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The HDMI Forum rejected AMD's open source HDMI 2.1 implementation
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This is a most excellent place for technology news and articles.
USB-C with Thunderbolt currently had a limit of 40Gbit/sec. Wikipedia has a table of what DisplayPort can do at that bandwidth:
https://en.wikipedia.org/wiki/DisplayPort
See the section "Resolution and refresh frequency limits". The table there shows it'd be able to do 4k/144hz/10bpp just fine, but can't keep above 60hz for 8k.
Its an uncompressed video signal, and that takes a lot of bandwidth. Though there is a simple lossless compression mode.