Support for WebVTT in captions output for apple HLS not working

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I am trying to convert an .mp4 file into an HLS output with captions/subtitles using AWS Mediaconvert. My end goal is to have multiple language selectors in the HLS video (ex. German and French) however I can only "Burn in" one set of captions. According to the AWS docs, if I am using .SRT file as input and HLS as my output container I should be able to also use WebVTT as a output caption format but when I go to create a template for that it gives me this error:

/outputGroups/0/outputs/0/captionDescriptions/0/destinationSettings/destinationType: Should be equal to one of the allowed values in ["BURN_IN","DVB_SUB","EMBEDDED","EMBEDDED_PLUS_SCTE20","SCTE20_PLUS_EMBEDDED","TELETEXT"]

According to another blog (https://www.vidbeo.com/blog/hls-subtitles-captions-webvtt/) it seems doable to insert subtitles into the .m3u8 manifest file like this:

#EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="subs",NAME="English",DEFAULT=NO,AUTOSELECT=YES,FORCED=NO,LANGUAGE="en",CHARACTERISTICS="public.accessibility.transcribes-spoken-dialog",URI="subtitles_en.m3u8"

It doesn't seem like I can do this via mediaconvert without choosing the burn in option which wont meet my requirement. Any assistance is much appreciated!

asked 18 days ago26 views
1 Answer
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Thank you for contacting AWS Re:post,

To answer your question, when selecting IMSC, TTML, or WebVTT as output captions, you need to NOTE that these are only supported as sidecar captions.

What that means is, for these 3 output caption formats, under your output group (say HLS in your case) you need to create separate caption-only outputs for as many caption languages as you need in your output. This is also known as DE-MUXING in media technical terms. To do this, you have to remove the Video and Audio from each caption output under your output-group.

Therefore, your output group should look like this:

Output-Group : HLS
     • Output-1 : Video (mp4) + Audio (xyz format)
     • Output-2 : Caption Eng (WebVTT)
     • Output-3 : Caption Ger (WebVTT)
        ...

Please check this public document that explains the above -> https://docs.aws.amazon.com/mediaconvert/latest/ug/ttml-and-webvtt-output-captions.html

SUPPORT ENGINEER
answered 17 days ago

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