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w3c / imsc-hrm

IMSC Hypothetical Render Model
https://w3c.github.io/imsc-hrm/spec/imsc-hrm.html
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Add note on conversion from CTA 608 signals #72

Closed palemieux closed 8 months ago

palemieux commented 9 months ago

Closes #69

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nigelmegitt commented 9 months ago

Adding this to the agenda for Thursday's call so we can hopefully agree wording.

css-meeting-bot commented 9 months ago

The Timed Text Working Group just discussed Add note on conversion from CTA 608 signals w3c/imsc-hrm#72, and agreed to the following:

The full IRC log of that discussion <nigel> Subtopic: Add note on conversion from CTA 608 signals w3c/imsc-hrm#72
<nigel> github: https://github.com/w3c/imsc-hrm/pull/72
<nigel> Nigel: There's been some debate on the wording for this editorial note, would
<nigel> .. be good to complete it today if we can.
<nigel> .. Pierre, any specific questions to be able to resolve this?
<nigel> Pierre: The general disagreement is if we should make a more general statement and
<nigel> .. I'm not excited by that for 2 reasons.
<nigel> .. 1 is, there's a case I'm comfortable talking about, which is the 608 line 21 example at 59.94fps
<nigel> .. I've seen a lot of streams like that.
<nigel> .. 2nd is, this is just a note and we should try to avoid designing a system in the note.
<nigel> .. The general question of conversion to IMSC is worth its own document.
<nigel> .. I'd like to keep it to just a specific example highlighting the challenges.
<nigel> .. That's my preference.
<nigel> .. I'm happy with the group decision ultimately.
<nigel> .. I can try to make another pass explaining that more clearly.
<nigel> Nigel: Worth noting that this note is right after an "e.g." pointing to 608 in the text immediately above it.
<nigel> .. Can we just remove the phrase about 59.94 Hz? The issue would remain at other frame rates
<nigel> .. such as 29.97.
<nigel> Pierre: If that resolves both your and Cyril's concerns I'm happy to do it.
<nigel> Cyril: I don't think adding the sample rate helps, and I don't want to add a note that is not totally accurate.
<nigel> Nigel: Maybe say "at the video field rate" to clarify that this isn't happening once every few seconds,
<nigel> .. but many many times per second.
<nigel> Pierre: There's a general misunderstanding - 608 is sampled at a field rate, but sometimes the packets
<nigel> .. are combined for other transport mechanisms. But 608 has no option other than sampling at the field rate,
<nigel> .. at least classic 608, like line 21.
<nigel> Nigel: Is this the change that we can accept then?
<nigel> Pierre: I'll put this on screen. [shares screen]
<nigel> .. Now says "... for every CEA-608 packet, which are sampled at the video field rate. ..."
<nigel> Cyril: I like that we removed the 59.94. Field rate is related to interlace. Should we say field or frame?
<nigel> .. I'm fine with this. A field is a frame when there's no interlace.
<nigel> .. And maybe we will have to change it again if we have Teletext issues.
<nigel> Nigel: Teletext doesn't have this "build up a buffer character by character" approach, so it doesn't arise.
<nigel> Pierre: That's modified, I'll ask for a re-review.
<nigel> SUMMARY: Live edits made, needs re-review