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With 4K TVs on the horizon and 4K monitors getting cheaper by the month, one of the limiting aspects of the new resolution standard is HDMI 1.4a – the standard that’s been around since 2010, which gave HDMI some new tricks for viewing 3D content. Although the HDMI forum was thinking ahead and allowed the standard to render to resolutions up to 4096 x 2160 at 30Hz, it’s not exactly an ideal solution for desktop use, which requires 60Hz by default.

Nvidia has now announced that they have a workaround in place that will be pushed out to their drivers very soon and it allows for viewing UltraHD 4K content at 60Hz over a regular, old HDMI 1.4a connection. Sounds too good to be true, eh? There’s a big catch to what they’ve done here.

HDMI 2.0, released to manufacturers in September 2013, deals with the 30Hz limitation by increasing bandwidth to 14.4Gb/s of actual throughput, or 18Gb/s if you’re including the TDMS data that allows devices to communicate with each other over the HDMI interface. There’s also a near-doubling of the maximum clock rate to allow the hardware to cope with the larger workload. UltraHD 4K is, after all, essentially four times the workload of a single 1080p stream. But HDMI 1.4 tops out at 8.16Gb/s of actual throughput, which is right on the edge of being able to drive a 4K monitor at 30Hz.

So while the market waits for HDMI 2.0 monitors and compatible graphics cards to be released, Nvidia’s come up with an old, but doable workaround.  Before the video stream is sent to the monitor, Nvidia cuts out unnecessary colour information to make the actual stream smaller in size, opening up enough bandwidth to fit in a 60Hz stream over the regular HDMI 1.4a interface. For most people, this shouldn’t affect their viewing of video or other online content because typically that extra colour information is chopped out anyway in the source video. The technique is called chroma subsampling and it’s been around for quite a while.

chroma subsampling

Here Tom’s Hardware explains chroma subsampling in terms of hammers.

Although nothing in the actual images is affected, there is the issue of colour loss. Chroma subsampling essentially reduces the range of colours that appear in the resulting video stream, which can be problematic for video content that has very dark scenes, or lots of colour in fine details. In Tom’s example above, a loss of the CbCr information might lead to the video card drivers or the TV interpreting the Y Component information incorrectly, over-brightening scenes or reducing the sharpness of text and things like road signs or fine detail.

You can get the same effect if you take a PNG 4K photo and edit it in GIMP, saving it as a JPEG with a file size exactly four times smaller. Not only are you losing information about the individual pixels, you’re also compressing the image, which will result in some distortion and possibly artifacting.

Nvidia says that the feature is now embedded into their beta Geforce 340.43 drivers and works for any of their Geforce Kepler and Maxwell cards, although no specific models are mentioned. I think it’s fairly safe to say that the NVENC video encoder is being used for this, so that means that anything from the Geforce GTX650 and up will be just fine.

Only a handful of 4K TVs coming out this year will support chroma subsampling for 4K video streams, namely the Sony XBR 55X900A and Samsung UE40HU6900. More models from other brands may be on the way, but this is as far as it goes for now. Some Smart TVs that can apply firmware updates to the scaler inside might develop this ability later down the road as well, although it’s not entirely ideal.

For gamers, this is an interesting option. You can buy one of these monitors and set the desktop to run at 30Hz, preserving all the colour and visual fidelity. Then when you get into a game, you can switch to 60Hz mode and enjoy the higher refresh rates, which is a decent tradeoff for the loss in colour information. It would be a better idea to just buy a single-stream Displayport-packing 60Hz monitor, though. That’s much less hassle to set up and support, but with the smaller sizes you may have to squint to read the text.

Source: Anandtech, Tom’s Hardware

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