HDR: Which format is which?

UHD or ‘Ultra High Definition’ television promises many things, among them high dynamic range (HDR), a wider colour gamut (ie getting closer to the huge range of colours that most people can see), higher frame rates (for super-smooth action, particularly in sport) and higher resolution (4K). Between them, they’re shaking up the TV technology landscape.

In HDR, there are three main standards you’ll have probably heard of. For delivery, the BBC and NHK have developed their Hybrid Log-Gamma system, HLG, while Dolby favour their own Dolby Vision (also known as Dolby PQ). Then, for more domestic delivery, there is also HDR 10.

The principle of using an alternate gamma so that you concentrate the bit-depth where you want the extra range is well established; our eyes do not perceive light the way cameras do. To recap, with a camera, when twice the number of photons hit the sensor, it receives twice the signal (a linear relationship). We, on the other hand, perceive twice the light as being only a fraction brighter — and this is increasingly true at higher light intensities (a nonlinear relationship).

Since gamma encoding redistributes tonal levels closer to how our eyes perceive them, fewer bits are needed to describe a given tonal range. Otherwise, an excess of bits would be devoted to describing the brighter tones (where the camera is relatively more sensitive), and a shortage of bits would be left to describe the darker tones (where the camera is relatively less sensitive). This means gamma encoded images store greyscale more efficiently.

ITU-R BT.2100

It does seem like all of the manufacturers will coalesce around BT.2100, which defines (amongst other things) how you handle the specular highlights: those very bright parts of the picture which really add to the look of pictures.

Specular highlights are typically defined to be >500 Cdm-2, which is much brighter than broadcast white! The idea is that in 10-bit HDR, the tenth bit of dynamic range (all values above 512) represents the highlights, and the other nine bits are akin to the usual video dynamic range.

Delivery formats

There are three delivery formats you need to consider.

HLG

HLG was developed by the BBC and their Japanese counterpart NHK. It is a scene-referred system, just like conventional television, and has been designed with the specific goal of making the transition to HDR easy on broadcasters and production crews – hence its compatibility with SDR, which means that broadcasters can continue to use their existing 10-bit SDI production installations (as with all video, levels are considered dimensionless).

HLG uses relative brightness values to dictate how an image is displayed – the display uses its knowledge of its own capabilities to interpret the relative, scene-referred information. This means that the image can be displayed on monitors with very different brightness capabilities without any impact on the artistic effect of the scene. Because it uses relative values, HLG does not need to carry metadata, and can be used with displays of differing brightness in a wide range of viewing environments.

HLG is supported in Rec. 2100 with a nominal peak luminance of 1000 Cdm-2 (though the BBC have said this is an artificial cap imposed by the monitors they use, and the real figure is more like 4000). It is also supported in HEVC.

Dolby Vision or DolbyPQ (Perceptual Quantiser)

Dolby Vision is the wider set of products that cover both digital cinema and video – Dolby PQ is the element that we’re concerned with. Unlike HLG, DolbyPQ is a display-referred system that uses absolute dimensioned values for the light captured.

The metadata that travels in the SDi payload defines how video levels equate to light levels, and how they should be reproduced at the DolbyPQ display end. The display then reports back to the playback device via EDID to convey its maximum light output.

DolbyPQ supports a maximum brightness of 10,000 Cdm-2.

HDR 10
HDR 10 is another format you’ll have heard of. It’s an open source version of PQ, developed by device manufacturers, but it has a lower video quality, mastered in 10-bit, and only up to 1000 nits cd/m(compared to Dolby’s potential 10,000). It does use metadata, but a far simpler form than Dolby, specifying one luminance level for the entire programme rather than frame by frame.
It’s not backwards compatible either (so can’t be viewed on SDR displays) but because its far more affordable, it’s a popular standard for domestic delivery, particularly on systems where it’s easy to host an SDR version to accompany the HDR (so UHD Blu-rays, set-top boxes and streaming services like Amazon). Sony and Microsoft have also gone down this route for PlayStation 4 and Xbox One S. So while it is big in home cinema, it ultimately has little relevance for the professional production environment.
Dynamic range

All of these formats have greater dynamic range than the human eye (about 14 stops) and SDR video (about six stops) and 10-bit pro SDR (about 10). When HLG footage is displayed on a 1,000 Cdm-2 display with a bit depth of 10 bits per sample, it has a dynamic range of 200,000:1 or 17.6 stops.

HLG also increases the dynamic range by not including the linear part of the conventional gamma curve used by Rec. 601 and Rec. 709. The linear part of the conventional gamma curve was used to limit camera noise in low light video, but is no longer needed with HDR cameras.

DolbyPQ has an even broader dynamic range of around 18 stops, but this is necessary when using display referral as you need to be able to accommodate different types of display and viewing conditions.

Which is right for you?

In a controlled environment like a movie theatre, Dolby Vision makes a lot of sense – it gives you very precise control, can take advantage of more advanced displays, and has a high degree of futureproofing thanks to that whopping 10,000 Cdm-2 upper limit. If you’re in a feature-focused facility with an existing Dolby workflow, it makes a lot of sense to roll out this system to other parts of your pipeline.

However, if your bread and butter jobs come from the BBC and you’re aware that a lot of your viewers will, ultimately, be watching your content in their living room, in the office at lunch or on their iPhone, aligning your setup to their standard seems sensible, as you’ll be in line with a major customer, and the adaptive nature of HLG means it’s well suited to the variety of viewing environments you need to cater for if you’re producing online or television content.

How can we help?

Well, we can make recommendations for acquisition, post-production and delivery of HDR content. We carry cameras, monitors and video interfaces appropriate to HDR workflows, and can often offer demo kit to test in customers’ workflows. To find out more, get in touch with the team on the details below.

If you want to know more, give us a call on 03332 409 306 or email broadcast@Jigsaw24.com. For all the latest news, follow @WeAreJigsaw24 on Twitter, or ‘Like’ us on Facebook.

Liz
Liz
Call us: 03332 409 306

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