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OK, so at first it might sound like a stupid question but I'm trying to get my head around all these definitions...

It seems that they don't have a specific resolution, but they do at the same time. It's as they have their own resolution but also are part of a group of resolutions.

Wiki says:

  • HD

    • 720i
    • 1080i
    • 1080p
  • UHD

    • 2160p
    • 4320p
    • 8640p

But generally we say that 1080i is HD and 1080p is FULLHD.

UHD has resolution 3840 x 2160 and 4K is 4096 x 2160 but 4K is UHD. Same vertical resolution (2160) but different aspect ratio, making the horizontal slightly longer by 256 pixels. And it also includes 8K...

Is UHD a resolution, a group of resolutions or both? If it's both, why?

10

The problem is that definitions such as HD, UHD and 4k etc. are partly hype. Marketers for AV equipment like to be rather elastic with the truth, hence the rather rubbery definitions. They're not standards such as ATSC or DVB, or graphic display resolutions such as VGA, WXGA etc., they're really just buzzwords.

There are other complications - like the different broadcast standards for SD — so that SD PAL is 576 visible lines and SD NTSC is 480 lines — or non-square pixels in digital video meaning that even a 16:9 HD 1080p image can be either 1440x1080 or 1920x1080*. So it's pretty understandable that you're confused.

For what it's worth, the general consensus is that:

  • SD refers to 576i / 480i (with the unusual EDTV variant being 576p and 480p). Display resolution is 768 × 576 PAL or 720 × 534 NTSC, often the actual pixel resolution is reduced, as in the case of DV: 720 × 576 PAL or 720 × 480 NTSC, with non-square pixels
  • HD refers to 720p and 1080i / 1080p. The display resolution is 1280 × 720 or 1920 × 1080, but sometimes the horizontal pixel resolution is reduced, e.g. HDV which is 1440 × 1080 with non-square pixels. You could also say that resolutions higher than this are also High Definition. So in one sense it's anything that isn't standard definition. Note that there is no 720i.
  • Full HD or FHD means the 1080 flavour of HD (either interlaced or progressive)
  • 2K describes the Digital Cinema Initiative resolution of 2048 × 1080, this resolution is based on the historical resolution of scanned super 35mm film. As with 4K it is a little slippery, and can sometimes be used to describe FHD. As the wikipedia article states it describes content "having horizontal resolution on the order of 2,000 pixels". So in other words: near enough is good enough.
  • QHD or Quad HD - 2560 × 1440 is a whacky one, it's twice the resolution of 720p. The "quad" refers to there being four times the pixels of a 720p frame. Not to be confused with QFHD - see below.
  • UHD refers to 3840 × 2160, and like HD also sometimes to higher resolutions like 4K and UHD 8K
  • QFHD is 3840 x 2160 "Quad Full HD", the quad referring to the fact that it has four times the pixels of a FHD frame, which sounds better than saying it has twice the horizontal and vertical resolution. QFHD = UHD
  • UHD 4K - 3840 × 2160, is double the HD 1080 frame size, and is also sometimes called 4K, even though the horizontal resolution is less than 4000 pixels. UHD 4K = UHD
  • UHD 8K or FUHD is 7680 × 4320. I'm guessing that as the FUHD sets hit the shops they'll be called "8K" by the salespeople.
  • 4K usually refers to Digital Cinema Initiatives 4K which is 4096 × 2160 but the term is sometimes used to refer to UHD 4K
  • Similarly 8K should properly refer to 8192 × 4320 but is also applied to FUHD

Note that there are cameras such as the Red cameras that shoot in-between resolutions such as 3K, 5K and 6K. But that's just for production, the finished media will usually be cropped / scaled to a more standard resolution for display or broadcast.

TL;DR If you're trying to define anything in technical terms and want to be unambiguous just give the pixel dimensions; terms like HD, UHD etc are really FUD. They're more for marketing than anything else.

Edit: Here's a handy chart:

here's a handy chart

pdf version available here

  • *not to mention that the broadcast signal has hidden lines, meaning for example that an analogue PAL broadcast picture actually has 625 lines, 49 of which are blanked – stib Feb 2 '15 at 6:58
  • The usual shorthand is to just count the vertical lines, with the assumption of a 16:9 aspect ratio. (4k may change this, since it's ~19:10, and counts the horizontal.) It's a pretty silly way to operate, because most movies are wider than 16:9, so they are 1920x880 or whatever, but still called 1080p. IDK if counting vertical rez is a holdover from interlacing (which seriously hurts vertical resolution, thus making it an important number). Or more likely it's a holdover from analog, where there were a specific number of vertical lines, but horizontal was fully analog. – Peter Cordes Feb 2 '15 at 13:17
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    TL;DR summary: unless you're trying to sell something with buzzwords, just say what you mean. Like 480i60, or 1080p24, or 3840x2160p30, to make up some examples. Or optionally leave off the refresh rate and say W x H, or 1080p. – Peter Cordes Feb 2 '15 at 13:22
  • 1
    @PeterCordes Counting vertical lines is a hangover from analogue. Horizontal resolution was measured in MHz, being the bandwidth of the signal. – stib Feb 2 '15 at 22:26
  • OK, so they are buzzwords. 4K is not better that UHD. @PeterCordes The problem with just saying the vertical is that the aspect ratio can change, meaning the resolution could be 2K aswell. Anyway, "buzzword" pretty much answers it. Thanks! – BBking Feb 4 '15 at 3:29

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