Wednesday, 28 September 2016

How to deliver colour accurate work for 4K television

How to deliver colour accurate work for 4K television

Colour expert Martin Parsons discusses the latest colour standards and how to achieve them.

In the world of sitting at home watching television, 4K is a wave that’s still to break. High Definition was designed to provide as much resolution as viewers need. In blind tests run by EBU back in 2013, audiences shown HD and 4K resolution TVs did not see enough difference to justify upgrading.

So it became clear to television manufacturers that the next generation of sets would have to offer more than just increased resolution, and they’ve driven the industry to improve content quality to make the new consumer technology more appealing.

There is a set of standards for image and sound quality in content for 4K TV. And the key differentiator between the HD and 4K television viewing experience is in colour, high dynamic range and higher frame rates as expert Martin Parsons explains.

Understanding colour standards

As humans we can see all the different wavelengths of visible light – in fact the full colour space discernible by us. But in a monitor or projector you’ve got only three primary colours, and the range of colours it can show depends on how pure, or saturated, those primary colours are.

This is why international standards are set by the International Telecommunication Union (ITU), which is part of the United Nations. Their remit is to bring television to everyone, regardless of their location or means. All your reference monitors should comply with their standards. So we’re already familiar with Rec. 709 (officially called ITU-R BT.709) for HD video. We line up all our monitors with that colour gamut and white point to ensure that they match HD television outputs.

With film, as opposed to television, we used to rely on what Fuji or Kodak or Agfa produced in their camera negatives, and from that the derived film print that would be projected in cinemas. When we switched from film to digital projection a group of people – mainly directors, DOPs and the studios – set up the DCI (Digital Cinema Initiatives) to establish a standard for the quality of digital movie projection. They also standardised the file formats and security to deliver to the cinema projectors. 

The DCI also wanted to make sure that cinema projectors were replicating as close as possible the colour gamut of film, so they set a new standard colour gamut, which we know as P3. Now all cinema projectors need to be able to show that colour gamut.

(Note that the P3 colour gamut is bigger than Rec. 709, but still doesn’t cover anywhere near the full range of colours that the average person can see.)

The standards for 4K TV

And now we come to Ultra High Definition (UHD) or ‘4K’ TV. The standards for this new UHD format include immersive audio (up to 22.2 from 5.1), higher frame rates (up to 100fps from 25fps in the UK and Europe), WCG (wide colour gamut) and HDR (high dynamic range). The ITU has just recently published ITU-R BT.2100, the standard for HDR. The new colour gamut for 4K TV is wider than both Rec. 709 and P3, and is called Rec. 2020 (a.k.a. ITU-R BT.2020). These are the standards that drive the production of better content for the next generation of televisions.

Unfortunately, no TV or monitor can fully achieve the new Rec. 2020 colour gamut. At the moment they can reach between 80% and 90% (or so it’s claimed) of Rec. 2020, depending on the measurement methodology used. The only device that can fully display Rec.2020 is a laser-illuminated digital movie projector, that can produce the purity of red, green and blue to show such a wide colour gamut. That device costs about $1million. But the aim of the new Rec.2020 colour gamut is for future technology to be able to attain this huge colour gamut.

Broadcasters are still working on meeting the new standards. Netflix and Amazon already deliver content in HDR, and Sky has just moved to UHD resolution. The BBC have also been doing research and in fact their HDR development has been included in the ITU-R BT.2100 HDR standard.

One of the important challenges broadcasters face is to deliver HDR and Wide Colour Gamut to the new UHD televisions whilst simultaneously maintaining backwards compatibility to legacy televisions that don’t support this technology. This is a very similar situation faced by broadcasters back in the late 1960’s with the introduction of colour TV transmissions which still had to be compatible with the legacy black and white TVs that the majority of the population still owned.

Ensure your content meets the standards

The crucial factors in colour management are calibration and consistency.

The only way you get to see your project is on your monitors, so it’s important you’re getting an accurate representation. If you don’t, you won’t know what you’re working with. It’s frustrating to calibrate everything perfectly then send your work to the client – only to get it back with colour gripes because they haven’t calibrated their monitor. It’s much worse to send out work that doesn’t look right because your monitor hasn’t been calibrated correctly.

For accurate colour work, invest in a good quality reference monitor. Look for a screen that is accurate across the whole of the panel. You need constant luminance and chromaticity, because the edges of the panel can be different from the centre and you can’t calibrate that out. EIZO monitors do a good job here by profiling the panel and applying correction to ensure 100% uniformity for luminance and chromaticity.

Normally you’d put a colour probe in the middle of the screen, but some of EIZO’s monitors have an inbuilt probe that swings down from the top and measures off centre. If the screen has been profiled and corrected, then that area will be representative of the whole screen.

In calibrating, you want to make sure that the colour primaries are accurate for the colour gamut you wish to portray (there are also print standards, including sRGB, Adobe RGB and ProPhoto RGB). You need to check that the greyscale tracking is accurate as well. Martin covers both of these in his calibration tutorial for Creative Rooms:

If you’re delivering projects with a high dynamic range then you’ll need a monitor that meets the ITU-R BT.2100 standard for HDR as well as being capable of a wide colour gamut. This means it has to support the Hybrid Log-Gamma and PQ (Perceptual Quantisation) HDR standards.

The importance of colour consistency between monitors

Getting the same image between monitors is vital for effective communication. Calibration across all monitors is essential from the beginning of the project. You want to be able to send the finishing department what the project is supposed to look like, since you’ve had directors and VFX supervisors look at the content and sign off on it.

If you can’t do the calibration internally, then use a service like Martin’s to come and do it for you. Image Eyes Ltd. supplies this service to 68 companies around London.

If you’re doing it yourself then carefully consider the accuracy of the probe you are using. Martin discusses this in the video above as well. He’s purchased one of only two colorimeters that can accurately measure HDR and it cost over £5000, so it can also be worth investing in the person who knows what it’s all about and can make sure that you’re working colour-accurately.

The new standards for 4K TV have set clear targets for content production, and helped companies and industries to communicate. But ultimately it comes down to how you perceive things too. Just think of last year’s internet phenomenon The Dress

Martin is running a public, open course in London this October about HDR and wide colour gamut.

To talk about investing in your colour pipeline, give us a call.

About Martin Parsons

Martin has worked as an engineer in film, broadcast and post production for over 30 years and for the last seven has run Image Eyes Ltd., providing solutions for new technology implementation as well as training. He also teaches at the NFTS and has created many courses for broadcasters and post-production facilities across a range of subjects.

www.image-eyes.com/training