DVB_TELETEXT to WEBVTT rolling issue.

[tresby]

When I convert dvb_teletext to webvtt, the 'rolling' subs get separated by about 40ms - causing players like Infuse to blink on and off for those 40ms instead of rolling the text out nicely.

Here is a small example - unsure if anything can be done about it though.

WEBVTT
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$1.6 million
Mount Pleasant apartment,

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plus over 11,000 other great prizes.

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Get your tickets now.

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Tomorrow

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Tomorrow night.

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Tomorrow night.
Life

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Life changing.

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Life changing. She's

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Life changing. She's lost

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Life changing. She's lost 90

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Life changing. She's lost 90 kilos

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Life changing. She's lost 90 kilos
now.

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now. Codie

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now. Codie wants

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now. Codie wants to

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now. Codie wants to feel

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now. Codie wants to feel good

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now. Codie wants to feel good in

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now. Codie wants to feel good in
the

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the skin

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the skin she's

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the skin she's in.

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the skin she's in. She

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the skin she's in. She knows

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the skin she's in. She knows the

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the skin she's in. She knows the
risk.

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risk. So

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risk. So why

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risk. So why is

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risk. So why is she

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risk. So why is she going

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risk. So why is she going under

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risk. So why is she going under the

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risk. So why is she going under the
knife

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knife overseas? <font color="#ffff00">I</font>

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knife overseas? <font color="#ffff00">I 100%</font>

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knife overseas? <font color="#ffff00">I 100% wish</font>

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knife overseas? <font color="#ffff00">I 100% wish I</font>

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knife overseas? <font color="#ffff00">I 100% wish I could</font>

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knife overseas? <font color="#ffff00">I 100% wish I could</font>
<font color="#ffff00">have</font>

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<font color="#ffff00">have had</font>

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<font color="#ffff00">have had this</font>

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<font color="#ffff00">have had this at</font>

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<font color="#ffff00">have had this at home.</font>

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<font color="#ffff00">have had this at home. Even</font>

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<font color="#ffff00">have had this at home. Even now,</font>

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<font color="#ffff00">have had this at home. Even now,</font>
<font color="#ffff00">like,</font>

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<font color="#ffff00">like, I'm</font>

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<font color="#ffff00">like, I'm getting</font>

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<font color="#ffff00">like, I'm getting a</font>

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<font color="#ffff00">like, I'm getting a little</font>

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<font color="#ffff00">like, I'm getting a little bit</font>

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<font color="#ffff00">like, I'm getting a little bit</font>
<font color="#ffff00">upset.</font>

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<font color="#ffff00">upset. Like</font>

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<font color="#ffff00">upset. Like I</font>

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<font color="#ffff00">upset. Like I was</font>

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<font color="#ffff00">upset. Like I was terrified</font>

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<font color="#ffff00">upset. Like I was terrified and</font>

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<font color="#ffff00">upset. Like I was terrified and I</font>
<font color="#ffff00">had</font>

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<font color="#ffff00">had no</font>

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<font color="#ffff00">had no option.</font>

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<font color="#ffff00">had no option. And</font>

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<font color="#ffff00">had no option. And for</font>

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<font color="#ffff00">had no option. And for anyone</font>

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<font color="#ffff00">had no option. And for anyone that</font>

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<font color="#ffff00">had no option. And for anyone that</font>
<font color="#ffff00">has</font>

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<font color="#ffff00">has any</font>

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<font color="#ffff00">has any type</font>

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<font color="#ffff00">has any type of</font>

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<font color="#ffff00">has any type of pull</font>

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<font color="#ffff00">has any type of pull in</font>

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<font color="#ffff00">has any type of pull in any</font>

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<font color="#ffff00">has any type of pull in any of</font>

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<font color="#ffff00">has any type of pull in any of this,</font>

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<font color="#ffff00">has any type of pull in any of this,</font>
<font color="#ffff00">something</font>

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<font color="#ffff00">something needs</font>

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<font color="#ffff00">something needs to</font>

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<font color="#ffff00">something needs to change.</font>

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<font color="#ffff00">something needs to change.</font> We'll

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<font color="#ffff00">something needs to change.</font> We'll
have

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have that

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have that tomorrow.

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have that tomorrow. And

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have that tomorrow. And that's

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have that tomorrow. And that's the

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have that tomorrow. And that's the
show

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show I'm

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show I'm Ally

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show I'm Ally Langdon.

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show I'm Ally Langdon. Thanks

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show I'm Ally Langdon. Thanks for

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show I'm Ally Langdon. Thanks for
your

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your company.

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your company. Enjoy

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your company. Enjoy your 

Thanks.

[alaotach]

Opening a PR to fix this.

The root cause is a hardcoded 40ms contraction in src/lib_ccx/telxcc.c:

// it would be nice, if subtitle hides on previous video frame, so we contract 40 ms (1 frame @25 fps)
ctx->page_buffer.hide_timestamp = timestamp - 40;

This was originally added so subtitles would hide one frame early on 25fps video. The problem is that the WebVTT exporter already subtracts 1ms from end timestamps to prevent boundary collisions between adjacent cues. So the two combine and create a ~41ms gap between consecutive rolling teletext cues.
For sparse dialogue subs with natural gaps this 40ms is invisible, but for rolling teletext that updates every ~40ms it's essentially a gap equal to one full cue duration.

The fix is to remove the 40ms contraction and let the WebVTT exporter's 1ms gap handle overlap prevention on its own. That 1ms is spec-compliant and imperceptible to players.

Also worth noting: the 40ms figure is only correct for 25fps content anyway and it's already wrong for 24fps (41.7ms/frame) or 30fps (33ms/frame).