60 frames per second.
4:4:4 chroma subsampling.
This trio has been used in combination for some time as a moniker to signify 18Gbps HDMI support. Simply stating ‘18G’ might have been simpler, but hey that’s another story…
The complication comes when HDR is added. From a menu where 4K is the main dish, and 60fps, 4:4:4 and HDR are the three side options, we can only choose two. The third comes at extra cost of which we can’t yet afford. That’ll come with HDMI 2.1.
Why is it so?
4K60 4:4:4 runs at 17.82Gbps, but only with the default 8-bit BT.709 color. That’s the same color gamut as 720p and 1080i/p. The whole point of HDR is to increase color volume, which it can’t do unless there’s first an increased color gamut. That means stepping up to BT.2020, which in turn means 10- or 12-bit deep color. The HDMI 2.0 spec also calls for a minimum 10-bit color with BT.2020.
Here's how it stands...
Main + 2 sides;
(Remember it has to be under 18Gbps…)
4K 60 4:4:4 (no HDR) = 17.82Gbps
4K 30 4:4:4 10b-HDR (no 60fps) = 11.14Gbps
4K 60 4:2:2 10b-HDR (no 4:4:4) = 17.82Gbps
Main + 3 sides;
4K 60 4:4:4 10b-HDR = 22.28Gbps
Bottom line – there currently is no 4K 60 4:4:4 HDR supported by HDMI 2.0. We need HDMI 2.1. The good news is that there’s not many applications that would require the combo of all three sides anyway. We're most commonly watching 24fps, 4:2:0 chroma, or both, in which case we're under the 18Gbps. Happy days. It’s just something to be aware of for now, whenever you see that moniker.
I’ll be going over this in some more context in the CEDIA course Delivering the 10K Video Payload.
Hope to see you there!