UHD or 4K – What’s it all about?
UHD or 4K – Resolution
First of all let’s look at the definition: Ultra High Definition (UHD) screens have twice the line count of full HD screens:
- Full HD = 1080 x 1920 pixels (aspect ratio = 16:9 or 1.78:1 approx)
- UHD = 2160 x 3840 pixels (aspect ratio = 16:9 or 1.78:1 approx)
UHD is commonly used synonomously with 4K, however 4K is a Digital Cinema Initiative (DCI) standard based around the DCI 4K definition which is 2160 x 4096 pixels, so there’s clearly a discrepancy there! 4K is now generally taken to mean any image with approximately 4000 horizontal pixels.
- DCI 4K = 2160 x 4096 pixels (aspect ratio = 1.9:1 approx)
While we’re on the subject of resolution, it’s worth taking a close look at the source material, as this is quite an eye opener. It is all very well having a display with UHD resolution, however if you are viewing lower resolution images on the display, you’re not getting the full benefit. As of writing this article, there are not that many players capable of producing full UHD images. So far (legally)we have:
- UHD Blu-Ray 2160 x 3840 pixels
- Amazon TV streaming at UHD (though highly compressed)
- BT Streaming at UHD (though highly compressed)
- Netflix Streaming at UHD (though highly compressed)
- Sky playback at UHD (later this year) (though likely to be highly compressed)
- Kaleidescape downloaded file playback at 2160 x 3840 pixels
UHD image resolution is not the whole story. There’s also frame rate, colour depth and dynamic range which all muddy the picture somewhat.
UHD or 4K – Frame rate
The UHD Blu-Ray format supports frame rates at 24, 25, 30 50 and 60 frames per second as does file download and playback, whereas the streamed services typically only support frame rates at 24, 25 and 30 frames per second.
UHD or 4K – Colour space
Colours are generated from a triangle of primary colours within the display, and how far these colours can go into the extremes of each of these primaries is governed by the colour gamut according to a number of standards. When calibrated, these standards ensure that the colour displayed is the correct colour compared to the source material. There are 3 main standards used:
- REC709 – colour space defined for HD colour television
- DCI P3 – colour space defined for DCI 4K
- REC2020 – colour space defined for UHD TV High Dynamic Range
REC709 (or ITU-R recommendation BT.709 or BT709) is the standard developed for HDTVs defined in 1990 and has served us well, however this only accounts for about 35% of the colours that are visible to the human eye. The Digital Cinema standard P3 was developed for digital cinema systems and expands this somewhat. The advent of UHD television has led to a new colour standard called REC2020 which has a much wider colour definition as can be seen from the charts below:
REC 709 colour gamut (left) compared with DCI P3 (centre) and REC2020 colour gamut (right)
What this means in practice is that a display capable of REC2020 will be able to show far more colours and to a deeper saturation than other displays. The different colour gamuts used by DVDs and regular HD blu-Rays compared to UHD Blu-Rays leads to a problem. If you set the display to REC709 standard (regular colour) the DVDs and Blu-ray movies will look normal, however UHD movies will appear washed out with little colour unless you set the display to REC2020 mode. In this mode however, regular DVDs and Blu-Rays will appear greatly over coloured and luminous. so far there doesn’t appear to be a way of the display automatically adjusting for the transmitted colour gamut, so you’ll need to swap between colour modes for each source material type.
UHD or 4K – Dynamic Range
You may have seen some displays advertising that they support High Dynamic Range (or HDR) this is a technique for expanding the effective dynamic range that an image is displayed. this effectively gives more realistic images when viewed in a cinema or on TV, and is needed because the film and TV equipment cannot even come close to the 6 orders of magnitude (1,000,000:1) dynamic range that the human eye can resolve (I think the best is likely to be a high end DSLR at ~28,000:1)
UHD or 4K – Content
Here’s the all that glitters is not gold part!
When you look at the UHD films that are available in detail, you discover that some of them were not even filmed in 4K, or even if they were filmed in 4k, they were edited in 2K or had a digital intermediate in 2K. It means that the gain in resolution of your shiny new TV or projector might not necessarily be used to the full by the UHD film you’re watching. Check out the realorfake4k web site for more details and lists of films to see what I mean.