The well-respected video guru tells AVForums how Ultra HD 4K offers a chance for a better overall TV format
We all know that Ultra HD 4K TV delivers four times the resolution of Full HD TV but did you know it has the potential to provide much more than just that?
In fact, Ultra HD 4K also offers a chance to create a TV standard that finally breaks free of its CRT (cathode ray tube) origins and embraces all of which modern display technologies are capable. The current industry standards of D65 for the colour temperature of white and Rec.709 for the colour gamut both stem from a previous generation of televisions. Some aspects such as 8-bit video and 4:2:0 chroma subsampling are directly related to limited bandwidth and storage capacity, whilst mastering video content at 120 cd/m2 is a throwback to the days of CRT.
With the exception of the colour temperature of white, all of these technical aspects of video delivery and presentation could possibly change over the next few years. The ITU (International Telecommunication Union) produced a series of recommendations that laid out a set of technical specifications for a future Ultra High Definition (UHD) format that encompassed both 4K and 8K resolutions. The white paper was called ITU-R BT.2020-1 but generally gets referred to as Rec.2020, although it's worth pointing out that these specifications cover more than just the colour space.
It should also be stressed that these are just recommendations and manufacturers, broadcasters and content providers are under no obligation to use all or indeed any of the new specifications. Whilst Rec.2020 recommends a comprehensive set of parameters for the future of Ultra HD TV, people have often fixated on the colour space aspect and perhaps lost sight of the overall intention of the recommendations themselves. In his presentation, Joe Kane was keen to stress all the benefits of Rec.2020 and the inherent problem of the proposed colour space itself.
However before he discussed Rec.2020 in detail, Joe wanted to quickly explain why it is we find ourselves in the rather confusing position of having both 4K and UHD to describe essentially the same thing. Joe said that 4K has been used in professional post-production for over twenty years and these days 4K is used extensively for archiving movies. Although 65/70mm archives are done at a resolution of 8K and 35mm can often be done at a resolution of 6K,but usually 4K is the preferred choice. However the most common resolution of a DCI (Digital Cinema Initiative) master is still 2K and a lot of special effects are rendered at 2K.
So why don't some TV manufacturers refer to 4K more often? Well, 4K started out with a resolution of 4096 x 2160 pixels, which equates to 4096 lines and hence the name 4K. However HDMI 1.4 had limitations so for the domestic market a resolution of 3840 x 2160 was chosen and called Ultra HD by the ITU and CEA (Consumer Electronic Association). This has now been adopted by the majority of manufacturers, although they still use 4K as well, which can prove rather confusing for the poor consumer. Joe himself thinks it should actually be referred to as 2160p instead, regardless of whether it's 4096 or 3840 to avoid confusion.
This resulted in the ITU creating Rec.2020 which was a designed to deliver a better experience for the consumer. Among the recommendations are two resolutions - 3840 x 2160 and - 7680 x 4320 both with 16:9 aspect ratios and square pixels. It also specifies frame rates ranging from 24p to 120p and bit-depths of 10 or 12 bits per a sample. There is also the option for 4:4:4, 4:2:2 and 4:2:0 chroma subsampling. Finally there is the Rec.2020 colour space which represents 75.8% of the visible colour spectrum, as opposed to Digital Cinema (DCI/P3) which is 53.6% and REc.709 which is 35.9%.
It’s the last point on that list that has caused the most controversy because the coordinates for red, green and blue actually hit the border of the CIE chart. This means there is a single wave length for each of the three primary colours. However, as Joe points out, broad spectrum colours can deliver better fidelity but with Rec.2020 there is almost no spectrum, just three specific points. Joe feels that the industry should use the overall specifications of Rec.2020 as a container within which other standards can be adopted.The introduction of Ultra HD 4K provides a golden opportunity to take television to the next level.This would mean that Ultra HD 4K could be adopted in gradual stages, which is what is happening to a certain extent. Early 4K used a resolution of 3840 x 2160 pixels at frame rates up to 24/25/30p, Rec.709 colour space, 8-bit video and 4:2:0 chroma subsampling. The new HDMI 2.0 supports a resolution of 3840 x 2160 and the Rec.2020 colour space. It can transmit 12-bit per sample RGB at a frame rate of 24/25/30p or it can transmit 12-bits per sample 4:2:2/4:2:0 YCbCr at a frame rate of 50/60p.
There is of course further to go and HEVC (High Efficiency Video Coding) can support 3840 x 2160 at up to 10-bit video and the Rec.2020 colour space. The BDA (Blu-ray Disc Association) announced that the 4K Blu-ray specification will support 3840 x 2160 at up to 60p, using HEVC, with support for Rec. 2020, high dynamic range and 10-bit colour depth. However whether the Rec.2020 colour space will ever become the standard remains to be seen because it is such a wide colour space. If a wider colour space was chosen, it might well be DCI/P3 which would bring consumer video into line with the standard used in commercial cinemas.
Joe is keen for the discussion to move away from just resolution or colour space and also look at the other improvements that Ultra HD 4K can bring. For example one area that Dolby has been pushing and a number of TV manufacturers are supporting is HDR (High Dynamic Range). This relates to TVs that have deeper blacks and a much brighter image (over 1000cd/m2) which results in a much wider dynamic range that better replicates the real world. The problem with this approach at the moment is that it requires a lot of energy and a lot of cooling.
Other areas that could make a difference are higher bit colour depth and higher frame rates, although the latter has proved controversial as far as feature films are concerned. What is eventually agreed upon will require the support of broadcasters, studios and manufacturers but as is often the case the cheapest or easiest approach often wins through. However even small changes can make a big difference, such as Rec.709 with 10-bit colour depth and 4:2:2 chroma subsampling.
The introduction of Ultra HD 4K is a golden opportunity to take television to the next level and create a new standard for the 21st century. Whether we will get such a standard - with a resolution of 3840 x 2160, a Rec.2020 colour space, a frame rate up to 120p, 12-bit colour depth and 4:2:2 - remains to be seen but we are on the edge of a brave new world. The prospect of a video experience that is the equal, if not better, than that in the cinema is now tantalisingly within our grasp, but may take several years to get here.
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