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What is HDR TV? All you need to know about High Dynamic Range, and why you should care

24 Feb 2020

HDR can make a huge difference to your favourite movies and TV programmes. Here’s why it’s so important

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What is HDR TV? You may already be familiar with the term in the context of cameras and smartphones, where clever processing combines multiple images to give the effect of more detail in both the darkest and brightest parts of the image. In the world of TVs, HD (High Dynamic Range) has a similar purpose. The various HDR standards (we’ll explain each in detail later) allow TVs to produce images which are more realistic and lifelike, with realistically dark shadows, squint-inducingly bright sunshine and every hue from the subtlest pastel shades to the most intense technicolour riot.

If you’re tempted to dismiss HDR as yet another gimmick like 3D or curved screens, then don’t. HDR is a technology that can transform your TV experience for the better, and especially so if you pick the right TV with the right kind of HDR support.

READ NEXT: The best 4K TVs you can buy

What makes an HDR TV different to a standard TV?

HDR TVs or projectors need to have a far greater contrast ratio (the ratio between the darkest black and brightest whites) and be able to reproduce a much wider range of colours.

HDR operates within the Rec.2020 colour space, and this means that its potential colour gamut (the palette of colours that it can produce) is actually 37% larger than the DCI-P3 colour space used by digital cinema projectors, and 72% larger than the Rec.709 colour space used by SDR TVs. As it happens, most current HDR content tends to restrict itself to colours within the smaller DCI-P3 gamut, but there are titles out there that take advantage of the wider colour range of Rec.2020. The more colours an HDR TV can reproduce, the better.

While you’ll hear people regularly banging on about 4K, HDR can make such a difference to picture quality that it’s arguably more important than those several million extra pixels. In fact, you may not even notice a difference between a good Full HD TV and a 4K TV. For instance, if you buy a 55in 4K TV, you’ll need to be no more than around 2 metres away to perceive the improvement, even if you do have perfect 20/20 vision. No matter where you sit, however, it’s impossible to ignore the improvements brought about by HDR. Providing, that is, that your TV is good enough to make the most of it.

Do all HDR TVs look better than 4K SDR or FHD TVs?

Not necessarily. Owning an HDR-capable TV does not guarantee impeccable image quality. TV manufacturers all put their own spin on the various display technologies, and individual models have different specifications and content playback capabilities. Budget 4K HDR TVs won’t be able to go as bright as more expensive sets, for instance, nor will they be able to reproduce the vast array of colours found in HDR content.

That’s fairly understandable, though. Not even the most expensive TVs on the market are capable of reproducing every single hue in the vast HDR palette of colours defined by the Rec. 2020 standard – in fact, you’ll see most touting their colour-reproducing prowess in terms of the percentage of colours they can reproduce from the smaller DCI-P3 gamut. When that DCI-P3 coverage gets over the 90% mark, a TV has one of the prerequisites for getting a shiny UHD Premium badge (more on which later).

Moreover, HDR content is routinely created in Hollywood’s mastering suites with a maximum brightness of 4,000 nits in mind – vastly higher than any consumer TV currently available. With the peak brightness of even the finest consumer sets topping out around the 2,000 nits mark – and significantly below 1,000 nits for OLED TVs – it’s no surprise to find that the budget LCD sets, which tend to peak at around 400 nits, simply can’t eke every drop of HDR goodness from the format.

READ NEXT: Netflix vs Now TV vs Amazon Prime Video

Do I need to spend a fortune to watch HDR content?

Thankfully, no. In truth, it’s not until you start looking at TVs at well over £1,000 that you’ll see what HDR can really do, but even some of the budget sets we’ve reviewed do a very good job thanks to the newer HDR formats such as Dolby Vision.

The reason for this is that Dolby Vision and other recent formats bring a feature called dynamic tone-mapping to the table. This is a particular boon for TVs that can’t go particularly bright, as it allows them to tweak the brightness levels and adjust the incoming HDR signal to best suit their capabilities. You might not get the brightest possible highlights or most vibrant colours that HDR content is capable of, but in our experience it means you should get some benefit, even on relatively humble HDR TVs.

Where can I watch HDR content?

HDR content is much easier to find than it used to be. There’s an ever-increasing number of 4K Blu-ray discs on the shelves, and streaming services such as Netflix and Amazon Prime Video have a decent library of HDR content in one format or another, although it can depend on how much you pay.

Netflix supports HDR10 and Dolby Vision, but only for those on a premium plan, while Amazon gives you HDR10, Dolby Vision and HDR10+ content playback on its standard payment plan. The HDR formats vary from title to title, and it goes without saying that you’ll need a TV that’s compatible with those formats in order to get the benefit.

Other streaming services are taking time to catch up, however. The BBC has dipped its toes into HDR’s shimmering waters with Seven Worlds, One Planet and Dracula both streaming in the HLG HDR format, but that’s your lot for the moment. Sky is currently working to introduce HDR playback through its Sky Q platform, but its Now TV service is certain to be much slower on the uptake – it only added 1080p streaming in November 2019.

What types of HDR are there?

HDR content is delivered in several different formats. HDR10 is the most common standard right now, but when buying a TV or browsing for HDR content to watch you’ll also encounter formats such as HDR10+, HLG (Hybrid Log-Gamma) and Dolby Vision. All of them essentially aim to deliver the same high-quality HDR images, but there are key differences worth knowing about, such as the support for dynamic tone-mapping that we mentioned earlier. Below you’ll find our quick guide to the key differences between these HDR formats.

HDR10 vs HDR10+ vs Dolby Vision vs HLG: What’s the difference?

HDR formats can be split into categories based on the information, or metadata, they provide to the TV. It’s this metadata that contains key information about the particular movie or programme, such as the maximum brightness level and the range of colours, or gamut, that it requires. This allows the TV to tailor its settings to best suit that HDR content.

The various HDR formats employ different types of metadata: static metadata, dynamic metadata or, in the case of HLG, none at all.

READ NEXT: Dolby Vision vs HDR10

What does HDR metadata actually do?

Static metadata, used by HDR10, is so-called because it only provides basic information that is applied for the entire duration of the movie. It doesn’t provide scene by scene or frame by frame information to the TV – it simply records the highest brightness and widest colour gamut required by any scene, so the content’s dynamic range and colour gamut may end up being much more compressed than when it was originally mastered. Therefore the smaller the colour volume (a combination of colour gamut and peak brightness) of the TV, the less impressive its HDR10 playback will be.

Dynamic metadata, on the other hand, provides peak brightness and colour gamut information on a scene-by-scene and frame-by-frame basis. This allows the TV to intelligently adjust its colour volume in a process called dynamic tone-mapping, in order to achieve the best possible results for every scene. Tone-mapping is achieved using a mathematical translation called a PQ EOTF curve, which is one aspect of the PQ EOTF system that was developed by Dolby; it translates the content’s colour volume to that of the TV’s in order get the best possible result out of the hardware. Static metadata also relies on tone mapping, though obviously it’s not dynamic.

Currently, dynamic tone-mapping is utilised by the Dolby Vision and HDR 10+ standards. In our experience, it can have a considerable impact on the quality of the HDR presentation when compared to HDR10; and particularly so on lower-end TVs that lack the peak brightness to really go to town on the HDR spectacle. In short, if a TV manufacturer implements the dynamic tone-mapping features effectively, then it’s possible to squeeze more performance out of any given TV, regardless of how much it costs or how bright it is.

And what about HLG HDR?

Hybrid-Log Gamma, HLG for short, is an alternative HDR system to the PQ EOTF system used by the other aforementioned HDR formats. It was created for television broadcast purposes and was developed jointly by the BBC and Japan’s NHK, with Planet Earth II serving as its first trial in the UK. HLG’s biggest benefit over metadata-based formats is that it can send HDR and SDR images using just one signal. This is, of course, a gross oversimplification of what HLG is; the main thing you need to know is that in the future it’ll be the HDR format of choice for live TV broadcasts, and most flagship TVs being made today are now HLG compatible.

Here’s a brief overview of the key features of each of the main HDR formats:

HDR10

  • Open-source
  • Uses static metadata
  • Supported by most HDR-capable equipment and streaming services
  • Up to 10-bit colour depth support (1 billion colours)
  • Up to 1,000nits max brightness support

HDR10+

  • Open-source
  • Uses dynamic metadata
  • The extension of HDR10
  • Hardware and streaming support growing fast
  • Up to 10-bit colour depth support (1 billion colours)
  • Up to 4,000 nits max brightness support

Dolby Vision

  • Licensed by Dolby
  • Uses dynamic metadata
  • Hardware and streaming support growing fast
  • Up to 12-bit colour depth support (68 billion colours)
  • Up to 10,000 nits max brightness support

HLG

  • Created by the BBC and Japan’s NHK
  • Not metadata-based
  • Limited content and hardware support
  • Up to 10-bit colour depth support (1 billion colours)
  • Up to 1,000nits (nominal)

As if that wasn't enough to get your head around already, we may start to see even more HDR technologies landing on TVs in the coming years. Advanced HDR, developed by Technicolour, is one such format.

Comprised of three Single Layer strands, it's a multi-layered format that has already found some use in the content creation sphere, although there's no commercially available Advanced HDR content so far. In a bid to future-proof its range, LG has added support for Advanced HDR to some of its high-end TVs. It's currently the only brand to have the feature and it's unclear how long owners of these TVs will have to wait before they can make use of it.

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HDR for TVs: Are some TVs better than others for HDR?

Even in the age of 4K, not all televisions are born equal. If your TV can’t meet the standards laid out by HDR formats then you won’t be able to view content in its true glory. That’s why we have certification schemes, such as UHD Premium, a specification laid out by the UHD Alliance – a group whose members include LG, Samsung, Netflix and Dolby.

To earn the UHD Premium badge a TV must meet or exceed a 4K resolution (3,840 x 2,160), support a 10-bit colour depth and offer at least HDR10 or Dolby Vision. It must also produce peak brightness levels of 1,000 nits and a maximum black luminance of 0.05 (or 540nit peak brightness with a maximum 0.0005nits black luminance) and be able to reproduce 90% of the DCI-P3 colour gamut. Although they operate within Rec. 2020 range, there’s no specified minimum for a UHD TV’s Rec. 2020 coverage. However, since they are all 90% DCI-P3 or above, they all hit at least 60% of the Rec. 2020 spectrum.

Do you really need an HDR TV with the UHD Premium certification?

In our experience, even budget TVs capable of reproducing a peak brightness of around 400 nits (well below the UHD Premium standard) can produce watchable HDR image – and especially so with the help of dynamic tone-mapping – but there’s no question that it takes a lot more to really do HDR justice. It’s worth emphasising, however, that peak brightness isn’t necessarily the be-all and end-all of HDR performance.

OLED (Organic Light-Emitting Diode) TVs manage to provide a stunning HDR spectacle with a modest peak brightness of around 600-700 nits. This is possible because every pixel in an OLED panel is effectively its own miniature backlight capable of turning on and off at will, and this allows OLED panels to provide a practically infinite contrast ratio with almost zero black luminance – which is a perfect scenario for HDR.

Traditional LCD-based televisions, on the other hand, have a tougher time with HDR. They require an LED backlight to illuminate the LCD panel and this means that their contrast ratio is limited by the fact that black isn’t actually black at all – at best it’s an extremely dark grey. One remedy is to use local dimming LED backlighting, a feature that partners hundreds of backlighting ‘zones’ that can be individually cranked up or turned down to effectively manage the contrast and dynamic range expected by HDR content. The downside is that it's more complex to manufacture, and it generally pushes the price of such models into four figures.

How do I know if a TV is good for HDR?

There’s only one way to establish whether a particular TV handles HDR effectively, and that’s to test it. We test HDR performance extensively in all of our TV reviews with everything from streamed HDR content to Blu-ray 4K discs and specialised test patterns. Only one thing is guaranteed: regardless of whether it’s a £500 TV or a £5,000 TV, every model varies dramatically in its handling of HDR content.

Some manufacturers use the metadata in the content to create the most ‘accurate’ presentation, while others tweak the end result to provide a brighter, more bombastic take on what the director may have initially intended. In any case, we always explain exactly what you can expect to see from any given display in our reviews.

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HDR for TVs: Why you should care

Not everyone will see a huge clarity improvement with the 4K resolutions of modern TVs – and especially not if you like to sit a good distance away from your TV – but HDR makes a difference regardless of where your sofa is. It can have a huge impact on films, TV shows and video games, and now that multiple streaming services and consoles like the Xbox One X and PS4 Pro have jumped aboard the HDR bandwagon, it’s probably time you hopped on too.