WebVTT files without X-TIMESTAMP-MAP=MPEGTS have sync issues

[raytiley]

Description

Testing an HLS Stream with WebVTT captions file that is missing a X-TIMESTAMP-MAP=MPEGTS:900000,LOCAL:00:00:00.000 entry. The captions were rendered early. I compared this to HLS.JS wich renders the captions at the correct time. Discussing with @gkatsev he asked me to open an issue.

yeah, it seems like it only applies various offsets if the timestamp map is present
6:14
it's probably a more sensible approach
6:14
can you open a ticket against VHS? And we'll be able to take a look at it, otherwise, we'll forget

Sources

Is a certain source or a certain segment affected? please provide a public (accessible over the internet) link to it below.

Steps to reproduce

Explain in detail the exact steps necessary to reproduce the issue.

1. Play a file with WebVTT captions missing X-TIMESTAMP-MAP
2. Toggle Captions
3. Captions will not be rendered at correct time

Results

Expected

Don't apply any offsets

Error output

N/A

Additional Information

N/A

videojs-http-streaming version

Latest here: https://videojs-http-streaming.netlify.app/

videojs version

Latest here: https://videojs-http-streaming.netlify.app/

Browsers

Latest Chrome

Platforms

OS X

Other Plugins

N/A

Other JavaScript

N/A

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[rajputvai]

Just checking if you do have any updates on this.

[darkHarry]

For me it got resolved either by:

1. Adding the X-TIMESTAMP-MAP=MPEGTS
2. Using this whilst transcoding https://stackoverflow.com/questions/61835223/ffmpeg-burnt-in-subtitles-out-of-sync-when-converting-to-hls/61899776#61899776

[damienalexandre]

To add X-TIMESTAMP-MAP=MPEGTS with the right value, I used ffprobe:

$ ffprobe -show_frames https://website.com/video.m3u8

[FRAME]
media_type=video
stream_index=1
key_frame=1
pts=133200
pts_time=1.480000
pkt_dts=133200
pkt_dts_time=1.480000
best_effort_timestamp=133200
best_effort_timestamp_time=1.480000
pkt_duration=3600
pkt_duration_time=0.040000
pkt_pos=564
pkt_size=949
width=1280
height=720
pix_fmt=yuv420p
sample_aspect_ratio=1:1
pict_type=I
coded_picture_number=0
display_picture_number=0
interlaced_frame=0
top_field_first=0
repeat_pict=0
color_range=tv
color_space=bt709
color_primaries=bt709
color_transfer=bt709
chroma_location=left
[SIDE_DATA]
side_data_type=H.26[45] User Data Unregistered SEI message
[/SIDE_DATA]
[/FRAME]

We can see the famous 1.4 second here:

pkt_dts=133200 pkt_dts_time=1.480000

That's exactly how much my subtitles are off!

So I need to add:

WEBVTT
X-TIMESTAMP-MAP=MPEGTS:133200,LOCAL:00:00:00.000

To the top of my subtitles files and it's fixed.