FFMpeg 嵌入 .srt 文件到 hls stream 播放列表作为 WebVTT

Question

I had an srt subtitle file see below.

1
00:00:06,840 --> 00:00:10,320
once long ago in the deep blue below

2
00:00:12,360 --> 00:00:14,370
Don't be silly, Gran Mother

3
00:00:14,870 --> 00:00:16,700
Everybody knows there's no such thing as

4
00:00:16,700 --> 00:00:17,200
Mermaids

5
00:00:17,620 --> 00:00:21,570
You wouldn't say that if you has meet one. You've meet a Mermaid

6
00:00:24,250 --> 00:00:25,180
This just came in

7
00:00:25,480 --> 00:00:26,630
I wanted to take priority

8
00:00:27,180 --> 00:00:28,500
And we followed stories like this before

9
00:00:28,640 --> 00:00:31,620
Would witches working mummies. No not like this

10
00:00:31,630 --> 00:00:33,190
Hundreds of people are testifying

11
00:00:33,370 --> 00:00:34,600
Welcome to Mississippi

12
00:00:42,390 --> 00:00:47,270
There's a powerful magic in you the spirit of the ocean

13
00:00:47,690 --> 00:00:53,940
Let me introduce you to your dreams, The Little Mermaid, off the Mississippi

14
00:01:01,880 --> 00:01:03,580
She was so beautiful

15
00:01:04,040 --> 00:01:05,270
And I know she was real

16
00:01:07,850 --> 00:01:08,550
I'm skeptical

17
00:01:13,460 --> 00:01:14,410
You shouldn't be here

18
00:01:17,560 --> 00:01:24,130
Mermaids do not exist

19
00:01:31,420 --> 00:01:34,300
Look isn't it beautiful? Sorry I have to go.

20
00:01:35,340 --> 00:01:48,570
Elizabeth, How can this be? It's Lock he knows i'm here he summoned me.

21
00:01:48,570 --> 00:01:50,570
We have to save here.

22
00:02:03,570 --> 00:02:05,570
Here soul is mine

23
00:02:05,570 --> 00:02:06,570
Take it

24
00:02:10,570 --> 00:02:12,570
It's the only way

25
00:02:15,000 --> 00:02:17,570
I believe

26
00:02:23,000 --> 00:02:25,570
Mississippi has always been magic

27
00:02:36,000 --> 00:02:38,000
Elizabeth please

If I use AWS Media convert to add this to my playlist it outputs the following.

#EXTM3U
#EXT-X-VERSION:3
#EXT-X-TARGETDURATION:300
#EXT-X-MEDIA-SEQUENCE:1
#EXT-X-PLAYLIST-TYPE:VOD
#EXTINF:170,
masterCaptionsEnglish/English.vtt
#EXT-X-ENDLIST

And the WebVTT file looks like this.

WEBVTT  #Elemental Media Engine(TM) 2.16.0.602399
X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:191970

00:00:06.840 --> 00:00:10.320
once long ago in the deep blue below

00:00:12.360 --> 00:00:14.370
Don't be silly, Gran Mother

00:00:14.870 --> 00:00:16.700
Everybody knows there's no such thing as

00:00:16.700 --> 00:00:17.200
Mermaids

00:00:17.620 --> 00:00:21.570
You wouldn't say that if you has meet one. You've meet a Mermaid

00:00:24.250 --> 00:00:25.180
This just came in

00:00:25.480 --> 00:00:26.630
I wanted to take priority

00:00:27.180 --> 00:00:28.500
And we followed stories like this before

00:00:28.640 --> 00:00:31.620
Would witches working mummies. No not like this

00:00:31.630 --> 00:00:33.190
Hundreds of people are testifying

00:00:33.370 --> 00:00:34.600
Welcome to Mississippi

00:00:42.390 --> 00:00:47.270
There's a powerful magic in you the spirit of the ocean

00:00:47.690 --> 00:00:53.940
Let me introduce you to your dreams, The Little Mermaid, off the Mississippi

00:01:01.880 --> 00:01:03.580
She was so beautiful

00:01:04.040 --> 00:01:05.270
And I know she was real

00:01:07.850 --> 00:01:08.550
I'm skeptical

00:01:13.460 --> 00:01:14.410
You shouldn't be here

00:01:17.560 --> 00:01:24.130
Mermaids do not exist

00:01:31.420 --> 00:01:34.300
Look isn't it beautiful? Sorry I have to go.

00:01:35.340 --> 00:01:48.570
Elizabeth, How can this be? It's Lock he knows i'm here he summoned me.

00:01:48.570 --> 00:01:50.570
We have to save here.

00:02:03.570 --> 00:02:05.570
Here soul is mine

00:02:05.570 --> 00:02:06.570
Take it

00:02:10.570 --> 00:02:12.570
It's the only way

00:02:15.000 --> 00:02:17.570
I believe

00:02:23.000 --> 00:02:25.570
Mississippi has always been magic

00:02:36.000 --> 00:02:38.000
Elizabeth please

I am trying to replicate this with FFMpeg. I have tried the following command.

ffmpeg -i little-mermaid.mov -i little-mermaid.srt -c copy -c:s webvtt -start_number 0 -hls_time 10000 -f hls mermaid/Eng/master.m3u8

This creates a WebVTT file but it is completely wrong and out of sync. See below.

All the times have changed and it's removed the first two digits from each section.

WEBVTT

00:06.882 --> 00:10.362
once long ago in the deep blue below

00:12.402 --> 00:14.412
Don't be silly, Gran Mother

00:14.912 --> 00:16.742
Everybody knows there's no such thing as

00:16.742 --> 00:17.242
Mermaids

00:00:17.620 -- 00:00:21.570
You wouldn't say that if you has meet one. You've meet a Mermaid

00:24.292 --> 00:25.222
This just came in

00:25.522 --> 00:26.672
I wanted to take priority

00:27.222 --> 00:28.542
And we followed stories like this before

00:28.682 --> 00:31.662
Would witches working mummies. No not like this

00:31.672 --> 00:33.232
Hundreds of people are testifying

00:33.412 --> 00:34.642
Welcome to Mississippi

00:42.432 --> 00:47.312
There's a powerful magic in you the spirit of the ocean

00:47.732 --> 00:53.982
Let me introduce you to your dreams, The Little Mermaid, off the Mississippi

01:01.922 --> 01:03.622
She was so beautiful

01:04.082 --> 01:05.312
And I know she was real

01:07.892 --> 01:08.592
I'm skeptical

01:13.502 --> 01:14.452
You shouldn't be here

01:17.602 --> 01:24.172
Mermaids do not exist

01:31.462 --> 01:34.342
Look isn't it beautiful? Sorry I have to go.

01:35.382 --> 01:48.612
Elizabeth, How can this be? It's Lock he knows i'm here he summoned me.

01:48.612 --> 01:50.612
We have to save here.

02:03.612 --> 02:05.612
Here soul is mine

02:05.612 --> 02:06.612
Take it

02:10.612 --> 02:12.612
It's the only way

02:15.042 --> 02:17.612
I believe

02:23.042 --> 02:25.612
Mississippi has always been magic

02:36.042 --> 02:38.042
Elizabeth please

I am trying to decide if it just makes sense to convert the.srt to WebVTT and use that file without encoding with FFMPEG just create a.m3u8 file with -hls_flag single_file that points to it but not sure what implementation this might cause.

Can anyone suggest how I could do this or where I might be going wrong?

Here is the video file I am using if anyone wants to run the FFMPEG command above to test.

http://www.hd-trailers.net/movie/the-little-mermaid/

Thanks

Answer 1

There's nothing really wrong here. All media data has two timestamps associated with it - the decoding timestamp (dts) and the presentation timestamp (pts). For non-video streams, these are usually identical. But since it's possible for video frames to refer to future frames for decoding purposes (B-frames), frames are stored in encoding/decoding order and not presentation order. So, the dts can be earlier than the pts. In your video, this is the case, the first video frame has a pts of 0.000000s and a dts of -0.041708s . Since MPEG-TS don't store negative timestamps, all timestamps are offset by 0.041708s forward, including subtitles. Sync isn't broken by this.