By Madeleine Noland (Blog originally published in BTR)
The Ultra HD movement has taken many twists and turns since its inception, with a gradual switch in emphasis towards better pixels rather than just higher resolution. With the best of intentions there has admittedly been some confusion not just among consumers but also pay TV operators, content owners and broadcasters, some of whom seem unsure about whether and when to commit to a given version of ultra HD at a time of flux.
Fortunately, some clarity has emerged from the confusion and we can say confidently that 2017 will be the first year of stability for Ultra HD as the infrastructure industry coalesces around the UHD Forum’s Phase A guidelines. These provide agreed-upon specifications for dynamic range, color space, resolution and frame rate, so all parties from equipment manufacturers to pay TV operators can converge on these, confident that their products or services will be interoperable and positioned for further improvements in display capabilities.
Phase A has been designed to be backwards compatible with as many recently developed TVs as possible, while setting the stage for ongoing evolution of the standards with the next Phase B in the pipeline for 2018-19. Indeed, one good thing to emerge from the period of reflection and debate over the course of Ultra HD standardization is a firm consensus around the need for the process to be evolutionary rather than revolutionary, allowing components to be added at the convenience of service providers and others without a corresponding need to ditch those that they already have.
The other major point of consensus (one that was originally more contentious) involves what might be called “bang per bit,” or the degree of perceived quality for each pixel of visual information. While viewers undoubtedly notice the difference between 4K resolution at 2160 x 3840 pixels and Full HD at 1080 x 1920 for premium content on large screens, numerous demonstrations and focus groups have now confirmed that this perceived quality jump is less than that achieved by HDR (High Dynamic Range) and to a slightly lesser extent HFR (High Frame Rate) and WCG (Wide Color Gamut). For roughly a 25% increase in bit rate, from 8 bits to 10 bits per sample, HDR achieves a massive – up to 1000 or more – increase in the range between peak brightness and darkness of pixels. 4K video generates nominally four times as many bits as full HD, even if that ratio can be reduced through use of advanced codecs such as HEVC that work well at high resolutions. Yet, the combination of Full HD and some flavor of HDR yields consistently higher MOS (Mean Opinion Scores) than 4K without HDR, even though the latter generates around three times as many bits per frame.
So, when service and content providers consider how best to invest their bits, more and more are concluding that HDR and WCG, followed perhaps by HFR, will yield a greater return in user satisfaction. This is reflected in the composition of the Phase A guidelines, which include both 1080p and 4K resolutions as options.
Perhaps one of the greatest sources of remaining confusion relates to the options for HDR, given that as many as three are included in the Phase A guidelines (PQ10, HLG10 and HDR10) with more variants to come in Phase B. It is true there would be just one in an ideal world, but in practice each has such advantages in given situations that it has been decided to support all of them.
In fact, the first two, HDR10 and PQ10, are essentially the same in terms of quality, although with the significant difference that the former includes metadata to describe how to display the video, while the latter does not. The third option is HLG10 (Hybrid Log-Gamma, 10-bit), with all three now supported by leading TV makers, including Samsung, LG and Panasonic.
HLG is very popular with broadcasters because it simplifies workflow and enables backwards compatibility with CPE equipment that supports the BT.2020 standard defining resolution, frame rate and color space for the first wave of TVs described as being UHD ready, which means models sold from about 2015 onwards. HLG is economical for on-demand content, consuming only storage space for a single asset without need for separate versions for HDR and SDR (Standard Dynamic Range). HDR10, on the other hand, has the advantage that it is already used for a lot of content, including Amazon and Netflix services, and comes with metadata that can improve quality a little, although not significant enough for that alone to make it worth having an additional standard.
There will be coexistence between several HDR formats, allowing service providers to select or mix and match the ones that meet their requirements. The bigger message though is that there is now a solid foundation for deployment of Ultra HD services without a big bandwidth cost. It has become clear that 1080p rather than 2160p will be much easier to deploy over existing infrastructures, including OTT and especially mobile services where support for the higher resolution is a long way off.
In addition, 1080p spatial resolution may enable earlier adoption of high frame rates (HFR – i.e., rates of 100 or 120), which is particularly interesting for sports content. Existing infrastructure is capable of handling the bitrates that 1080p/HFR would require, but not the bitrates of 2160p/HFR. It has also become clear that resolution can be upscaled effectively, more so probably than dynamic range or frame rate, so that when 1080p content is upgraded for display on 2160p displays, the quality comes close to what can be achieved with native 2160p content.
The Ultra HD Forum will be showing various aspects of the Phase A guidelines at NAB 2017, including HLG content displayed on HDR and SDR TV sets that support BT.2020. This will highlight the backwards compatibility that is so important given that it will be some years before even most consumers have HDR-capable TV sets. The Forum will also be giving a preview of the forthcoming phase B guidelines in action, as well as being available to discuss issues such as cooperation with the parallel UHD alliance, which focuses on the content and CPE rather than the infrastructure. These aspects will be previewed in further blogs over the coming weeks.
Madeleine Noland chairs the UHD Forum’s Guidelines Work Group. Noland, of LG Electronics, also chairs the Advanced Television Systems Committee’s S34 Specialist Group on ATSC 3.0 Applications & Presentation