UHD… But what is actually UHD?

The beginning of the year saw a milestone in the Spanish audiovisual sector: the transition of all terrestrial broadcasting to HD. The wrongly so-called ‘analogue blackout’ -nor that SD DTT was analogue, anyway- has also driven public broadcaster RTVE to finally begin permanent broadcasting in UHD. And if from SD to HD we have no doubts, -as resolution was multiplied by four- the move to UHD can be a little more awkward.


It wasn’t that simple from SD to HD.

In the evolution from SD to UHD, the first milestone -HD- was quite ‘simple’. Although concepts such as ‘HD ready’ appeared in an attempt to establish an intermediate resolution, which amounted to little more than a commercial strategy from TV manufacturers that led to some confusion, it actually entailed a leap in resolution.

Taking into account the two existing standards for analog signals -some of whose parameters have been inherited by digital signals-, under the NTSC standard SD resolution was 720 x 480, while the PAL standard had a resolution of 720 x 576. Originally, aspect ratio was 4:3 but, thanks to the anamorphic artifact, it was possible to have an aspect ratio of 16:9, which would be the only one to this day. As you can see, it seemed ‘only’ a change in resolution, but it involved some complexity.

In an environment of coexistence of analog and digital signals under these parameters, and although in countries such as Japan NHK did some analog HD experiments -as it was also the case here in Spain with Retevisión in the 1992 Olympics- the leap to digital HD signals was finally taken. We left behind any aspect ratio other than 16:9 -which was a joy- and a single resolution of 1920 x 1080 was introduced. So far everything perfect. TV manufacturers struggled to meet the demand and pulled out their sleeves -that’s how it came about- the ‘HD Ready’ resolution. This sort of commercial invention established an intermediate resolution of 1280 x 720 by which chiefly small format television sets were capable of receiving HD signals without the need for more expensive panels. Thanks to everyone, this was short-lived and quickly fell into oblivion without affecting the production environments.

HD brought us another concept that until then had no greater complexity: scanning. In SD all the signals were interlaced, since that was the way that cathode ray tube TVs displayed the images, a method that was a legacy until the emergence digital signals; but HD meant an opportunity to evolve. Progressive scanning was introduced, the PAL standard saw signals featuring 25 frames per second interlaced in 50 fields increase to 25 progressive or complete frames. The amount of information transmitted was the same, but in a different way.

And then came UHD.


The five parameters of UHD

And the thing is that ultra high definition is much more than a leap in resolution. With analog times well behind us and TV sets capable of displaying much higher resolution, colorimetry, images per second and a greater range of brightness, it is time to leave behind past legacies and take a real leap.

The first parameter that evolved, just like when going from SD to HD, is resolution. This is the easiest one. We again multiply the number of pixels in our image by four and move to a resolution of 3840 x 2160, but finally now featuring progressive scanning and, just like in HD, always with a 16:9 aspect ratio. But with a little rub. There is another kind resolution, called DCI 4K, which sets forth a resolution of 4096 x 2160 and an aspect ratio slightly higher than 17:9 and oriented to cinema but, as far as we are concerned, UHD resolution can be considered a single one.

Let’s move on to the second parameter of UHD: color. Both in SD and HD -although in HD we can also have signals with extended color space- the BT.709 color space was defined, again a legacy due to the colorimetric reproduction capacity of the phosphors fitted in analog cathode ray tube screens. A new BT.2020 color space is defined in UHD. It is a color space that maintains the white point D65, 6500K, as in BT.709, but extends its primary colors, thus making them purer, to cover more than 90% of the color range visible to the human eye. This new color environment is defined as WCG, or Wide Color Gamut.

The next parameter, and perhaps the best known for the improvement it entails for the image, is HDR, High Dynamic Range. In this case it consists of an increase -a very considerable one- of the contrast range that an image is capable of displaying. There are different standards and this range can be adapted to the image to be represented through static and dynamic metadata systems that set points of black and white, minimum and maximum luminosity, in a changing way. And it turns out that SDR, Standard Dynamic Range, was one of the biggest differences between the signals on our screens and the perception of the human eye, and with HDR they become narrower. much more so.

These parameters -WCG and HDR- will lead us to increase the number of bits that each pixel must contain in order to represent a greater dynamic range and color range, as well as to facilitate a better representation of textures on surfaces, making them more realistic. The 8 bits that were used until now are not enough any longer and standards appear in 10 and even in 12 bits per pixel. In fact, HDR implies that the signal is WCG, so this backward compatibility that we will review later is not such or, at least, not so straightforward.

The fourth parameter that characterizes UHD is New Generation Audio, or NGA. Compared to stereo or multichannel audio -5.1 for example-, NGA defines a new method for capturing and propagating sound by using objects instead of channels. This will allow us to offer new experiences that are much more immersive, adapt playout systems, and expand the offering of sound customization and accessibility. Again, there are several standards on the market, such as Dolby Atmos or MPEG-H, but all of them are aimed at providing these new functionalities that were not possible through sound before.

And the fifth parameter in UHD image resolution is HFR, or High Frame Rate. In this case, the concept is simple: at least twice the number of frames per second that are displayed, going from 25 or 30 images per second to 50 and 60 respectively, or even higher. It has been shown that, at very high resolutions, such as 4K, and, above all, with high dynamic range and the normal screen viewing distances in a typical living room, the different frames gradually become perceptible, especially for images with a lot of motion such as motor sports or certain fiction scenes, if the current standard of 25 or 30 frames per second is used. The direct implication of this is to double the amount of information needed and this is the case, although there are compression and coding techniques that allow to increase efficiency and not to quite double said information flow.

As we can see, the leap from SD to HD is much more straightforward, although not without some complexity, as compared to the transition from HD to UHD. In fact, posing the question that gives the title to this article, “What is actually UHD?” starts to become appropriate. Let’s get to the controversy, but first let’s see what the industry standards say.


What the standards say

The two standards applicable to UHD signals are BT.2020 and BT.2100.

The BT.2020 standard, also known as Rec. ITU-R BT.2020, sets the parameters for transmission of ultra-high resolution (UHD) video signals. This standard defines a wide range of features that a UHD signal must meet to ensure optimal image quality. Key parameters include a minimum resolution of 3840 x 2160 pixels -four times the resolution of Full HD- as well as a color depth of at least 10 bits per channel, thus allowing for more accurate and detailed color reproduction.

In addition, as we discussed earlier, BT.2020 specifies a broader primary color space, known as BT.2020, which encompasses a wider spectrum of colors than previous standards, such as BT.709, this resulting in a richer and more vibrant color gamut.

The BT.2100 standard, or Rec. 2100, represents a significant progress in video signal transmission technology, focusing specifically on improving the viewing experience of High Dynamic Range (HDR) and Wide Color Gamut (WCG). This standard introduces the concepts of PQ (Perceptual Quantizer) and HLG (Hybrid Log-Gamma) as two major methods for HDR representation, each of them designed to meet different viewing needs and conditions. PQ offers a gamma curve perceptually more adapted to the human eyesight, thus resulting in a more realistic reproduction of brightness and contrast, while HLG is a more compatible option that allows reproduction on SDR (Standard Dynamic Range) devices without losing detail in areas of high brightness or shadowing, only from the point of view of luminance since, as we have mentioned, HDR currently requires a signal to be WCG. In addition, BT.2100 provides tools to prevent the loss of luminosity in highly saturated colors and in areas of the image with high spatial frequencies (highly contrasted edges). This standard also defines the technical parameters for transmission of ultra-high resolution (UHD) video signals, including a minimum resolution of 3840 x 2160 pixels and a color depth of at least 10 bits per channel, just like BT.2020.


So, what is actually UHD?

Can we consider a 2160p50 SDR signal featuring BT.709 color space and stereo audio as UHD? Well, there are those who say yes, and there are those who say no. If we are objective and consider that the five parameters described here must be met, a signal like the one described above only meets one of the five -resolution- but it is not WCG, it is not HFR, nor HDR or NGA, so it cannot be considered UHD. On the other hand, a signal with 3840 x 2160 resolution does not fit in any previous standard that defines HD signals, which could be considered UHD, especially when it comes down to selling TV sets.

Let’s not merely scratch the surface and ask the experts in the field to try to shed some light on this.



The opinion of the experts

And there is nothing better than to ask those who know. To do this, we have relied on two colleagues who have been studying and working on these new formats for a long time and have not only ideas, but conclusions about them. Jose Manuel Menéndez, professor and director of the RTVE Chair at Polytechnic University of Madrid, and Emili Planas, CTO of Grup Mediapro.

«Given the number of times the question ‘What is UHD?’ has come up at the meetings we have had between friends and colleagues in the industry, it is starting to become the million-dollar question. It is usually associated with the question that relates to UHD signal and size for home screens, but this second question is better addressed in another article, in depth and at length.

Defining what a UHD signal is turns out to be very easy if we stick to the regulations and recommendations that have been published by entities that regulate the sector, such as ITU (International Telecommunication Union) and EBU (European Broadcasting Union). According to the BT.2020 recommendation, “UHD -ultra-high-definition television- will offer viewers a better viewing experience mainly due to a wide field of view, both horizontally and vertically, on screens of adequate size for use in homes and public places. The applications of UHD require system parameters that exceed those of HD.” One thing is clear. UHD beats HD and it is made explicit that there is a difference between the two. In addition, the same recommendation defines the parameters that are considered for UHD: resolution of 3840 x 2160 pixels (referred to in the EBU TR 037 document as UHD-1 or 4K) and 7680 x 4320 pixels (UHD-2 or 8K according to the EBU glossary), plus WCG, HFR ranging from 24/1,001 to 120 frames/second, NGA and use of 10 or 12 bits (often referred to as HBD – High Bit Depth). So far, everything is clear about the differences between HD and UHD. Then came the BT.2100 recommendation, which brings HDR to both UHD and HD signals, and everything got messed up.

But of course, migrating from HD to UHD is not a trivial matter, neither in production, nor in contribution links or regarding dissemination to users. For this reason, EBU suggested to carry out the migration in several stages:

  • Migrate from HD to 1080i25 conventional interlaced, then to 1080p24-60 (progressive, with 24 to 50 or 60 frames/second refresh rate) with HBD, HDR, WCG and NGA, and this was called “1080p Advanced 1”.
  • Then move on to “1080p Advanced 2”, which involved cranking HFR up to 100 or 120 frames/second
  • Then move on to “UHD-1 Phase 2”, which involved upgrading resolution to 3840 x 2160, using HBD, HDR, WCG, HFR up to 50 or 60 frames/second and NGA.
  • And then finalize the transition with full “UHD-1 Phase 2”, including HFR up to 100 or 120 frames per second.

UHD-2 is reserved for resolutions of 7680 x 4320, and it is assumed that it will still take some time for it to become widespread in homes, given the limitations of the broadcast systems.

It is all very clear. HDTV is 1080p (we get rid of interlacing, at last!), and UHDTV is 4K and 8K. But, it is true that both HDTV and UHDTV must evolve to use full phase 2 (HBD HDR WCG HFR NGA). Some call HDTV with full phase 2 ‘enhanced HD’, but EBU only sees it as one more step towards reaching full UHD-1 phase 2.

And, if you ask me, then I think I’ll second EBU’s proposal, if only to define clear targets for progress. There may be entities that make more progress and others less but, at any event, it is for sure that signal quality and QoE offered will be very good. What’s more, it’s always a good thing to have competition in offering the best signal…»

José Manuel Menéndez


«I think it is good that, in the more technological environment, we analyze each of the parameters that make up the UHD concept from a practical point of view. In my opinion, of the five parameters you mentioned, HDR and WCG are essential, as they can be appreciated in any type of production and at any viewing distance. HFR is the most demanding extra feature in terms of bandwidth and we should be flexible when considering it indispensable for a particular number of frames per second. If dealing with cinema content, 25fps can be accepted exceptionally, but 50fps should be the minimum required to formally be considered UHD. Increasing to 100 or 120fps may be a recommended improvement, but this involves such a large financial effort in relation to the improvement, that it is unrealistic to consider it mandatory. Quite the same goes for NGA, which happens to be a much-needed added value, but considering the reality of user sound devices, I don’t think we can claim it as indispensable for attainment of the UHD label.

And I’ve left the issue of resolution for last, as this is where the controversy increases. Is a resolution 3840 x 2160 required for UHD? Well, clearly not! Add to this the fact that the capabilities and limitations of the human visual perception system (HPPS) are what they are. At a certain distance from the screen (whatever the screen), the number of pixels we are able to see is limited. There are some tables on which more or less everyone agrees that allow us to find out what the optimal resolution is based on screen size and the viewing distance. Let’s take a 65-inch screen as an example: in order to distinguish between a 2160p or 1080p resolution, we must be at less than 3 meters from it. At more than 3 meters, most humans will not be able to tell an image of 1080 x 1920 from the same image with higher resolution. Therefore, if we have bandwidth and technical-economic feasibility to produce and broadcast in 2160p, let’s go ahead, we will be able to get very close to the TV. Otherwise, a 1080p resolution is sufficient.
From my point of view, the most versatile and realistic format for the coming years is 1080p50 HDR-WCG. Broadcasting should always feature NGA audio and each production will use the most appropriate functionalities in each case.»

Emili Planas



In our detailed analysis on Ultra High Definition (UHD), we have explored the various parameters that define this advanced image and sound format. From the move from Standard Definition (SD) to High Definition (HD) to the advent of UHD, we’ve seen how every technological leap involves not only higher resolution, but also significant improvements in color, dynamic range, frame rate, and audio experience.

UHD, with a main resolution of 3840 x 2160 pixels, goes beyond simply multiplying the number of pixels on the screen. It is a set of features designed to deliver a more immersive and realistic visual and listening experience. From WCG, which broadens the color spectrum, to HDR, which improves contrast and brightness, each component is designed to take image and sound quality to a higher level.

In this sense, the opinions of experts in the field give us a valuable perspective. José Manuel Menéndez, professor and director of the RTVE Chair at the Polytechnic University of Madrid, and Emili Planas, CTO of Grup Mediapro, shared their views, which shed light on the definition and applicability of UHD in practice.

José Manuel Menéndez, with his vast experience in the field, emphasizes the importance of sticking to the regulations put in place by entities such as ITU and EBU. According to the BT.2020 recommendation, which defines the parameters of UHD, it is clearly established that this format improves upon HD and the key requirements are detailed: resolution of 3840 x 2160 pixels, WCG, HFR from 24 to 120 frames/second, NGA and 10 or 12 bits per pixel. Menendez explained how EBU has proposed a gradual migration from HD to UHD, splitting the process into stages such as ‘1080p Advanced’ and ‘Full UHD-1 Phase 2 Complete’. Their perspective points to a clear technological evolution towards full UHD.

On the other hand, Emili Planas provided a practical vision, highlighting the importance of HDR and WCG as essential elements for a meaningful UHD experience. In addition, he considers HFR and NGA to be valuable features, although not necessarily indispensable in all cases. As for resolution, he raised an interesting debate by questioning whether 3840 x 2160 pixels is always necessary to be considered UHD. He argued that a 1080p resolution with HDR-WCG at 50 frames per second could be a more realistic and versatile option for many scenarios.

These insights from experts show us that the path to UHD involves not only adopting new technical standards, but also adapting content production and consumption to take full advantage of these advanced capabilities. As technology and infrastructure continue to develop, we are likely to see greater diversity in UHD deployments, each of them looking to deliver the best possible picture and sound quality for viewers.

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