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Hardware doesn't upgrade itself because you are using a better cable. In other words, unless the TV and the AppleTV were built with HDMI 2.1 spec ports, an HDMI 2.1 cable will revert to the spec that the ports support. So if they are HDMI 2.0 ports... it will drop to HDMI 2.0 spec even though it is an HDMI 2.1 cable.
HDMI Types:
Each cable types are available with or without Ethernet. In practice, HDMI with Ethernet isn't embraced by most and I wouldn't bother with it unless the cost premium is next to nothing.
HDMI Wikipedia lists minimum bandwidth for various resolution, frame rates, and colorspace.
The short version is that Apple TV 4K is a HDMI 2.0b device, and while 4K Dolby Vision at 4:4:4 subsampling rate requires 24.06 Gbps for 48 frames per second or higher, it won't output anything beyond 18 Gbps.
My recommendation. If you are buying a new cable, I would pay the nominal price increase and buy ultra high-speed cable. They are only few bucks more. But if you already have premium high-speed cable, you are all set.
| Name | Bandwidth | Ethernet | HDMI Compliance |
| Standard | 5 Gbps | Optional | 1.2a |
| High-Speed | 10 Gbps | Optional | 1.4a |
| Premium High-Speed | 18 Gbps | Optional | 2.0b |
| Ultra High-Speed | 48 Gbps | Optional | 2.1 |
Each cable types are available with or without Ethernet. In practice, HDMI with Ethernet isn't embraced by most and I wouldn't bother with it unless the cost premium is next to nothing.
HDMI Wikipedia lists minimum bandwidth for various resolution, frame rates, and colorspace.
The short version is that Apple TV 4K is a HDMI 2.0b device, and while 4K Dolby Vision at 4:4:4 subsampling rate requires 24.06 Gbps for 48 frames per second or higher, it won't output anything beyond 18 Gbps.
My recommendation. If you are buying a new cable, I would pay the nominal price increase and buy ultra high-speed cable. They are only few bucks more. But if you already have premium high-speed cable, you are all set.
