By Peter Schut, CTO of Axon
The development of TV technology has been a continuing search for greater picture resolution. During the 21st century, the rate of innovation and research has increased, resulting in HD now being challenged by Ultra High Definition (UHD) and its 4K higher resolution. The pace has accelerated considerably over the past five years, as over-the-top (OTT) and streaming services have changed the broadcasting landscape, bringing a cinematic, immersive experience into the home.
While 4K/UHD itself is still in the very early stages of roll-out for both production and transmission, the new breed of entertainment distribution services, notably Netflix and Amazon Prime, have made 4K/UHD a primary specification for programming and are delivering it to homes as standard rather than being a special feature or test channel.
In order to compete, broadcasters and manufacturers must now provide the viewer with an absolutely stunning picture (regardless of platform). However, with broadcast image technology currently in a state of flux, with several formats and systems — both old and new — likely to co-exist for some time to come, the industry is still learning what all this means and what needs to be done.
4K/UHD has already brought us more pixels. The next logical step therefore is not additional pixels but better pixels that give a picture with more definition and, most important of all, improved colour reproduction and a higher dynamic range. Which is why the initials HDR (high dynamic range) and WCG (wide colour gamut) have been challenging those of 4K/UHD for prominence on booths at trade shows and in new TV set advertisements.
HDR is able to produce a dynamic range of 200,000:1 (or 17.6 stops in camera terms) when shown on a 2000 cd/m2 display with a bit depth of 10-bits per sample. This compares to the 64:1/approximately 6 stops from standard dynamic range (SDR) on a conventional gamma curve with a bit depth of 8-bits per sample. By extending the dynamic range, more information can be accommodated in an image, bringing with it more detail to an image. In a similar way, WCG allows more colours to be displayed because it can store a wider range of colour values than established RGB (Red Green Blue) colour spaces.
Both HDR and WCG are being discussed in professional and consumer electronics circles but HDR is the more recognisable of the two terms. It has almost become a catch-all for improved image quality and greater definition — with the immersive feel of 4K/UHD creating a sense of reality — and life-like colour. As a result, many broadcasters and consumers are saying they will move to HDR but by that do they mean just that format on its own or a combination of HDR and WCG?
It is very likely it will mean both. HDR means a blacker black and a whiter white. If we go to WCG as well, the reds will be more red, the greens greener and the blues more blue, with everything else in between. The whole (image) space is bigger and more beautiful.
The broadcast industry needs to move into more pixels, more colours and HDR. We need to provide better pixels and do it right first time. We have one chance and if we get it wrong, the audience will switch off.
The technical challenge behind this is the move from the HDTV colour space (BT.709) to the colour space for 4K/UHD plus HDR and WCG (BT.2020). In doing this, we are going from a relatively small area (refer to diagram: HDR BT.2020 & BT.709 Colour Space) with a maximum brightness/luminance of 100 nits.
Compared to BT.709, BT.2020 has a much larger colour space to work in, with luminescence measuring 1,000/10,000 nits. In effect, we are trying to put a square peg in a round hole. This is impossible without using brute force, but with the correct forms of conversion and compression, we can make it right.
The whole issue becomes more complicated if WCG is part of the equation. HDR with some sort of backwards-compatibility is only possible when everything is working in the 709 colour space. But because the broadcast market has to be part of the real world, services will have to include WCG, which will mean the whole issue of backwards-compatibility is down the drain.
