What is HDR?
Is High Dynamic Range a game changer?
Over the last few months you may have heard various TV manufacturers, film studios and streaming content providers talking about High Dynamic Range or HDR but what is it?The broadcasting and consumer electronics industries are going through a significant period of change and one of the main drivers of this revolution is the introduction of 4K or, as it’s otherwise known, Ultra High Definition (UHD). However unlike the move from standard definition to high definition, where the increase in resolution was quite pronounced, broadcasters and manufacturers felt the increased resolution of 4K Ultra HD wouldn't be enough on its own to convince people to upgrade from HD to 4K. So the decision was made to look at other ways of improving the image, aside from simply increasing the resolution.
These included improving the video bit depth and moving from 8- to 10-bit video, changing the colour space from the current standard of Rec.709 to something wider, increasing the frame rate from 50/60Hz to 100/120Hz and introducing High Dynamic Range (HDR). It was thought that a combination of two or more of these approaches would create an image that would better appeal to consumers. HDR was seen as an option that could prove popular with consumers because it would have a far greater and more obvious impact on perceived image quality and could thus be an effective way of differentiating 4K Ultra HD from HD.
4K Ultra HD is about more than just increased resolution, it also means a wider colour space and increased dynamic range.In simple terms, HDR is a way of displaying the wider dynamic range inherent in the original capture of content. A film camera or a modern digital camera is capable of capturing a higher level of brightness (luminance) and perceivably deeper blacks than a current video display is capable of showing. However if a transfer can be created that retains all this luminance detail and a display is capable of showing it, then the resulting image will be a better representation of what the human eye is capable of resolving.
It’s important to realise that HDR isn’t simply about increasing the brightness of an image, although that is a common misconception. The idea is to increase the overall dynamic range between black and white by making the dark parts darker and the bright parts brighter, whilst retaining detail when both are in the frame. So for example if the image was of the inside of a room you could see details in the dark shadows but also make out details through the brightly-lit window.
However it isn’t just about the difference between the darkest and brightest elements in an image, HDR also calls for greater expression and detail within colours too. So HDR will use a wider colour space than the current standard of Rec.709. Exactly which colour space will be used hasn’t actually been announced yet but probably the DCI standard that is used in the cinema, although it could theoretically go as wide as Rec.2020.
If you look at the example above, where the bell shape represents the total visible spectrum of the human eye, the smallest triangle within it represents the current limitations of the Rec.709 standard. The larger triangle represents the new Rec.2020 standard for colour, which is the maximum colour space proposed for the new 4K Ultra HD standards, and it's obviously much wider in scope than what we have been seeing up until now.
The current 8-bit video standard allows for a maximum of 256 shades of any one primary colour to be displayed but under the new 4K Ultra HD standards, displays will need to be capable of handling colours at a depth of 10-bits, allowing for 1,024 shades of every primary colour. The combination of a wider colour space and greater bit depth will result in a more natural looking and detailed images than can be obtained at present.
Although HDR and wider colour spaces are technically separate aspects of image reproduction, the two are becoming linked by standards bodies into what is often referred to as colour volume. This is essentially a three-dimension version of the graph above with colour and luminance combined. The bigger the luminance range and the wider the colour space, the larger the overall colour volume and thus the greater the impact on the perceived image.The combination of increased dynamic range and a wider colour space results in a larger colour volume and a better perceived image.Until recently the standard for creating video transfers has largely remained unchanged for decades and it is based upon the capabilities off the old CRT (cathode ray tube) monitors that were used in the mastering studios. As a result of the historical limitations of the display technology, all high definition transfers are created using an 8-bit video depth, the Rec.709 colour space and at a peak brightness of 100 Nits (which is a measure of luminance).
This has been frustrating for colourists working in the film industry because the actual colour and luminance detail in the original footage captured (be it on film or digitally) is far greater than the current standards are able to convey. In addition display technology has moved on and is now able to handle standards far in excess of what is currently being used. Therefore as part of the move towards 4K Ultra HD, the standards are being changed to reflect the capabilities of modern displays.
What this essentially means is that new 4K Ultra HD transfers are now being created using a 10-bit video depth and a wider colour space. Although there hasn’t been confirmation of the exact colour space to be used, since films use the DCI colour space for theatrical presentation at the cinema, that would seem the obvious choice. The final aspect of this new standard is the addition of HDR, which means using a much higher peak luminance than 100 Nits when creating new transfers.
By combining all these elements, film colourists will be able to create transfers that can take full advantage of the luminance and colour detail inherent in the original content. In particular, HDR will allow them to distinguish bright details in highlights that are often compressed in traditional video transfers as well as more perceptible details in shadows. In addition there will be greater separation of colour details in diffuse near-white colours and in strongly saturated parts of the image.
Unfortunately this is where it gets confusing because there are actually competing versions of HDR. Whilst they all essentially aim to deliver a similar experience, they take slightly different approaches to how HDR content is mastered and delivered to the consumer. The concern is that these different versions of HDR might not be compatible and as a result SMPTE (Society of Motion Picture and Television Engineers) is trying to develop a single HDR ecosystem.There are a number of different HDR systems but SMPTE are hoping to develop a single HDR ecosystem.As is so often the case in the broadcasting and electronics world, more than one way of creating and delivering HDR to the consumer has been developed. All of these versions are essentially trying to deliver a similar HDR experience with an extended dynamic range, a wider colour space and increased bit depth. However their approaches are slightly different and whilst some are open, others are proprietary.
HDR10 - This is an open platform version of HDR that has been adopted by the Blu-ray Disc Association (BDA) for 4K Ultra HD Blu-ray. Under the specifications for 4K UHD Blu-ray HDR will use a 10-bit video depth and up to the Rec.2020 colour space. It also uses the SMPTE 2084 EOTF (Electro-Optical Transfer Function, which is the new name for gamma) and SMPTE 2086 metadata for delivering the extended dynamic range, which is mastered using a peak brightness of 1000 Nits. This version of HDR has been adopted by Twentieth Century Fox for the 4K Ultra HD Blu-rays that they announced recently and it is also being used by both Amazon Instant and Netflix to deliver HDR content.
Dolby Vision - This is a competing version of HDR that was developed by Dolby Labs, who have been an active proponent for extended dynamic range for years. Dolby Vision is also included in the specifications for 4K Ultra HD Blu-ray and uses a 12-bit master with a colour space that can go up to Rec.2020. It also uses the SMPTE 2084 EOTF and and a peak brightness that could theoretically go as high as 10,000 Nits. However current Dolby professional monitors can ‘only’ go as high as 4,000 Nits and use the DCI colour space. Dolby have taken a different approach when it comes to delivery with a 10-bit base layer and a 2-bit enhancement layer that can deliver increased colour volumes up to 10,000 Nits. Dolby also have a single layer 10-bit solution that is aimed at broadcast and OTT applications. To date both Warner Brothers and Sony Pictures have adopted Dolby Vision as their preferred method of extending dynamic range.
Technicolor/Philips HDR - Technicolor and Philips are working together to create an efficient HDR workflow/ecosystem based upon three key pragmatic tenets - that two content masters are created, one graded to HDR and one to SDR; that the process is as efficient as possible; and that it preserves the artistic intent. The system will also use 10-bit HEVC encode/decode combined with the SMPTE 2084 EOTF and other EOTF curves better suited for television such as the one proposed by the BBC and NHK. The system would initially use a single layer SDR/HDR encoding with SMPTE 2086 metadata in order to ensure backwards compatibility, with a dual layer solution suggested for possible use in the future. The idea is that a single layer/stream combined with metadata can deliver HDR by adding low bit rate information to allow the transformation of content to the optimal grade for any display peak luminance and dynamic range.
Hybrid Log-Gamma - The BBC’s development of HDR is concentrated more on an open, royalty-free approach that is both practical and cost effective. The BBC feels that the SMPTE 2084 EOTF isn’t best suited to broadcast and, in conjunction with NHK, they have developed a revised EOTF which allows the signal to be displayed on a wide range of displays, with varying peak luminance, black level and viewing environments, to produce a high quality picture. The BBC approach which is called 'Hybrid Log-Gamma' is designed to incorporate many of the attributes of HDR already mentioned but to deliver them in the most efficient and cost-effective manner possible.
Whilst all these HDR systems take a slightly different approach, the majority of them do share certain characteristics and standards. The hope is that the industry can agree on a unified workflow and ecosystem to ensure the best, most efficient and cost-effective approach. However regardless of which HDR system is used, the various HDR TVs all have different luminance capabilities. As a result tone mapping technology needs to be used to map the colour space and dynamic range to the display itself.
The idea behind tone mapping is to use metadata in the HDR content to map it to the actual capabilities of the display and thus retain as much of the artistic intent as possible. The current tone mapping methods use static metadata but Samsung have proposed a dynamic metadata as a way of better retaining the original HDR intent on displays with smaller dynamic ranges and narrower colour gamuts.There are already a number of TVs available that support HDR but the majority sold to date won't be compatible.As it stands the majority of 4K Ultra HD displays that have been released do not support HDR and are unlikely to have this feature added in the future. So the answer to that question is probably no, unless you own a flagship TV for 2015 from Samsung (JS9500, JS9000 and JS8500), LG (EG960 and EF950), Panasonic (CX802, CR852 and CZ952), Sony (X94, X93 X91, X90, X85 and S85) and Philips (9600). As far as we know these TVs support the open source HDR 10 and don't currently support any of the other HDR systems but that might change as more HDR content is released in 2016.
However, even if your TV can support HDR that doesn’t mean you’ll be able to take advantage of all the HDR sources available. That’s because some TVs have been upgraded to HDR via a firmware update but they may still only have HDMI 2.0 inputs. This means that whilst these TVs can take advantage of HDR content from streaming services like Amazon Instant and Netflix, they won’t be able to support HDR content from external devices such as 4K Ultra HD Blu-ray players. This is because for an HDMI input to pass the necessary HDMI metadata the connection needs to be HDMI 2.0a, so you'll need to check what type of HDMI input your TV uses.
So does that mean my new 4K Ultra HD TV is obsolete?
No, your current 4K TV will always be able to display a picture and, provided it is HDMI 2.0/HDCP 2.2 compatible and capable of HEVC decoding, you should always have Ultra HD content, via streaming services or on 4K Ultra HD Blu-ray. Your TV might also use a 10-bit panel, so you'll still be able to take advantage of content that uses a higher bit-depth. As we mentioned earlier, there are also plans to include tone mapping so the content can be mapped to match the dynamic range and native colour space of your TV, which would mean that even if your TV doesn't actually support HDR you can still enjoy many of the benefits of the proposed new standards.If you have a fast enough broadband connection, you will already be able to access HDR content from both Amazon Instant and Netflix. Amazon Instant are already allowing subscribers to watch shows like Mozart in the Jungle in HDR and Netflix will offer something similar with their original series like Marco Polo, which has already being shown in HDR. Doubtless other streaming services will also add HDR content as things progress into 2016.
The other major potential source of HDR content is 4K Ultra HD Blu-ray, which includes support for HDR in its specifications. Twentieth Century Fox has already announced that their 4K UHD Blu-rays will include HDR and they have been creating HDR masters for the last couple of years in anticipation. In addition both Warner Bros. and Sony Pictures have announced support for Dolby Vision, with examples of HDR versions of films from all three studios being shown at trade shows.
So there will be a reasonable catalogue of content available in HDR, including films and more recently produced TV series. It's also worth remembering that 35mm film has a very wide dynamic range, so anything shot on film can be transferred in HDR and that's exactly what the studios have been doing as they've restored their older titles. This has been going on for the last few years, so there should be plenty of new and older titles available in HDR.
The final source of HDR is broadcast TV and although this is still in its early stages, public broadcasters like the BBC and NHK have been working on an HDR standard that is specially optimised for broadcast TV, in terms of both workflow and distribution. The BBC has already conducted numerous tests and recently covered the America's Cup in HDR in a joint venture with BT Sport, who believe that HDR will play an important part in TV broadcasting going forward.The catalogue of content is increasing, with Amazon Instant and Netflix already supporting HDR and 4K UHD Blu-ray coming soon.There's no denying that all the major industry participants from studios, manufacturers, broadcasters and streaming services all believe that HDR has the potential to be a real game changer. In fact it was the realisation that 4K Ultra HD would require more than just increased resolution to be a success that led to the formation of the Ultra HD Alliance. This body is made up of just about every major participant in the industry and they are all working towards a more unified approach in terms of the technology used and how it's promoted. The UHD Alliance plan to certify new TVs, thus guaranteeing that they meet a base level of performance and ensuring that consumers can take full advantage of the new format.
HDR has certainly come a long way very quickly and in the space of a year it has become one of the dominant new TV technologies. The TV industry is going through a major period of adjustment as it moves from the old standards (HD, 8-bit, Rec.709 and SDR) to a new one based upon 4K Ultra HD, 10-bit, DCI and HDR. There will undoubtedly be some issues at first, especially relating to the new standards, the different HDR systems and how best to deliver the content whilst remaining backwards compatible to the existing base of 4K Ultra HD TVs. However once that initially difficult period has passed we will have a new standard that can deliver better images than we've ever seen before.
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