More, Faster, Better Pixels!
As we have reported over the last few weeks on the 8K Association website, Intel’s Ravindra (Ravi) Velhal often talks about ‘more, faster and better pixels’. We thought we’d highlight what this means in practice.
More Pixels
‘More pixels’ is quite simple. The pixels are the dots that make up a picture. The more pixels you have, the higher level of detail that can be shown in the image. More pixels shows more detail providing you have enough contrast between the adjacent pixels in the display. We’re not going to dig into this here, but this is a topic that we have highlighted on the 8K Association website.
Although it is generally true that any bigger number of pixels is an advantage, in an ideal world, there would be consistency in the image format from capture or creation, through editing to final rendering, storage, transmission and display. If you don’t have consistent formats, processing needs to be done to match different formats and in the process, errors and image degradation can creep in. To make formats consistent, the International Telecommunications Union (ITU) acts as the standardization body for broadcasters. The ITU has different levels of format for broadcast in high definition (HD). All use a 16:9 aspect ratio (the ratio between the horizontal size/resolution and the vertical)
- 1920 x 1080 is recommended in ITU-R BT.709-6 and is often known as FullHD (to differentiate from some companies’ broadcasts called HD at 1280 x 720) . 1920 x 1080 gives just over two million pixels
- 3840 x 2160 is the lowest format in the ITU-R BT.2020 definition of Ultra High Definition or UltraHD. It is sometimes known as 4K resolution. This level of UltraHD has just over eight million pixels – four times the level of FullHD
- 7620 x 2160 is the highest format in ITU-R BT.2020 UltraHD and is usually known as 8K. 8K has 33 million pixels or 16 times the level of FullHD and four times the level of 4K UltraHD
Sometimes you may see reference to DCI-2K or DCI-4K. These are 17:9 aspect ratio formats defined by the Digital Cinema Initiative and represent 2048 x 1080 and 4096 x 2160, respectively. At the time of publishing, no DCI-8K format had been published, although it might be expected to be 8192 x 4320.
Faster Pixels
Motion video on TV or at the movies relies on the fact that if the human visual system is presented with a sequence of separate images at high speed, the brain can be fooled into believing it is seeing continuous motion. The level needed to fool the brain is a complex topic (look up ‘Critical Flicker Fusion‘ if you want to understand more).
In an ideal world, enough images would always be provided to ensure that there is no flicker or judder. However, the more frames that are needed to be delivered, the higher the cost of creation, storage and transmission. At one extreme, the movie business standardized on 24fps almost 100 years ago (in 1926 with the introduction of sound) and cinematographers carefully plan their shots to avoid showing judder at this low rate. At the other extreme, PC game players like refresh rates with hundreds of frames per second.
In TV, initially the transmission systems were very restricted in bandwidth and so a concept of ‘interlacing’ was used. In this system, the odd and even rows that make up the image are sent alternately, allowing frame rates of 50 or 60Hz (depending on the region of the world) although each only had, effectively, half the resolution. This worked well when TV systems were based on cathode ray tubes (CRTs) and analogue electronics, but less well with flat panel displays and digital electronics. The changes in displays and electronics have meant a trend towards ‘progressive’ formats where the image is created directly from top to bottom. (look here for a really detailed treatment of interlacing)
Higher frame rates (HFR) give smoother motion display, which is particularly useful in sports and other fast moving content which is less predictable than movie content. However, some like the 24fps frame rate as being ‘more cinematic’ for movies, so the.best TVs often have special technology to look good in this mode.
Illustrating the desire of broadcasters to stick within a particular bandwidth limit to meet cost requirements, some show sports in 1280 x 720 format (HD rather than ‘FullHD’) with 50/60fps in progressive formats. This is known as 720P in contrast to 1080i.
As formats increase in resolution and clarity, and as displays get bigger, it becomes even more important to have higher frame rates (human peripheral vision tends to be more sensitive to flicker than the central viewing area). This is one of the reasons that Intel’s recent 2024 Paris Olympic 8K stream was created at 60fps in a progressive format to capture the motion as well as possible.
The way that the human perception system responds to resolution and frame rates is complex, so some are even working on 8K 120Hz and above. The BBC’s technology researchers have previously suggested moving to 300 fps to make the conversion to 50fps or 60fps even better.
Tradiitional broadcasters using terrestrial and satellite transmissions have big cost and technology barriers in moving to higher frame rates and the bandwidth that they require, but streaming is just about network speed and history has shown that this can increase very rapidly. Further, streamers can use the latest codecs which can help reduce the bandwidth needed.
Better Pixels
The topic of ‘Better Pixels’ tends to be divided into two areas, brighter pixels with more contrast between bright and dark pixels and better color. The two topics are often referred to under the terms High Dynamic Range (HDR) and Wide Color Gamut (WGC).
Brighter and More Contrasty Pixels
Our visual systems are capable of adapting to a huge range of different lighting conditions, although they are more limited at any particular time as it takes time for vision to adapt. Viewers really like to see images with high contrast between the darker parts of the image and the brighter ones as this makes an image more like the real world. Increasingly TV specialists talk about high dynamic range rather than contrast. The term contrast was often used in the past to compare the display will a full screen of black compared to a full screen of white. On the other hand, dynamic range is the difference between the lightest and darkest parts of a single frame.
In the first few decades of the developent of TVs, the brightest part of an image was fixed by the performance of the CRTs used to make TVs. CRTs could be made bright, but there was a trade-off between brightness and focus, so brighter images got fuzzier. To optimize the image formats, the ITU had a standard for the dynamic range at quite a low level of brightness and known as Standard Dynamic Range. SDR is specified in ITU-R BT.709 along with standard definition. Maximum brightness for SDR was specified at just 100 cd/m2 with black at 0.1 cd/m2.
However, display technology has dramatically improved and LCDs in particular are capable of very high brightness without losing other aspects of image quality. Tests by Dolby found that some viewers liked to see images with as much as 10,000 cd/m2 of peak brightness – 100 times the level of SDR. To allow systems from content to display to accurately show what the creator intended, a number of HDR formats have been developed. It’s beyond the scope of this article to dig deeply into this complex topic, but the most common formats are HLG (broadly preferred by some broadcasters) and PQ (broadly preferred for pre-defined content such as movies and standardized as ST.2084 but also seen in TVs as Dolby Vision and HDR10). Modern TVs are advertised with peak brightness up to 5,000 cd/m2.
OLED and miniLED TVs can effectively deliver almost infinite dynamic range and so HDR has been a big feature for TV makers. Optimizing HDR performance and the different ways of converting SDR to HDR or HDR from one format to another for display are among the factors that differentiate TV sets from different brands. To deliver HDR usually requires a little more data (typically a switch from 24 color bits per pixel to around 30) than SDR, but that may be less than the boost needed for HFR so broadcasters sometimes choose to stick with lower resolution and frame rates, but add HDR.
More Colorful Pixels
Just as the brightness of pixels in TV was initially limited by the capabilities of CRTs, so was the display of color. CRTs had only a limited range of color capability and the SD format used color in a format known as ITU-R BT.709 – often referred to as Rec.709. A similar range of colors was also used to standardized PC displays as sRGB.
Additional wider color gamuts were developed such as AdobeRGB for print applications and DCI-P3 for digital cinema displays which were not restricted by CRT capabilities. It was clear that new display technologies (OLED, microLED, laser projection etc) would be able to display a much broader range in the future, so in 2012, the ITU published the initial specifiation for the Rec.2020 color gamut, which was intended to be a much wider specification for the very long term.
Because of the purity of the red, green and blue needed to meet the full Rec.2020 gamut, at the time of publication only laser-based projection systems have been able to show the entire Rec,2020 gamut. However, the specification now exists as a target for display makers and technologists and samples and prototypes have been shown at technical conferences that start to get over 90% of Rec. 2020 without needing to use lasers.
No broadcasters or streamers currently provide content with the full range of Rec. 2020 (although technical demonstrations in collaboration with Hollywood studios have been made). However content is often encoded using the Rec. 2020 format.
As with HDR, the different TV brands have different approaches to how they translate content that was formatted and created as Rec.709, DCI-P3 or Rec.2020 to the specific color range on their sets. To minimize the variation introduced by set makers, the UHD alliance promotes a ‘Film Maker Mode‘ in TVs to bypass special adaptations made by set makers and present a consistent image to the viewer.
How Do You Get More, Faster, Better Pixels?
TV sets that include 8K displays will cost more than sets with lower resolutions, so set makers will usually reserve 8K for their top of the range sets in other areas as the panel is by far the most expensive component. It is usually the case that the simplest way to ensure that you are getting the faster and better pixels as well as the most pixels is to choose an 8K set.
(If you’d like to dig into the state of the industry, just as we went to press, independent consultant Ian Hendy, a display specialist, published an update to his annual report on colour and HDR on LinkedIn. It’s worth a look if you are curious to learn more.)
