel: Yes I can see the validator errors are terrible.
   … But can we still publish?

   Atsushi: Yes, for the initial publication of FP DNote it is
   done manually so we can state
  … that these errors are okay.
   … It should be possible to publish by hand.
   … You may see in that page I pasted above that Charter
   extension has not yet been announced yet,
   … so there's an error about the group. I'm waiting for that -
   should happen today or tomorrow I believe.

   Nigel: Right.

   Nigel: So status now is Not Published Yet.
   … Atsushi will you ask about fixing the HTML Validator?

   Atsushi: I will write today or tomorrow to sys team or spec
   prod. Someone might know something.

   Nigel: Ok, thanks.
   … Any more on this agenda topic?

  Timed Text in Low Latency Streaming applications

   [11]Thread that began this.

     [11] https://lists.w3.org/Archives/Public/public-tt/2022Mar/0010.html


   Gary: I want to note that everything that we're discussing here
   for IMSC would also apply
   … to WebVTT in terms of HLS segmentation etc.
   … It's something we have started to investigate too, how to do
   this.

   Mike: It's a timely topic. I'm aware of one encoder vendor that
   implemented something.
   … Still learning what they did exactly.

   Nigel: Would be good to start this with a summary of the
   problem that needs solving.

   Mike: The problem is that TTML documents can't really be
   incrementally parsed and displayed.
   … You have to get to the end of the document and then you can
   start to present.
   … You can make it better by omitting things like set and
   animation.
   … In general the model is you get a document, you parse it and
   display it.
   … That process introduces delay in packaging the segment.
   … You can't start to emit the segment until you have a complete
   document,
   … and if it's a 2s segment say, you can't emit anything until
   those 2s are up.
   … This is only for low latency live. If you have VOD and don't
   care about low latency you don't care about this.
   … In a TV studio like today for ATSC the encoders are taking an
   MPEG transport stream with 608 captions,
   … and starts transcoding into ATSC 3 including IMSC 1.
   … For video and audio you can encode incrementally with random
   access points.
   … In the case of IMSC 1 you have to wait until the segment time
   is over before shipping it off.
   … In practice that imposes a delay in the audio and video
   today.

   Nigel: And you could reduce the segment duration, but there's
   something limiting that?

   Mike: Every time you make the segment duration smaller there's
   a lot higher ratio of signalling to content.
   … Some schemes like CMAF limit the minimum duration of a
   segment to 960ms.

   Cyril: Mike, you mentioned 2 things that got me thinking.
   … In live low latency, what would be the constraint that would
   make the progressive rendering not work?
   … As I mentioned on the thread, it is possible to have a
   progressive SAX parser and renderer.
   … The 2nd question is: nothing prevents low latency chunks with
   documents. The overhead increases,
   … but is it that significant compared to audio and video?

   Mike: I'm coming at this from the desire to have a backward
   compatible solution with existing
   … equipment in the field. One of the solutions is chunking IMSC
   1.
   … In theory a smarter decoder could parse that and present it.

   Cyril: I don't follow - you're saying implementations in the
   field don't support progressive decode?

   Mike: No, the incremental encode into packets...

   Cyril: No, I mean 1 segment = 1 sample = 1 document but you
   deliver this with HTTP chunked transfer,
   … progressively, and you let the renderer handle the SAX
   parsing and incremental rendering.

   Mike: Non-backwards compatible.

   Cyril: Why?

   Mike: A decoder today wouldn't know how to handle that.

   Gary: The expectation is that the document is done after
   writing and won't get updated afterwards.

   Mike: Yes, it's too late in the 1-2s timeframe.

   Cyril: So to paraphrase, decoders do not support chunked
   download and parse?

   Mike: They could, but they don't.

   Cyril: And why not increase overhead with multiple small
   samples?

   Mike: My reading of 14496-30 says there's 1 document per
   segment.

   Cyril: I disagree, part 30 does not mention segmentation, it
   talks about carriage into samples.

   Mike: I'll check that, maybe I was referring to CMAF. It would
   solve the problem.
   … But today, a backwards compatible solution would be to start
   putting out documents incremental
   … and a smart decoder could parse and decode as it goes along,
   whereas older ones have to wait.
   … Need to take into account that there are some devices that
   can never be updated.
   … Need to support their behaviour for a while until end of
   support period.
   … Harder than software decoders, for example.
   … Idea is that the sum of the samples would still be a legal
   segment.

   Cyril: That's not compliant with part 30.

   Mike: Not multiple samples, just chunked delivery in different
   packets, for a single sample.
   … That's one idea that would keep compliance and backwards
   compat

   Nigel: Complicated interplay between standards here, with
   constraints coming from
   … specifications not defined here. How can we constrain our
   discussion to things that we can control?

   Mike: Need to include constraints from ISOBMFF.

   Nigel: But that doesn't impose any issues - the key constraint
   here is from CMAF not ISOBMFF.

   Mike: The two largest implementations are from DASH and HLS
   with CMAF.

   Cyril: CMAF does not constrain anything here though - 960ms is
   a recommendation only.
   … It's the same for audio and video and this is no different.
   … To Nigel's point, what is needed in TTML to solve this use
   case?

   Mike: Different ways to solve this.
   … We could introduce non-backwards-compat features in TTML to
   help solve this.

   <cyril> ack

   Cyril: For example?

   Mike: Like assumptions in the case of a partial document, for
   example.
   … Not sure I want to go there, it's just an example
   … I hadn't given it a lot of thought until our exchange.
   … Based on a private conversation I thought of a couple of
   ideas of how it could work.
   … I have a somewhat unique problem with respect to broadcast,
   which adds a layer
   … in terms of what can and can't be expected to work.

   <Zakim> gkatsev, you wanted to ask about whether short segments
   provide a good UX. Might have really short lines?

   Gary: One of the potential solutions is to have really short
   segments, and a document per segment.
   … My issue with that is a potentially bad user experience, if
   there is not enough text to
   … provide a full line. Unless you did a whole bunch of work to
   position properly.
   … Because you can't append to an existing cue.

   Cyril: It's difficult to know the position because you don't
   know the device font.

   Gary: It's like how to handle partial cues.
   … You might want to send stuff to the client before you have
   all the information you want to have.

   Nigel: Strict formatting formats make this easier - in the BBC
   we use word by word live subtitles
   … all the time in IMSC and it works fine as long as the text
   alignment is set correctly.
   … In situations where, for example, text is centre aligned, it
   is unreadable.

   Gary: Yes, that does make it easier.

   Cyril: It would really help to have a concrete example, not
   talking about
   … documents, chunking, segmentation, just what you want to
   display at what time,
   … then people can propose ways to do it, and we can see where
   the spec compliance issues arise.

   <cyril> ack

   Mike: Happy to put that together.
   … First and foremost, the encode delay.
   … No problem with TTML syntax for timing. That's not the issue.
   … Given a paint-on progressive delay, within a single document
   you can add text in small numbers of letters.
   … Most decoders work pretty well that way.
   … Good question, so I'll try to explain the problem in more
   detail in writing, with a picture or two.
   … I'll take that as an action item.

   Cyril: Thank you.

   Nigel: Just a clarification question about the requirements: is
   anyone worrying about data rates?

   Mike: In general no, but one of the things we have had to do is
   reconstruct the screen for a
   … random access point in every segment. The first thing to
   worry about is recreating the screen
   … from scratch. That could be tedious. So far that hasn't hurt
   the processors but it gets
   … quickly out of control, hence the focus on the HRM.
   … The initial implementations to make this work were terrible
   and violated the HRM.
   … Now the output is HRM conformant that helped bound the
   complexity.

   Nigel: Thanks.

   g

   <Zakim> gkatsev, you wanted to ask about character/word
   deletions from say 608

   Gary: Maybe not to answer right now, but one thing brought up
   previously
   … with regards to live captioning is that sometimes the
   captioner can delete words or
   … characters in 608, and depending on how you're doing live
   captions you might,
   … say if you're emitting VTT or IMSC caption right before a
   delete command, how would you
   … go about deleting that word?

   Mike: This is an issue with live captioning. So far the
   encoders have ignored it.
   … I haven't produced any tests with backspace.

   Gary: This is probably a longer term thing, we don't need to
   worry in the short term.

   Mike: For now it's a general problem not a low latency live
   one.

   Nigel: In IMSC just as you can add text you can remove it, so I
   don't think that's a format problem per se.
   … If you're in a cue based environment where cues cannot easily
   be changed, I don't know how to resolve that.
   … For time, let's close off now unless there's anything else
   burning on this topic?

  Rechartering status update

   [plh joins]

   Nigel: Atsushi already advised that we're expecting a Charter
   extension.

   Philippe: It's on my list - your Charter runs out at the end of
   the month?

   Nigel: No, two weeks ago!

   Philippe: Oops - Atsushi, do you want to do it? It has approval
   already.
   … I'll get on it - I have a bunch of announcements to AC to go
   too.

   Nigel: The other side of this is dealing with the FOs.

   nigel: last time we talked, we had tried to contact apple to
   get more info for the FO
   … and we got a response.
   … Asked if Adobe's proposal of indenpendenant would be
   good/enough.
   … Response was no, but good change.
   … Independenace of implementation was the big issue for them.
   … Could be that the way we defined facts and verifications
   doesn't look like implementations to Apple.
   … No further follow-up from Apple yet.

   plh: I had a talk with Apple last week.
   … big change in charter.
   … From their point of view, the new change doesn't satisfy
   "adequate implementation experience"
   … If spec says in CR for longer, that's fine.

   nigel: AC rep from Google also objected.
   … asked what's defined as an implementation.

   <plh> plh

   nigel: He stated that treating content as an implementation
   isn't good enough.
   … Ralph's view seemed to support our charter updates.

   plh: they want to raise the bar.
   … You don't have to agree with their views.
   … People who read the specs should be able to reproduce without
   talking to the spec maintainers

   nigel: following up from being able to implement from the spec
   … but some specs do allow implementation choices, deliberately,
   e.g. CSS
   … Should we wrap it up for today?
   … Should we adjourn for today?
   … [adjourns meeting]


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Received on Thursday, 14 April 2022 16:15:16 UTC