Regardless of how fancy or basic the features on UHD TVs are, the common foundation they share is a resolution of 3840x2160 pixels—the UHD pixel resolution. Add HDCP 2.2 and the ability to handle up to 60 frames per second at that resolution, and you've got a 4K Ultra HD TV as defined by the CTA (Consumer Technology Association, the folks who produce CES). The purpose of the accompanying logo is to assure consumers that the TV they are buying can properly handle UHD video.
The 4K Ultra HD logo.
Last year, there was a kerfuffle regarding whether or not certain LCD TVs from LG—which used a novel RGBW sub-pixel arrangement—can actually achieve UHD resolution. Digital ink was spilled in various forums and publications arguing the pros and cons of the technology. Unsurprisingly, Samsung took a critical stance, and LG defended its new LCD panels from that criticism.
Unlike LG's OLED TVs, in which each pixel includes a white sub-pixel in addition to red, green, blue, its RGBW LCDs are based on a scheme where every fourth sub-pixel in a row is white, which means that each white sub-pixel is shared by adjacent pixels. This arrangement requires the panel to jump through some hoops in order to render 3840x2160 UHD video.
Because of their peculiar sub-pixel arrangement, RGBW LCDs cannot achieve full UHD resolution in color, they can only pull it off with monochromatic resolution test patterns. LG does not deny this; rather, the company argues that achieving UHD resolution is really only important for the luma channel (brightness). However, when I recently checked out an LG RGBW UHDTV, my findings called into question how effectively it can render monochromatic UHD resolution.
Quite literally, reading the fine print is at the core of the RGBW-versus-RGB debate. If the critics are correct, the shared white sub-pixel arrangement renders RGBW TVs incapable of achieving true UHD resolution—defined by the CTA as 3840x2160 active pixels with 8-bit color. This negatively impacts the rendering of fine text, making it a bit fuzzier, which is especially noticeable when a TV is used as a PC monitor.
There's also a figurative element to the debate, in that it's really about what defines resolution. Is it discrete pixels? Or is it subjective perception that matters, as long as a TV can ingest UHD video and output something sharper than a 1080p HDTV?
One thing I have discovered for sure, RGBW LCDs produce some funky anomalies when working with 4K test patterns. It's not too tough to find moire patterns in what should be seen as solid, uniform colors.
You may wonder whether this issue matters to a typical TV viewer. As you would expect, LG had denied that RGBW presents any problems at all. Since image-quality issues are in the eye of the beholder, I spent some time experimenting with an LG UH6150 RGBW LCD. It's listed as a 4K Ultra HD TV on the LG website, which explicitly claims that it offers 8.3 megapixel resolution "for 4x the Resolution of Full HD TVs." I compared how it rendered various test patterns—plus real-world content—to a couple of Samsung RGB LCD UHDTVs: a JU6400F and a JU7100F.
I tested the TVs using a variety of sources including a DVDO AV Lab TPG 4K signal generator, a PC sporting an NVIDIA GTX980 video card and a Samsung UBD-K8500 Ultra HD Blu-ray player. To ensure each TV received the same signal, I used a Cable Matters UHD 4-port HDMI splitter.
Let's start with one of the simplest test patterns, a 1-pixel checkerboard grid coming from the DVDO. On the RGB LCDs, that pattern looked gray and uniform—no surprise, since the pattern consists of 50% white and 50% black pixels. On the RGBW LCD, instead of gray, I saw hue-shifted vertical stripes (alternating between green and magenta) that were several pixels wide. I had trouble counting exactly how many pixels wide because with LG's RGBW sub-pixel arrangement, it was tough to tell where one pixel ended and another pixel began.
RGB LCD sub-pixels handled a 1-pixel checkerboard pattern much better than RGBW LCD sub-pixels did (actual photos).
There was no such ambiguity with the RGB LCD panels, where each black and white pixel was clearly defined and discrete. Furthermore, the RGB panels rendered the pattern perfectly in both PC mode (with 4:4:4 color) and other modes (4:2:2 color), while the RGBW panel had issues regardless of whether it was in PC mode or not.
If an RGBW LCD can't reproduce a simple monochromatic checkerboard pattern accurately, how can it claim to achieve true UHD resolution? Apparently, because it can render a vertical black-and-white alternating-line pattern at 3840-pixel resolution, and also a horizontal alternating-line pattern at 2160-pixel resolution. I saw this with my own eyes! However, when I flipped between those two patterns I noticed a huge shift in hue, when in reality both patterns should have remained neutral gray since, like the checkerboard pattern, they consist of 50% black and 50% white pixels.
The crux of the debate comes down to what defines a pixel. If it's just a hypothetical concept, a mere proxy for a subjective experience, then perhaps RGBW LCD could be considered capable of rendering some flavor of UHD resolution. However, if you demand that each pixel be a discrete, independently-addressable unit capable of expressing a full range of color, then the current crop of RGBW LCDs doesn't cut it.
If the limitations of RGBW LCDs were restricted to test patterns, perhaps you could argue that it might not matter to many consumers whether it's a "true" UHD display or not. But I found that the sub-pixel scheme used by RGBW LCDs can impact some content. While cable TV looks essentially the same on RGB and RGBW LCDs, in other scenarios, I spotted clearly identifiable differences in how details were rendered. For example, when viewing slideshows of high-resolution aerial golf-course images, I noticed anomalies in how the greenery was rendered—it was harder to see leaf and grass blade patterns on the RGBW display—and often golf balls appeared to be stretched-out horizontally, making them resemble divots.
I tried switching the TVs in and out of PC (4:4:4) mode. On the RGB panels, I saw no difference in detail rendition with the golf photos. However, the LG's detail rendition did improve quite a bit in PC mode. It did not achieve the resolution of the RGB panels, but it came close. The real issues arose when the LG was not in PC mode—that's when I saw clearly visible loss of detail when compared to the other TVs.
Photo of the Samsung JU7100F RGB LCD's screen.
Photo of the LG UH6150's RGBW LCD screen. Note how the golf balls don't look like golf balls.
I did not see a similar effect on any other display I own, whether it was a TV, laptop, tablet, or phone. Only the LG RGBW LCD with shared white sub-pixels struggled to faithfully render the organic textures in those photos.
What I noticed in those golf shots is highly relevant given that, just a few days ago, The Masters was broadcast in UHD/4K resolution, a first for a live sporting event in the US. With ultra high-definition golf, you want to be able to tell divots apart from golf balls, otherwise what's the point? The RGBW display added ambiguity that was not present with the RGB displays.
Another issue I noticed during my testing pertains to color luminance. The issue here is elementary—RGBW TVs can't achieve their maximum peak luminance without the help of the white sub-pixels. That's great if you watch ice hockey or skiing all the time, but not ideal for content that's full of rich, saturated colors. It's hard to make a judgement call on how severely color-starved highlights affect total image quality, but it exists as an issue in parallel to the chroma (color) resolution limitation of RGBW LCD.
The conundrum lies in how much to care about RGBW LCD's shortfalls. If you only lost a bit of chroma resolution, I would say it was no concern. If RGB and RGBW LCDs looked essentially identical when playing 4:2:2 or 4:2:0 UHD video and test patterns, I'd say fuggedaboutit. But the reality is more nuanced; in the end, I was able to find visible image-quality degradation caused by the RGBW LCD using both test patterns and real content, and especially when the TV was not in PC mode.
I won't claim that the differences between RGB and RGBW LCD are super-obvious. From 11 feet away and with cable as the source, you won't spot a difference on a 55" TV. Then again, at that distance, you'll never see the full resolution offered by any 55" UHDTV—you may as well buy a 1080p TV and pocket the difference.
If you spend additional money for the extra resolution offered by a UHD/4K TV, and especially one that's identified as a 4K Ultra HD model, that TV should deliver the resolution the CTA logo promises: 3840x2160 pixels. For that matter, in my view, any TV that claims UHD resolution ought to be able to do so without adding qualifiers and conditions to how it gets there. With both real-life content and test patterns, RGB LCDs had no problem hitting the UHD resolution target.
Do you have any experience with LG's RGBW LCDs? If so, please let me know your impressions in the comments.