HDMI 2.1 Allows for Dynamic Metadata

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In addition to increasing the bandwidth from 18Gbps to 48Gbps, allowing for 8K at 60P and 4K at 120P, and improved audio handling, the new 2.1 HDMI standard improves dynamic range and HDR potentials significantly by allowing scene by scene settings rather than a global setting at the start.  See this article for more.