Supporting Closed Caption in DASH for Live content

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  • Updated 2 years ago
  • (Edited)
So you are thinking about supporting Closed Captioning for your live content in DASH? Well like everything else in DASH there are a couple of options:
  • Leave the EIA-608 or EIA-708 data in the H.264 SEI data
  • Convert the data from EIA-608 or EIA-708 into WebVTT carry it as a separate track either
    • Carry it as text
    • Carry as a MP4 wrapped Timed Text Track 
In VOS 360 we have made your life simple by defaulting to WebVTT carried as text as this has the highest level of support from the player community.
  • ExoPlayer
  • Shaka
  • Adobe
  • etc.
Lets have a quick look at why we rejected the other two options:

Simply leaving in the video element puts significant extra development requirements on the player decoder and even if they solve it perfectly there is no easy way to map the CC number to the language so the user experience is compromised or it requires additional out of band metadata. 

For the mp4 wrapped Timed Text track lets look at what happens. You have to segment the WebVTT anyway and then insert it into an mp4 fragment on the encode side then on the player side you have to strip the mp4 fragment before you can get at the WebVTT segment which is what you actually wanted. So you are adding complexity and bandwidth for no reason.
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Moore, Product Architect

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Posted 2 years ago

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