With 4K technology now mainstream, let's look at two terms that have become conflated with one another: 4K and UHD, or Ultra HD. TV makers, broadcasters, and tech blogs are using them interchangeably, but they didn't start as the same thing, and technically still aren't. From a viewer standpoint, there isn't a massive difference between 4K vs UHD. 4K is a more popular term than UHD, but 4K-capable Blu-ray drives are marketed as Ultra HD drives.
4K vs. UHD
In simple terms, 4K is a professional production and cinema standard. UHD is a consumer display and broadcast standard. To discover how they became so confused, let's look at the history of the two terms.
"4K" was initially derived from the Digital Cinema Initiatives (DCI). This consortium standardized a spec for producing and digitally projecting 4K content. In this case, 4K is 4,096 by 2,160 and is precisely 4x the previous standard for digital editing and projection (2K, or 2,048 by 1,080). 4K refers to the horizontal pixel count (4,096), or roughly four thousand. The 4K standard is not just a resolution; It also defines how 4K content is encoded. A DCI 4K stream is compressed using JPEG2000, can have a bitrate of up to 250Mbps, and employs 12-bit 4:4:4 color depth.
The next step up from HD is UHD, or Ultra High Definition. It's the official name for the display resolution of 1,920 by 1,080. UHD quadruples that resolution to 3,840 by 2,160. It's not the same as the 4K resolution made above. Despite this, almost every TV or monitor advertised as 4K is UHD. Some panels are 4,096 by 2,160, which adds up to an aspect ratio of 1.9:1. But the vast majority are 3,840 by 2,160, for a 1.78:1 aspect ratio.
Why Not 2160p?
It's not as if TV manufacturers are unaware of the differences between 4K and UHD. But presumably, they seem to be sticking with 4K for marketing reasons. To avoid conflict with the DCI's actual 4K standard, some TV makers seem to be using the phrase "4K UHD," though some are just using "4K."
The UHD standard is really two standards. There's 3,840 by 2,160, and then there's a big step up to 7,680 by 4,320, also called UHD. It's reasonable to refer to these two UHD variants as 4K UHD and 8K UHD. The 8K UHD spec should probably be renamed QUHD (Quad Ultra HD).
The real solution would have been to abandon the 4K moniker entirely and instead use the designation 2160p. Display and broadcast resolutions have always referred to resolution in horizontal lines. The letters "i" and "p" refer to interlacing, which skips every other line, and progressive scan, which doesn't. Common standards are 576i (PAL), 480i (NTSC), 576p (DVD), 720p, 1080i, 1080p, and so on.
This didn't happen because the number didn't match the size of the resolution increase. "2160p" implies that the resolution is double that of 1080p HD, while the actual increase is a factor of 4. The gap between 720p and 1080p is significantly smaller than between 4K and 1080p, though how much you notice the upgrade will depend on the quality of your TV and where you sit.
Further complicating matters is that just because a display has a 2160p vertical resolution doesn't mean it supports a 3,840- or 4,096-pixel horizontal width. You're only likely to see 2160p listed as a monitor resolution, if at all. These sorts of displays target professionals. Now that there are 4K TVs everywhere, it would take a concerted effort from at least one prominent TV manufacturer to right the ship and abandon using 4K in favor of UHD. In all honesty, though, it's too late. The branding ship has sailed.
YouTube is a bit of an exception to this. YouTube labels video as both UHD and 2160p. The labels above attempt to be both technically accurate (2160p50 means 2160p at 50 frames per second) and tell regular viewers that this is a 4K stream.
When people use 4K and UHD in regular conversation, they mainly refer to the same thing. The distinction matters more in technical contexts like the ones discussed above.
Editor's Note: Sebastian Anthony also contributed to this article.
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