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  • What''s the difference between 4K and UHD ?

    Time:2014-12-31

    What's the difference between 4K and UHD ?

    31 December 2014


    Q: What's the difference 4K VS. UHD ?

    A: To put it simply, 4K is a standard for professional video production and cinema, while UHD is a standard for consumer displays and broadcast television. Both 4K and UHD refer to high pixel density video, but there is a slight difference: a 4K image (4096x2160) has 256 more lines of vertical resolution than UHD (3840x2160), and as a result, has a different native aspect ratio. The term “4K” originally derives from the Digital Cinema Initiatives (DCI), a consortium of motion picture studios that standardized a spec for the production and digital projection of 4K content. In this case, 4K is 4096×2160, and is exactly twice the previous standard for digital editing and projection (2K – 2048×1080). As you can see, 4K clearly refers to the fact that the vertical resolution (4096) is just over four thousand. 


    1920 multiplied by 1080 = 2,073,600 pixels. 

    - 4096 multiplied by 2160 = 8,847,360 pixels. - They may call 4096x2160 4k because of the 4096 (4k x 2k). 
    - 3840 multiplied by 2160 = 8,294,400 pixels divided by 4 = 2,073,600 pixels. - They may call 3840x2160 4k because it has 4 times the pixels as 1920x1080. 


    More answers:

    The term 4K is generally used to refer to video signals with a horizontal resolution on the order of 4,000 pixels. To be technically accurate, 4K is a resolution of 4,096 pixels horizontally by 2,160 pixels vertically, which is 8.8 million pixels. This is a cinematic standard for 4K film projection (Digital Cinema Initiatives [DCI] 4K) and carries a specific aspect ratio that is not used for commercial or consumer displays. Previous generations of video resolutions were described by the vertical resolution (i.e., 1080p refers to a signal with 1080 vertical lines). Had the naming convention for standard definition (SD) and HD been used, 4K video might instead have been referred to as 2160p. 


    What a lot of video consumers do not realize is that DCI 4K is a 17:9 aspect ratio, whereas most displays today — including those supporting 720p, 1080i, and 1080p — are built for a 16:9 aspect ratio (see table 1). Because essentially all sources and displays have used this aspect ratio, manufacturers and integrators have seen an era of relative simplicity. For several years, there has been no need to accommodate various other aspect ratios. 4K brought with it a challenge of managing two different standard resolutions. 


    In 2014, International Telecommunication Union’s Radiocommunication Sector (ITU-R) defined an alternative, ultra high-definition (UHD) resolution that would fit a 4K-like image into the standard display ratio. Such an image has 7 percent fewer pixels in the horizontal aspect. On the left and right edges, 3.5 percent of pixels are eliminated without compression or stretching so the image would have a 16:9 ratio (see figure 1). 


    In Recommendation BT.2020-1 (popularly known as Rec. 2020), ITU-R defines two UHD television resolutions: 3840×2160 (4K) and 7680×4320 (8K). The recommendation further confirms that UHDTV applications require system parameters that go beyond the levels of HDTV (Rec. 709, 1993) — larger screens, higher spatial/temporal resolutions, a wider color gamut, and a wider dynamic range.


    Most of the 4K recording equipment today records natively in DCI 4K. The chipsets in those devices are built for native 4K. Usually, when they are writing to the disc, the chipsets record in the 16:9 aspect ratio instead. Considering the sparse need for DCI 4K, 16:9 UHD chipsets are becoming the norm. Manufacturers are building recording devices either in the 16:9 UHD aspect ratio or DCI 4K (with some type of internal conversion). DCI 4K devices are then cropping the image horizontally and writing the result to disc. 


    Adding to the complexity of having two 4K resolutions is the existence of other resolutions between 2K (2,048×1,080) and 4K. Quad HD resolution is 2560×1440 pixels and most popular in the mass consumer market because it is the HD resolution (1820×720) multiplied by 4 exactly (see figure 2). 


    Resolutions 2560×1600 and 2880×1800 are becoming popular on some high-end source devices, such as the MacBook Pro, high-end Windows laptops, and even some mobile devices. The problem is, even though those resolutions are not actually 4K — they are not 4,000 pixels wide — they require more bandwidth than 2K and 1080p. Displaying those resolutions, therefore, requires 4K-class switching and distribution gear.


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