Has UHD been slow to roll out? Not so, we hear in this talk which explains the work to date in standardising, testing and broadcasting in UHD by the BBC and associated organisations such as the EBU.
Simon Thompson from BBC R&D points out that HD took decades to translate from an IBC demo to an on-air service, whereas UHD channels surfaced only two years after the first IBC demonstration of UHD video. UHD has had a number of updates from the initial resolution focused definition which created UHD-1, 2160p lines high and UHD-2 which is often called 8K. Later, HDR with Wide Colour Gamut (WCG) was added which allowed the image to much better replicate the brightnesses the eye is used to and almost all of the naturally-occurring colours; it turns out that HD TV (using REC.709 colour) can not reproduce many colours commonly seen at football matches.
In fact, the design brief for HDR UHD was specifically to keep images looking natural which would allow better control over the artistic effect. In terms of HDR, the aim was to have a greater range than the human eye for any one adpation state. The human eye can see an incredible range of brightnesses, but it does this by adapting to different brightness levels – for instance by changing the pupil size. When in a fixed state the eye can only access a subset of sensitivity without further adapting. The aim of HDR is to have the eye in one adaptation state due to the ambient brightness, then allow the TV to show any brightness the eye can then hold.
Simon explains the two HDR formats: Dolby’s PQ widely adopted by the film industry and the Hybrid Log-Gamma format which is usually favoured by broadcasters who show live programming. PQ, we hear from Simon, covers the whole range of the human visual system meaning that any PQ stream has the capability to describe images from 1 to 10,000 Nits. In order to make this work properly, the mix needs to know the average brightness level of the video which will not be available until the end of the recording. It also requires sending metadata and is dependent on the ambient light levels in the room.
Hybrid Log-Gamma, by contrast, works on the fly. It doesn’t attempt to send the whole range of human eye and no metadata needed. This lends itself well to delivering HDR for live productions. To learn more about the details of PQ and HLG, check out this video.
Simon outlines the extensive testing and productions done in UHD and looks at the workflows possible. The trick has been finding the best way to produce both an SDR and an HDR production at the same time. The latest version that Simon highlights had all the 70 cameras being racked in HDR by people looking at the SDR down-mix version. The aim here is to ensure that the SDR version looks perfect, as it still serves over 90% of the viewership. However, the aim is to move to a 100% HDR production with SDR being derived off the back of that without any active monitoring. The video ends with a look to the challenges yet to be overcome in UHD and HDR production.
There seem no let up in the number of technologies coming to market and whilst some, like HDR, have been slowly advancing on us for many years, the technologies that enable them such as Dolby Vision, HDR10+ and the metadata handling technologies further upstream are more recent. So it’s no surprise that there is some confusion over what’s possible and what’s not.
In this video, Bitmovin and Dolby the truth behind 5 myths surrounding the implementation and financial impact of Dolby Vision and HDR in general. Bitmovin sets the scene by with Sean McCarthy giving an overview on their research into the market. He explains why quality remains important, simply put to either keep up with competitors or be a differentiator. Sean then gives an overview of the ‘better pixels’ principle underlining that improving the pixels themselves is often more effective than higher resolution, technologies such as wide colour gamut (WCG) and HDR.
David Brooks then explains why HDR looks better, explaining the biology and psychology behind the effect as well as the technology itself. The trick with HDR is that there are no extra brightness values for the pixels, rather the brightness of each pixel is mapped onto a larger range. It’s this mapping which is the strength of the technology, altering the mapping gives different results, ultimately allowing you to run SDR and HDR workflows in parallel. David explains how HDR can be mapped down to low-brightness displays,
The last half of this video is dedicated to the myths. Each myth has several slides of explanation, for instance, the one suggests that the workflows are very complex. Hangen Last walks through a number of scenarios showing how dual (or even three-way) workflows can be achieved. The other myths, and the questions at the end, talk about resolution, licensing cost, metadata, managing dual SDR/HDR assets and live workflows with Dolby Vision.
Measuring video quality automatically is invaluable and, for many uses, essential. But as video evolves with higher frame rates, HDR, a wider colour gamut (WCG) and higher resolutions, we need to make sure the automatic evaluations evolve too. Called ‘Objective Metrics’, these computer-based assessments go by the name of PSNR, DMOS, VMAF and others. One use for these metrics is to automatically analyse an encoded video to determine if it looks good enough and should be re-encoded. This allows for the bitrate to be optimised for quality. Rafael Sotelo, from the Universidad de Montevideo, explains how his university helped work on an update to Predicted MOS to do just this.
MOS is the Mean Opinion Score and is a result derived from a group of people watching some content in a controlled environment. They vote to say how they feel about the content and the data, when combined gives an indication of the quality of the video. The trick is to enable a computer to predict what people will say. Rafael explains how this is done looking at some of the maths behind the predicted score.
In order to test any ‘upgrades’ to the objective metric, you need to test it against people’s actual score. So Rafael explains how he set up his viewing environments in both Uruguay and Italy to be compliant with BT.500. BT.500 is a standard which explains how a room should be in order to have viewing conditions which maximise the ability of the viewers to appreciate the pros and cons of the content. For instance, it explains how dim the room should be, how reflective the screens and how they should be calibrated. The guidelines don’t apply to HDR, 4K etc. so the team devised an extension to the standard in order to carryout the testing. This is called ‘subjective testing’.
With all of this work done, Rafael shows us the benefits of using this extended metric and the results achieved.
Update: This webinar is now available on-demand. Links in this article have been updated to match.
Brightness, luminance, luma, NITS and candela. What are the differences between these similar terms? If you’ve not been working closely with displays and video, you may not know but as HDR grows in adoption, it pays to have at least a passing understanding of the terms in use.
Date: Thursday January 23rd – 11am ET / 16:00 GMT
Last week, The Broadcast Knowledge covered the difference between Luma and Luminance in this video from YouTube channel DisplacedGamers. It’s a wide ranging video which explains many of the related fundamentals of human vision and analogue video much of which is relevant in this webinar.
To explain the detail of not only what these mean, but also how we use them to set up our displays, the IABM have asked Norm Hurst from SRI, often known as Sarnoff, to come in and discuss his work researching test patterns. SRI make many test patterns which show up how your display is/isn’t working and also expose some of the processing that the signal has gone through on its journey before it even got to the display. In many cases these test patterns tell their story without electronic meters or analysers, but when brightness is concerned, there can still be place for photometers, colour analysers and other associated meters.
HDR and its associated Wide Colour Gamut (WCG) bring extra complexity in ensuring your monitor is set up correctly particularly as many monitors can’t show some brightness levels and have to do their best to accommodate these requests from the incoming signal. Being able to operationally and academically assess and understand how the display is performing and affecting the video is of prime importance. Similarly colours, as ever, a prone to shifting as they are processed, attenuated and/or clipped.
This free webinar from the IABM is led by CTO Stan Moote.
Senior Principal Research Engineer,
SRI International SARNOFF
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