Support live subtitles stream from transport stream

[jadarve]

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I'm trying to transmit live subtitles from our transcoder to GPAC using Transport Stream. Our transcoder does not support MP4 Text Stream, and from what I have read of GPAC's code, it does not support demuxing it either.

Instead of implementing the whole spec, which at the moment is overkill for my use case, I tried to mux SRT subtitles as private data in PES packets.

• The Elementary Stream uses PES packets private data with a registration_descriptor for the SRT format.
  
  

• Each PES packet contains one SRT event (aligned with PES start):

0
00:00:00,000 --> 00:00:01,000
This is the event zero

In GPAC, I managed to demux the stream in this branch: master...jadarve:gpac:NS_ts_subtitles

running a command like:

./bin/gcc/gpac \
  -i test_data/out_subtitle_only.ts \
  txtin \
  -o test_data/subtitles.vtt \
  -graph

prints the following graph:

Filters connected:
fin (src=test_data/out_subtitle_only.ts) (idx=1)
-(PID out_subtitle_only.ts) m2tsdmx (dyn_idx=4)
--(PID P1T2) writeuf (dyn_idx=5)
---(PID P1T2) writegen (dyn_idx=6)
----(PID P1T2) txtin (idx=2)
-----(PID P1T2) tx3g2vtt (dyn_idx=7)
------(PID P1T2) writegen (dyn_idx=8)
-------(PID P1T2) fout (dst=test_data/subtitles.vtt) (idx=3)

However, the generated WebVTT is not what I expected, it looks as:

WEBVTT

90:15:36.023 --> 90:15:39.623
1
01:00:00,000 --> 01:00:00,000
0
00:00:00,000 --> 00:00:00,040
current counter: 00000000000000000000000000000000000000000000000000

where I expected lots of subtitles lines.

This is just an example. My final goal is to be able to mux the TS subtitles into a CMAF track as WebVTT for live streaming. I would like to know if something is already done in that direction (live subtitle muxing), or if the approach I'm taking makes sense, and if you could advise me on how to proceed.