4K 60 4:4:4 HDR

By David Meyer posted 06-22-2017 01:58

4K 60 4:4:4 HDR

4K resolution.
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!





06-25-2017 19:28

Hi Walter, totally agree about HFR being an overlooked benefit. I'm all for that!! My point was more about the combination, where all 4 elements of 4K 60 4:4:4 HDR exceeds HDMI's current ceiling. HDMI defaults to 8-bit BT.709 even at 4K UHD, but stepping up to BT.2020 requires 10-bit minimum (well, by spec anyway). HDR can work with 8b 709, but then it's only about brightness and contrast, not color volume being where the greatest benefit of HDR resides.

The culprit in this combo is actually 4:4:4 chroma subsampling. If 1 of the 4 has to give, I'd rather it be that. The ideal scenario would be 4K 60 4:2:2 HDR, where either 10- or 12-bit color is supported within the 18Gbps HDMI limit. We'd get the best of the current capacity, until such time as HDMI 2.1 serves out more. 

As for HFR, the next challenge is to get more DP's of this world to move on from 24fps film! If only...

06-25-2017 04:40

David, I agree with your points, but Color Gamut is the total size of the color space, bit depth (8, 10, 12) is the sample size or number of steps.  You could theoretically have wide color gamut (BT2020) and 8 bit, but the steps would be big and you would likely see vignetting, much like 10 bit sampling at REC 709 provides smoother color transitions.  I think more time should be spent on the benefits between 60 Hz and 30Hz (or 24p).  HFR results in a much more "viewable" picture.    It's really all about how we best utilize our bandwidth