When DisplayPort 1.4 Isn’t Enough: Chroma Subsampling

One of the key elements that even makes G-Sync HDR monitors possible – and yet still holds them back at the same time – is the amount of bandwidth available between a video card and a monitor. DisplayPort 1.3/1.4 increased this to just shy of 26Gbps of video data, which is a rather significant amount of data to send over a passive, two-meter cable. Still, the combination of high refresh rates, high bit depths, and HDR metadata pushes the bandwidth requirements much higher than DisplayPort 1.4 can handle.

All told, DisplayPort 1.4 was designed with just enough bandwidth to support 3840x2160 at 120Hz with 8bpc color, coming in at 25.81Gbps of 25.92Gbps of bandwidth. Notably, this isn’t enough bandwidth for any higher refresh rates, particularly not 144MHz. Meanwhile when using HDR paired with the P3 color space, where you’ll almost certainly want 10bpc color, there’s only enough bandwidth to drive it at 98Hz.

DisplayPort Bandwidth
Standard Raw Effective
DisplayPort 1.1 (HBR1) 10.8Gbps 8.64Gbps
DisplayPort 1.2 (HBR2) 21.8Gbps 17.28Gbps
DisplayPort 1.3/1.4 (HBR3) 32.4Gbps 25.92Gbps

As a result, for these first generation of monitors at least, NVIDIA has resorted to a couple of tricks to make a 144Hz 4K monitor work within the confines of current display technologies. Chief among these is support for chroma subsampling.

Chroma subsampling is a term that has become a little better known in the last few years, but the odds are most PC users have never really noticed the technique. In a nutshell, chroma subsampling is a means to reduce the amount of chroma (color) data in an image, allowing images and video data to either be stored in less space or transmitted over constrained links. I’ve seen it referred to compression at some points, and while the concept is indeed similar it’s important to note that chroma subsampling doesn’t try to recover lost color information nor does it even intelligently discard color information, so it’s perhaps thought better as a semi-graceful means of throwing out color data. In any case, the use of chroma subsampling is as old as color television, however its use in anything approaching mainstream monitors is much newer.

So how does chroma subsampling work? To understand chroma subsampling, it’s important to understand the Y'CbCr color space it operates on. As opposed to tried and true (and traditional) RGB – which stores the intensity of each color subpixel in a separate channel – Y'CbCr instead stores luma (light intensity) and chroma (color) separately. While the transformation process is not important, at the end of the day you have one channel of luma (Y) and two channels of color (CbCr), which add up to an image equivalent to RGB.

Chroma subsampling, in turn, is essentially a visual hack on the human visual system. Humans are more sensitive to luma than chroma, so as it goes, some chroma information can be discarded without significantly reducing the quality of an image.

The technique covers a range of different patterns, but by far the most common patterns, in order of image quality, are 4:4:4, 4:2:2:, and 4:2:0. 4:4:4 is a full chroma image, equivalent to RGB. 4:2:2 is a half chroma image that discards half of the horizontal color information, and requires just 66% of the data as 4:4:4/RGB. Finally 4:2:0 is a quarter chroma image, which discards half of the horizontal and half of the vertical color information. In turn it achieves a full 50% reduction in the amount of data required versus 4:4:4/RGB.

Wikipedia: diagram on chroma subsampling

In the PC space, chroma subsampling is primarily used for storage purposes. JPEG employs various modes to save on space, and virtually every video you’ve ever seen, from YouTube to Blu-rays, has been encoded with 4:2:0 chroma. In practice chroma subsampling is bad for text because of the fine detail involved – which is why PCs don’t use it for desktop work – but for images it works remarkably well.

Getting back to the matter of G-Sync then, the same principle applies to bandwidth savings over the DisplayPort connection. If DP 1.4 can only deliver enough bandwidth to get to 98Hz with RGB/4:4:4 subsampling, then going down one level, to 4:2:2, can free up enough bandwidth to reach 144Hz.

Users, in turn, are given a choice between the two options. When using HDR they can either pick to stick with a 98Hz refresh rate and get full 4:4:4 subsampling, or drop to 4:2:2 for 144Hz.

In practice for desktop usage, most users are going to be running without HDR due to Windows’ shaky color management, so the issue is moot and they can run at 120Hz without any colorspace compromises. It’s in games and media playback where HDR will be used, and at that point the quality tradeoffs for 4:2:2 subsampling will be less obvious, or so NVIDIA’s reasoning goes. Adding an extra wrinkle, even on an RTX 2080 Ti few high-fidelity HDR-enabled games will be able to pass 98fps to begin with, so the higher refresh rate isn’t likely to be needed right now. Still, if you want HDR and access to 120Hz+ refresh rates – or SDR and 144Hz for that matter – then there are tradeoffs to be made.

On that note, it’s worth pointing out that to actually go past 120Hz, the current crop of G-Sync HDR monitors require overclocking. This appears to be a limitation of the panel itself; with 4:2:2 subsampling there’s enough bandwidth for 144Hz even with HDR, so it’s not another bandwidth limitation that’s stopping these monitors at 120Hz. Rather the purpose of overclocking is to push the panel above its specifications (something it seems plenty capable of doing), allowing the panel to catch up with the DisplayPort connection to drive the entire device at 144Hz.

Meanwhile on a quick tangent, I know a few people have asked why NVIDIA hasn’t used the VESA’s actual compression technology, Display Stream Compression (DSC). NVIDIA hasn’t officially commented on the matter, and I don’t really expect they will.

However from talking to other sources, DSC had something of a rough birth. The version of the DSC specification used in DP 1.4 lacked support for some features manufacturers wanted like 4:2:0 chroma subsampling, while DP1.4 itself lacked a clear definition of how Forward Error Correction would work with DSC. As a result, manufacturers have been holding off on supporting DSC. To that end, the VESA quietly released the DisplayPort 1.4a specification back in April to resolve the issue, with the latest standard essentially serving as the “production version” of DisplayPort with DSC. As a result, DSC implementation and adoption is just now taking off.

As NVIDIA controls the entire G-Sync HDR ecosystem, they aren’t necessarily reliant on common standards. None the less, if DSC wasn’t in good shape to use in 2016/2017 when G-Sync HDR was being developed, then it’s as good a reason as any that I’ve heard for why we’re not seeing G-Sync HDR using DSC.

From G-Sync Variable Refresh To G-Sync HDR Gaming Experience The (Asus) G-Sync HDR Experience: Premium Panel for Premium Price
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  • FreckledTrout - Tuesday, October 2, 2018 - link

    AUO have stated it lands this fall so should be very soon. They made it sound like they will have a shipping monitor by the end of 2018 albeit who really knows but im sure its under 1 year away at this point.

    Can Google: "AUO Expects to Launch Mini LED Gaming Monitor in 2H18"
  • imaheadcase - Wednesday, October 3, 2018 - link

    Don't keep hopes hope, remember this monitor in this very review was delayed 6+ months
  • Lolimaster - Tuesday, October 2, 2018 - link

    384 zones is just CRAP, you only find that number of zones on low end cheapo TV's with FALD just o be a bit more "premium". For that price is should have 1000 AT LEAST.

    Seems we will need to wait for LCD with minileds to actually start seeing monitors with 5000+zones.
  • know of fence - Tuesday, October 2, 2018 - link

    Consoles started to push that 4K / HDR nonsense and now the monopoly provides a monitor to match for the more money than sense crowd. The obscure but sensible strobing backlight / ULMB got sacrificed for the blasted buzzwords and Gsync. Is it because the panel is barely fast enough for Gsync or is it a general shift in direction, doubling down on proprietary G-stink and the ridiculously superfluous 4K native. Is it because with failing VR, high frame rates are off the table completely?
    Is there any mention on how 1920x1080 looks on that monitor (too bad), because the pixel density is decidedly useless and non standard. But scaled down to half it could be 81.5 ppi and this thing can actually be used to read text.
  • godrilla - Tuesday, October 2, 2018 - link

    $1799 at micr1 fyi!
  • godrilla - Tuesday, October 2, 2018 - link

  • Hectandan - Tuesday, October 2, 2018 - link

    "the most desired and visible aspects of modern gaming monitors: ultra high resolution (4K)"
    No it's not. At least on Windows where UI scaling still sucks. At least on "slow" graphics card like 2080 Ti where 4K doesn't run 144fps. And 4K monitors can't do 1440p natively, so a huge deal breaker.
  • Zan Lynx - Wednesday, October 3, 2018 - link

    If you had a graphics card that could always run 144 Hz then you would have no need for GSync.
  • imaheadcase - Wednesday, October 3, 2018 - link

    Why would you care about scaling for gaming? Besides, plenty of 3rd party apps to correct windows bullshit.
  • Hectandan - Thursday, October 4, 2018 - link

    No I don't care about scaling in games, but I do care about 144fps in games. Only possible in 4K with SLI 2080 Ti and good game SLI support. Plenty of games don't.
    Also plenty of 3rd party apps not correcting Windows bullshit, and I gain no extra working space if I do scale.
    Simply too many downsides and too little benefit.

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