ARIB (MPEG-TS) subtitles are displayed with a delay when read from a stream

[corsaire]

related to
https://forum.videolan.org/viewtopic.php?f=7&t=137566

I am french living in Japan for several years.

To watch and record TV using my devices I use a setup made of popular Earthlink PT3 PCI-e cards to grab the MPEG2 TS streams of broadcast signals.

The nightly builds of VLC 3.0 includes support for subtitles in the ARIB format, however in my tests there always is a 10s delay in the display of the subtitle making it very impracticable.

If I grab the stream in a file (named .ts or .m2ts depending of the soft) and read it there is no such delay.

I read the stream from a URL of this kind (once again depending of the backend software, but same result):
http://192.168.1.10:40772/api/channels/GR/27/stream
http://192.168.1.10:8800/27

[Jean-Baptiste Kempf]

It's weird that it works with the ts dump but not with the live stream.
How do you dump the stream?

[corsaire]

It's weird that it works with the ts dump but not with the live stream.
How do you dump the stream?

I use a recorder software called recpt1
https://github.com/stz2012/recpt1

[Jean-Baptiste Kempf]

It's weird that it works with the ts dump but not with the live stream.
How do you dump the stream?

I use a recorder software called recpt1
https://github.com/stz2012/recpt1

I hope it does not change the subtitles timings. This is very weird...

...and annoying: we support ARIB subtitles (it took a bit of time) and we don't support it live....

[InTheWings]

I don't see any timing specific stuff in the code.
Is that delayed by 10s, or,
delayed until next subtitle is transmitted (not shown yet),
or first is synced, but others are delayed ?

[corsaire]

Hey everyone,
I found the reason for the delay... and it just blows your mind.

First, VLC isn't the culprit, everything is working fine with it.

This being said, Japan is a country where regulations for accessibility (like for persons hearing impaired for this matter) are very strong.
More over on the public service like the NHK (some channels doesn't provide subtitle service at all).
So, they take the effort to make subtitles for everything, I mean EVERYTHING. So what do you do for programs that are broadcasted live ?

Well, you have some people with alien typing capacities on a outer-space like keyboard, typing live !
The 10s or so delay is the time for this person to type what's going on, some signs even indicating "laughs", "out of field", and other visual indications.
An explanation here :
https://matome.naver.jp/odai/2143745025890438001
https://matome.naver.jp/odai/2143743478768464501

So in our case what happened.
Well I was unfortunate to step on live typed subtitles on three different channels when I first made the test...
Each of them had this delay of course.

Knowing this I tried a much larger panel on programs being recorded without a doubt, and this time, the subtitle and the sound are in sync.

Sorry for the mess, and my two cents on providing this information

[InTheWings]

If you know what you're doing, and can read fluently japanese,
It would be nice to stay around. There's some technical details in untranslated ARIB specs which sometimes are hard to figure out.

[corsaire]

If you know what you're doing, and can read fluently japanese,
It would be nice to stay around. There's some technical details in untranslated ARIB specs which sometimes are hard to figure out.

Could you provide an example ?

[InTheWings]

Could you provide an example ?

No current issue.
BTW if you have a recording which occurred during an emergency broadcast (quake or typhoon..) that would be interesting.