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
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
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