To date, we’ve discussed many of the technical aspects of HDR, including brightness and color gamut. Now it’s time to discuss how we get the signal to the viewer so that they can enjoy the step change improvement in the viewing experience. For that, we need delivery standards — and luckily for us, there are a number of them (that’s the great thing about standards — there are so many to choose from!).

INCREASED BIT DEPTH

The first thing to note in all of the delivery standards is that without exception, they use a higher number of bits to carry each component, the minimum being 10-bit. A subtle point here is that we have dramatically increased the gamut of colors that the system has to reproduce, so of course we need more bits in order to be able to numerically represent that wider color space.

EOTF

Many of us are well versed in the idea of gamma. This was the term for the nonlinear curve that was applied to the linear light output of video sources to account for nonlinearities in the delivery and display systems. The idea is simple, and illustrated in Fig. 1: if the display output is nonlinear, then by applying the inverse nonlinearity to the signal, the output to the viewer would then be linear (there is way more to it than this, but this explanation will do for now).

This assumes, of course, that all display devices have the exact same nonlinearity: a generalization to be sure, but it stood the test of time pretty well. You could have put this correction in the TV set itself, but that would have increased the cost of every TV set ever made, so it was more economical to do this at the source. In the HDR world, this correction is called the “Electro Optical Transfer Function,” or EOTF for short. SMPTE has published a standard for this transfer function in ST-2084 (commonly referred to as “Perceptual Quantization” or “PQ” for short), and this has been adopted by three of the delivery standards.

Other transfer functions have also been proposed and adopted — specifically HLG, or “Hybrid Log Gamma.” The idea behind HLG is to offer some level of backwards compatibility with SDR displays (PQ is not backwards compatible with SDR displays). It offers somewhat reduced luminance capabilities, but retains all of the color detail — of course, the SDR set needs to understand Rec.2020 color space in order to reproduce all of those colors.

METADATA

Modern displays have different properties and capabilities, depending on their underlying technologies. It is therefore crucial that we give the display information about the image, so that it can maximize its ability to produce the best possible picture. The minimum metadata set that the display needs are: (1) The RGB values of the black point, (2) The RGB values of the white point, (3) “MaxCLL” — Maximum Content Luminance Level — the value (in nits) of the brightest pixel in the entire clip and (4) “MaxFALL” — Maximum Frame Average Luminance Level — the value (in nits) of any single frame. MaxFALL is generally smaller than MaxCLL, and is a parameter used by the display to prevent damage if a frame exceeds its safe handling capability — OLED pixels, for example, can burn out if run at maximum intAppliensity for a period of time.

There are two versions of this metadata: Static (for the entire clip) and Dynamic (can change scene-by-scene or even frame-by-frame). Static metadata was adopted first, but, of course, is “set and forget.” Dynamic metadata allows the colorist to adjust settings on a scene-by-scene basis and therefore gives greater creative freedom.

We now have the information we need in order to be able to compare the individual formats.

HDR10

This is the base level standard. It uses 10-bit resolution (hence the name), the PQ transfer function and static metadata, and is targeted at broadcast, Blu-ray and VOD applications. It is not compatible with SDR displays, so any media company who wishes their content to be available in both HDR and SDR must process the media through two separate paths (or at least a final conversion stage).

HDR10+

This is HDR 10 with the addition of dynamic metadata. The clip will still contain the static metadata, though, so HDR10 displays that lack the dynamic capabilities can still reproduce the image

HLG

This format is being championed by the BBC and Japan’s NHK. It does not use metadata at all, relying on the HLG transfer function to manage the image. It is particularly targeted at live broadcast, but its main advantage is that it is backwardly compatible with SDR. The HLG transfer function can result in color shifts or desaturation, however.

DOLBY VISION

This format uses 12-bit quantization (which in theory means that a compatible display could go as bright as 10,000 nits!), so it has quite a bit more resolution than the other formats. It uses the PQ transfer function and provides both static and dynamic metadata. This is a proprietary, licensed system however.

Paul Turner is founder of Turner Media Consulting and can be reached at pault@turnerconsulting.tv.

Other articles in this series:

HDR and the Science of Color

The Practical Side of HDR

Candelas and Lumens and Nits – Oh, My!!

How We See: The Human Visual System

HDR: What Is It And Why Do We Need It?

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Unless you’ve had your head in the sand, you can’t help but have heard about HDR (High Dynamic Range) TV. There’s been no end of announcements from manufacturers about how they are (or will be) supporting this phenomenal new technology.

One thing we know to be constant in our industry is that there is always going to be change. We’ve moved from black and white to color, from analog to digital and from SD to HD (now moving on to UHD and up), in the constant desire to bring higher and higher fidelity images to the home. Talk of higher and higher resolutions and frame rates continue to stretch the boundaries of delivery bandwidths—with, some might say, decreasing rates of return as each of these technology advancements comes with a real price tag for the creators and distributors of media.

THE PROOF IS IN THE IMAGE

It’s hard to argue the value of HDR TV, though. Even the least discriminating viewer notices immediately the improvement when they see their first HDR images. Large screen or small, high frame rate or standard, the visible improvement through HDR is immediately apparent and, more importantly, significantly improves customer engagement with the program. The details, though, still appear somewhat veiled—our industry is highly experienced in adopting new technologies and simply expecting those producing, lighting, shooting and distributing the resultant media to adopt it and excel in their various duties without taking the time to really explain the basics. Over the next few postings, I’m going to be examining this topic, with the aim of clearly explaining what it is intended to do, what it is not intended to do, how it works, what the differences are between the various formats are and ultimately (hopefully) answering the question “Why should I care?”

[Is HDR Worth It?]

If we are going to discuss “High Dynamic Range,” we have to also accept the term “Standard Dynamic Range.” As you might expect, these two terms differentiate between different tiers of dynamic range within the media industry. So we should start by understanding the term “Dynamic Range.” With that understanding, we can extend the discussion to cover these tiers of performance.

Dynamic Range is a term engineers and physicists use to define the full range of signal that a piece of technology or system is designed to handle (preferably without distortion—dynamic range doesn’t specifically detail that the system should be linear). For example, in film, the dynamic range is the range between the lowest amount of light that a piece of film can capture (if you go any darker, you can’t reproduce the difference when you present it) to the largest amount of light that the film can tolerate before it saturates (go any brighter, and you can’t detect the increase—the film has absorbed as much light as it can). In audio, the dynamic range goes from silence to the loudest level that a piece of equipment can record, tolerate or present (play back).

SYSTEM DESIGN IS CRUCIAL

It’s important to note that different pieces of equipment at different stages in the media production/delivery process could, in theory, have different dynamic ranges. In that case, the true dynamic range of the system will be limited by the performance of the lowest dynamic range component. System design is therefore crucial—there’s little point in putting an HDR component in the middle of an SDR workflow. For now, just remember that in any discussion of dynamic range or HDR, it’s important to specify if you are talking about a system, or an individual component of that system.

[ITU Announces HDR Standard for TV]

In video, it’s often convenient to discuss dynamic range in terms of contrast ratio. As we’ll discuss in a later post, HDR systems can offer up to a 50x increase in contrast ratio in comparison to an SDR system. But—and here’s the really important point—SDR systems could achieve similar gross contrast ratios if the final display can be made bright enough. This is analogous to turning up the amplifier on your stereo: the sound gets louder, not better. The big difference is that an HDR system is cable of reproducing far more of the intermediate values than an SDR system can—ensuring that all of the nuances of the signal make it through to the viewer’s display. This is the most important point—HDR allows you to reproduce images that convey a much more realistic representation of the original scene. Absolute light level plays an important part in that, to be sure—specular highlights can be breathtaking—but it’s the ability to recreate all of the intermedia values that makes HDR material so compelling.

In the next post, we’ll examine the characteristics of light in the natural world and what it takes to reproduce that on TV.

Paul Turner is founder ofTurner Media Consulting and can be reached atpault@turnerconsulting.tv.