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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  • Ryan Smith - Wednesday, October 3, 2018 - link

    Aye. The FALD array puts out plenty of heat, but it's distributed, so it can be dissipated over a large area. The FPGA for controlling G-Sync HDR is generates much less heat, but it's concentrated. So passive cooling would seem to be non-viable here.
  • a5cent - Wednesday, October 3, 2018 - link

    Yeah, nVidia's DP1.4 VRR solution is baffelingly poor/non-competitive, not just due to the requirement for active cooling.

    nVidia's DP1.4 g-sync module is speculated to contribute a lot to the monitor's price (FPGA alone is estimated to be ~ $500). If true, I just don't see how g-sync isn't on a path towards extinction. That simply isn't a price premium over FreeSync that the consumer market will accept.

    If g-sync isn't at least somewhat widespread and (via customer lock in) helping nVidia sell more g-sync enabled GPUs, then g-sync also isn't serving any role for nVidia. They might as well drop it and go with VESA's VRR standard.

    So, although I'm actually thinking of shelling out $2000 for a monitor, I don't want to invest in technology it seems has priced itself out of the market and is bound to become irrelevant.

    Maybe you could shed some light on where nVidia is going with their latest g-sync solution? At least for now it doesn't seem viable.
  • Impulses - Wednesday, October 3, 2018 - link

    How would anyone outside of NV know where they're going with this tho? I imagine it does help sell more hardware to one extent or another (be it GPUs, FPGAs to display makers, or a combination of profits thru the side deals) AND they'll stay the course as long as AMD isn't competitive at the high end...

    Just the sad reality. I just bought a G-Sync display but it wasn't one of these or even $1K, and it's still a nice display regardless of whether it has G-Sync or not. I don't intend to pay this kinda premium without a clear path forward either but I guess plenty of people are or both Acer and Asus wouldn't be selling this and plenty of other G-Sync displays with a premium over the Freesync ones.
  • a5cent - Wednesday, October 3, 2018 - link

    "How would anyone outside of NV know where they're going with this tho?"

    Anandtech could talk with their contacts at nVidia, discuss the situation with monitor OEMs, or take any one of a dozen other approaches. Anandtech does a lot of good market research and analysis. There is no reason they can't do that here too. If Anandtech confronted nVidia with the concern of DP1.4 g-sync being priced into irrelevancy, they would surely get some response.

    "I don't intend to pay this kinda premium without a clear path forward either but I guess plenty of people are or both Acer and Asus wouldn't be selling this and plenty of other G-Sync displays with a premium over the Freesync ones."

    You're mistakenly assuming the DP1.2 g-sync is in any way comparable to DP1.4 g-sync. It's not.

    First, nobody sells plenty of g-sync monitors. The $200 price premium over FreeSync has made g-sync monitors (comparatively) low volume niche products. For DP1.4 that premium goes up to over $500. There is no way that will fly in a market where the entire product typically sells for less than $500. This is made worse by the fact that ONLY DP1.4 supports HDR. That means even a measly DisplayHDR 400 monitor, which will soon retail for around $400, will cost at least $900 if you want it with g-sync.

    Almost nobody, for whom price is even a little bit of an issue, will pay that.

    While DP1.2 g-sync monitors were niche products, DP1.4 g-sync monitors will be irrelevant products (in terms of market penetration). Acer's and Asus' $2000 monitors aren't and will not sell in significant numbers. Nothing using nVidia's DP1.4 g-sync module will.

    To be clear, this isn't a rant about price. It's a rant about strategy. The whole point of g-sync is customer lock-in. Nobody, not even nVidia, earns anything selling g-sync hardware. For nVidia, the potential of g-sync is only realized when a person with a g-sync monitor upgrades to a new nVidia card who would otherwise have bought an AMD card. If DP1.4 g-sync isn't adopted in at least somewhat meaningful numbers, g-sync loses its purpose. That is when I'd expect nVidia to either trash g-sync and start supporting FreeSync, OR build a better g-sync module without the insanely expensive FPGA.

    Neither of those two scenarios motivates me to buy a $2000 g-sync monitor today. That's the problem.
  • a5cent - Wednesday, October 3, 2018 - link

    To clarify the above...

    If I'm spending $2000 on a g-sync monitor today, I'd like some reassurance that g-sync will still be relevant and supported three years from now.

    For the reasons mentioned, from where I stand, g-sync looks like "dead technology walking". With DP1.4 it's priced itself out of the market. I'm sure many would appreciate some background on where nVidia is going with this...
  • lilkwarrior - Monday, October 8, 2018 - link

    Nvidia's solution is objectively better besides not being open. Similarly NVLINK is better than any other multi-GPU hardware wise.

    With HDMI 2.1, Nvidia will likely support it unless it's simply underwhelming.

    Once standards catch up, Nvidia hasn't been afraid to deprecate their own previous effort somewhat besides continuing to support it for wide-spread support / loyalty or a balanced approach (i.e. NVLINK for Geforce cards but delegate memory pooling to DX12 & Vulkan)
  • Impulses - Tuesday, October 2, 2018 - link

    If NVidia started supporting standard adaptive sync at the same time that would be great... Pipe dream I know. Things like G-Sync vs Freesync, fans inside displays, and dubious HDR support don't inspire much confidence in these new displays. I'd gladly drop the two grand if I *knew* this was the way forward and would easily last me 5+ years, but I dunno if that would really pan out.
  • DanNeely - Tuesday, October 2, 2018 - link

    Thank you for including the explanation on why DSC hasn't shown up in any products to date.
  • Heavenly71 - Tuesday, October 2, 2018 - link

    I'm pretty disappointed that a gaming monitor with this price still has only 8 bits of native color resolution (plus FRC, I know).

    Compare this to the ASUS PA32UC which – while not mainly targetted at gamers – has 10 bits, no fan noise, is 5 inches bigger (32" total) and many more inputs (including USB-C DP). For about the same price.
  • milkod2001 - Tuesday, October 2, 2018 - link

    Wonder if they make native 10bit monitors. Would you be able to output 10bit colours from gaming GPU or only professional GPU?

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