06 Aug 2015

HDR: a quick primer on a very significant technology

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The Biggest Advance In Television for a Decade The Biggest Advance In Television for a Decade Dolby Vision


Just in case you're not fully up to speed with HDR and its importance, a quick rewind to just after IBC2014 when Dolby first introduced Dolby Vision and a look at some of the underlying principles.

Attached to a wall in a sterile exhibition hall in Amsterdam, we’ve just seen the future of video. That’s a big statement to make, but we genuinely believe it. It’s bigger than 4K and 8K, and very much bigger than 3D. In a way, you could argue that it’s a greater change than the one from film to video.

It’s Dolby Vision, one of the very first examples of High Dynamic Range TV.

Now, “High Dynamic Range TV” may sounds like a rather dry expression for a revolution, but stay with me. It’s a genuinely dramatic increase in the quality of moving images.

The human eye has a remarkably ability to deal with different levels of light. We can see perfectly happily in bright, direct sunlight and in moonlight. Within a scene, we can make out detail in dark areas and at the same time be startled at a glint of sunlight coming off a polished chrome detail on a 1950s Chevy. We can enjoy seeing the light force its way through a back lit leaf or flower, and be entertained by a firework display: intensely bright explosions of light against a velvet-black sky.

Video displays have struggled to keep up with this, so we pre-package our video into “standards” like Rec 709, which is for HD TV transmission and display systems. It does a very good job of making television look rich and vibrant, but that’s only (quite literally) part of the picture, and it’s all relative.

Look at it this way: video is normally delivered to the home (via terrestrial broadcast, satellite, cable, DVD or internet download) in 8 bit format. The number of bits is normally equated to the number of colours that can be reproduced, but, ultimately, it also determines the dynamic range. You can actually reproduce virtually any dynamic range with as many or as few bits as you like, but if you use too few, and you have a large dynamic range, you’ll get unpleasant contours, and the picture will degrade quickly if you apply too many stages of processing to it.

We don't have linear eyes



The problem is worse than it might actually seem because although sensors tend to be roughly linear, the way we perceive light certainly isn’t. Here’s a clip from Phil Rhodes' recent article on Log Cinema modes to explain:

The human eye responds to light more or less linearly as the real light intensity doubles. This is familiar to photographers as an exposure value, where increasing an exposure value by one always looks like the same amount of additional brightness, even though every increase in visible brightness actually represents a doubling of light intensity. If we view a scene with a single 100W light aiming at it, we could call light 100. Let's assume the scene is comfortably exposed in these circumstances, so we can call it brightness 1. To double the amount of light, we might add another 100W light, so light is 200 and brightness is 2, which is fairly intuitive. But to increase the apparent brightness by the same amount again, so that brightness is 3, we need to double the amount of light again, so that light is 400. This is of course not new; when the system of f stops and exposure value was developed, we knew that linear apparent brightness increase requires successive doubling of the amount of light.

Digital cameras are approaching the point where they can emulate the sensitivity and the dynamic range of the human eye, with as many as 14 stops available between black and white. Since each stop represents a doubling of light, that’s a very wide range indeed.

But none of this solves the problem of displays that can’t handle such a range of brightness. To be able to display this huge dynamic range calls for new technology. And there’s good news on this, as you’ll read later in the article.

But first I want to clear up one possible point of confusion.

This is not just "HDR" photography

HDR photography has been around for some time. But this is different to what we’re talking about here. In HDR photography, multiple shots at different exposures are combined to create an image that shows detail in the whole brightness range, from shimmering highlights to gloomy shadows. But here’s the thing - this enormous range is compressed to fit into the capabilities of either a display or a printed page. The results can certainly look spectacular, but they often look unreal - like a computer game. That’s because we’re not seeing a real dynamic range but merely the details that are revealed in a very wide range of light, reproduced on a display that has only a “normal” dynamic range.

To see what the real world (ie real objects in realistic levels of light seen by real eyes) looks like, we need displays that can match the dynamic range of our eyes and, in latter times, our cameras.

And that’s a problem, because, lovely though flat screens are, they aren’t even as good, in some respects, as the Cathode Ray Tubes that preceded them.

But they have got much better over time. Properly set up, and with good, contrasty material, the results can appear very good indeed. And, yes, 4K has made them even better, even if it’s done little to improve dynamic range.

To match real-world images, displays need to have deeper blacks, and much, much brighter whites. The improvement needed is actually quite brutal: they need to be 1000 times better.

Put like that, it might seem like an unattainable goal, but remarkably, we’re just seeing the first examples of genuine HDR displays, such as the one from Dolby, called Dolby Vision, that we saw in Amsterdam the other day.

Dolby’s a clever company. Rather than relying on manufacturing and selling their products, they’ve preferred to spend money on research and development, and to gain their income through licensing.

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David Shapton

David is the Editor In Chief of RedShark Publications. He's been a professional columnist and author since 1998, when he started writing for the European Music Technology magazine Sound on Sound. David has worked with professional digital audio and video for the last 25 years.

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