TV producers, broadcasters, and tech media are using the phrases UHD and 4K interchangeably since they have grown so interchangeable; yet, they are not the same.
Considering that 4K technology is now widely available, let us examine two terms that have come to be confused with each other: 4K and UHD, or Ultra HD. Although TV producers, broadcasters, and tech blogs use these terms interchangeably, they were not originally the same and technically still aren’t. From the perspective of the viewer, 4K and UHD are not that different; 4K is a more widely used term than UHD, but 4K-capable Blu-ray drives are sold as Ultra HD drives.
UHD vs. 4K
To put it simply, UHD is a consumer display and broadcast standard, and 4K is a professional production and cinema standard. Let us take a look at the origins of this confusion by examining the two names’ respective histories.
The term “4K” was first coined by the Digital Cinema Initiatives (DCI), a consortium that standardized a spec for the production and digital projection of 4K content. Specifically, 4K is 4,096 by 2,160, which is exactly 4 times larger than the previous standard for digital editing and projection (2K, or 2,048 by 1,080). The term “4K” refers to the fact that the number of horizontal pixels (4,096) is approximately four thousand. Moreover, the 4K standard specifies how 4K content is encoded; a DCI 4K stream is compressed using JPEG2000, and it can have a bitrate of up to 250Mbps.
Ultra High Definition, or UHD for short, is the next step up from High Definition. It is the official term for the 1,920 by 1,080 display resolution, which is quadrupled to 3,840 by 2,160. This is not the same as the 4K resolution mentioned above, but despite this, nearly all TVs and monitors that you see advertised as 4K are actually UHD. Some panels have an aspect ratio of 1.9:1, because they are 4,096 by 2,160. However, the vast majority have an aspect ratio of 1.78:1.
Not 2160p, Why?
It is not as if TV manufacturers do not know the difference between 4K and UHD; yet, they seem to be sticking with 4K, probably for marketing purposes. Some TV makers seem to be using the phrase “4K UHD,” however some are just using “4K,” to avoid conflict with the DCI’s genuine 4K standard.
It is 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). (Read: 8K UHDTV: How do you send a 48Gbps TV signal over terrestrial airwaves?) The UHD standard is actually two standards: 3,840 by 2,160, and then there is a big step up, to 7,680 by 4,320, also called UHD.
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. The true solution would have been to completely renounce the 4K moniker and use the designation 2160p instead.
The reason this did not happen is that the number did not correspond with the size of the resolution increase: “2160p” suggests that the resolution is twice as high as 1080p HD, but in reality, it is increased by a factor of 4. The difference between 1080p and 720p is much less than that between 4K and 1080p, but how much of an improvement you notice will depend on your TV’s quality and your position.
In addition, the fact that a display with a 2160p vertical resolution does not necessarily support a 3,840 or 4,096-pixel horizontal width further complicates matters. Professionals are the target audience for these kinds of displays, and since 4K TVs are now commonplace, it would take a determined effort from at least one well-known TV manufacturer to turn the tide and stop using 4K in favor of UHD. But, in all honesty, it is too late; the branding ship has sailed.
A notable exception to this is YouTube, which labels videos as both 2160p and UHD. The labels above are an attempt to be as technically accurate as possible (2160p50 means 2160p at 50 frames per second) while also informing casual viewers that this is a 4K stream. In everyday speech, 4K and UHD are essentially synonymous; the distinction only becomes significant in technical contexts such as the ones mentioned above.