HDR and WCG: The new vision alphabet
Ultra High Definition Blu-ray is doing more than bringing even higher definition to the home theatre, says Stephen Dawson. It’s adding brighter whites, darker blacks, subtler shades and new colours
UHD, HDR and WCG* go together, more or less. This new set of acronyms is being introduced into the home mostly by the latest high quality movie source: Ultra High Definition (UHD) Blu-ray.
The UHD part has been well rehearsed over the past few years, thanks to us long-since having had UHD TVs available. That is just the raw picture resolution: 3,840 pixels across, 2,160 pixels vertically.
The interesting part is the other two sets of letters – HDR and WCG – which address long standing defects in home entertainment
What are they?
HDR stands for High Dynamic Range. WCG stands for Wide Colour Gamut.
High Dynamic Range refers to the brightness scale of the video signal: how bright it can go, how dark it can go and how finely graduated it can be between the two extremes.
Wide Colour Gamut concerns the range of colours that the video signal can deliver, and being ‘wide’, it clearly means a greater range of colours than presently delivered.
Read on for further details, but before doing that we must make one thing clear: HDR and WCG refer to the video signal. There are precise specifications. An HDR TV, for example, must be able to accept an HDR signal. That’s not negotiable.
But how well it displays it is another matter. Can the TV physically go brighter than a regular TV? Or darker? Are there a sufficient number of brightness levels physically able to be delivered by the screen to allow those finer graduations? Or, in the case of WCG, does the design of the display permit the wider range of colours implied by WCG to actually be produced?
These aspects of real world performance will continue to need to be assessed on a product-by-product basis.
To take an example, LG’s latest OLED TVs boast of a wide colour range of 130% of the Rec. 709 standard. That is impressive indeed, and I am of the view that these are the best consumer TVs currently available. But that is still only 62% of the colour space supported by UHD Blu-ray.
No doubt there will continue to be improvements in display handling of HDR and WCG signals as the years ago by.
Nonetheless, the support of HDR and WCG signals is important, because no longer will home entertainment quality be limited by the signal standards themselves.
Blacks, whites and greys
So, HDR. What’s all that about? First let’s look at the current situation. Blu-ray and DVD and digital TV all use similar standards to define the signal. It’s called component video and represented as YCbCr. Y, Cb and Cr are those components. Y is the luminance signal – it’s black and white. Cb and Cr are the blue and red, respectively, colour difference signals. By adding and subtracting these three signals together in well-defined proportions a display produces the RGB signal that it needs for its red, green and blue subpixels.
Why this complexity? In part because YCbCr was compatible with black and white TV. In larger part because this way colour can be delivered at lower resolution (the sharpness of colour perception in the human eye is much lower than luminance). Combined YCbCr requires only around half as much data as RGB, so this makes for a cheap data saving.
In DVD, DTV and Blu-ray, the Y (luminance) signal was defined in 8 digital bits. That means it had a maximum of 256 levels. But some of those levels were reserved for other purposes, and that brought it down to 220 levels. If you look at a ‘continuous’ grey scale ramp from full black to full white, you won’t see a smooth graduation. You’ll see steps.
Now, in a sense a new TV technology could deliver brighter whites and blacker blacks while still using the same old signals. But that would make the steps even more distinct since the same number of steps would be used to cover the increased range.
HDR bumps up the number of steps. The UHD Blu-ray standard is HDR10, where the 10 stands for 10 bits. This brings us up to 1024 levels of brightness. In most circumstances that should allow completely smooth brightness ramping from full black to full white.
HDR10 is the norm and it is a requirement of the Blu-ray format that UHD Blu-ray players support it. But the specification optionally allows the incorporation of the even higher specification Dolby Vision. This uses 12 bits for over 4000 brightness levels, necessary since Dolby envisages displays capable of brightness levels more than ten times those available in present day TVs.
The take home from all this? Most of the talk about HDR is about darker blacks and brighter whites, but while those boundaries are being pushed, HDR (and even more so Dolby Vision) provides for smooth graduations of brightness across the full scale. That is particularly important for allowing the display of detail in dark scenes.
Many, many more colours
Along with UHD and HDR, we’re also getting WCG – wide colour gamut.
As it happens, no commonly used video signal standard is able to represent all the colours the human eye is capable of perceiving in the real world. This full capability is represented graphically by a colour diagram showing the CIE 1931 colour space, a shark-fin-shape, tilted slightly to the left.
The current Rec. 709 colour space specified for HDTV and Blu-ray encompasses just 36% of this space. Clearly there’s room for improvement there. One of the standards for computer monitors – Adobe RGB – manages 52%. But with UHD Blu-ray comes the Rec. 2020 colour space which manages nearly 76% of CIE 1931, stretching the boundaries somewhat in the reds and blues, and a long way in the greens.
As with HDR, stretching the boundaries is all well and good, but with no other change means that the steps between the extremes will be more obvious to the eye since they will also be stretched further apart.
Blu-ray, DVD and DTV all use 8 bits to define colour (that is, in the Cr and Cb parts of the component signal. So we come back to those 256 levels (although, again, the extremes are not used, so there are 225 actual levels). Even with the relatively modest colour gamut of regular Blu-ray this limited number of levels often leaves visible banding. There is simply insufficient numerical resolution to allow smoother colour graduations in all circumstances, especially in the darker shades of primary colours.
Solution? More bits, as always. Roughly speaking, one extra bit is required simply to keep the same numerical resolution in the colour, given the expanded boundaries, as that provided by Rec. 709. However, the Rec. 2020 colour space provided by UHD Blu-ray gives us ten bits so again we’ve got a touch over a thousand levels. Remember, those are per colour. There are three colours so something like a billion different colours are supported by the signal, rather than the eleven million-odd covered by standard Blu-ray.
With consumer technology until now, providing improving the range capabilities of a TV with regard to colour and brightness has been to a degree pointless, given that the signal they produce their pictures from have been limited. For years TVs have been boasting of being able to show a billion colours. But the only way they could do that in practice would be by guessing at new colours based on the limited range in the signals.
But now with HDR and WCG, there is a real point to improving display technologies. That’s something that I for one am looking forward to.
* UHD Blu-ray also provides support for HFR – High Frame Rate video. Rather than just 24 frames per second, there is support for 30, 50 and 60 frames per second. Get ready for a super smooth rendition of The Hobbit.