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For the Coronation, over 100 UHD HDR cameras were deployed, covering the procession between Buckingham Palace and Westminster Abbey, Wellington Barracks and inside Westminster Abbey itself. That is very similar in size to the TV coverage of the US Super Bowl but, unlike the Super Bowl which was produced in UltraHD (1080p HDR), The Coronation was produced in full UHD (2160p) HDR. Producing in full UHD HDR adds considerable complexity to such a large-scale production, but it was considered important to produce and archive such a historic event in the highest possible quality.

My colleague Simon Thompson described how our coverage utilised seven outside broadcast (OB) trucks from four different providers – one covering the abbey, five covering the procession route and Wellington Barracks, and a final ‘presentation’ truck covering the ±«Óãtv studio at Canada Gate and the main ±«Óãtv and international programme feeds.

±«Óãtv Research & Development were approached by ±«Óãtv Studios in October 2022, to advise on the production workflow and help with equipment approval and configuration. With different equipment being used in each of the seven outside broadcasts trucks, we needed to ensure consistent results from each production unit.

The entire production used what’s become known as the ‘single-stream’ UHD HDR production workflow, that we developed through our UHD 2019 FA Cup trials. It is, of course, based on the ±«Óãtv/NHK HLG (Hybrid Log-Gamma) HDR format, standardised in the Emmy Award winning Recommendation ITU-R . It has now been adopted worldwide, for live HDR TV production and a simplified version is illustrated below.

HDR cameras are controlled by vision engineers looking at a standard dynamic range (SDR) shading (or racking) monitor, fed with the HDR camera’s output via an HDR to SDR ‘down-mapping’ converter. Only the UHD HDR camera signal is passed through the production switcher (vision mixer). Any SDR outputs available from the cameras are ignored and not made available for the production, although they might be used to drive the camera’s viewfinder. Exactly the same type of HDR to SDR converter takes the switcher’s HDR ‘programme output’ (PGM Output) to create the HD SDR version of the programme. So, the HD SDR signal that is viewed by the largest audience is identical to that seen by the vision engineers who are crafting the look of the pictures.

The OB trucks may only have a single UHD HDR ‘check’ monitor to allow accurate adjustment of the UHD cameras’ ‘detail’ (sharpness) settings and ensure that the UHD signals are in focus and not too noisy – adjustments that would be very difficult on an HD SDR monitor. Some trucks also provide UHD HDR ‘Programme’ and ‘Preview’ monitors for the director, but that’s not essential. Most often, the entire production gallery monitor ‘stack’ is running in SDR.

In practice, most UHD HDR cameras can provide both UHD (2160p) HLG and HD (1080p) HLG outputs. To reduce the need for costly UHD HDR to SDR down-mappers, at the expense of some additional cabling, the 1080p HLG camera output is down-mapped for the camera shading. As the shading monitors are usually 17" displays, 1080p resolution is sufficient.

The workflow is documented in greater detail in Section 7.2.2 of Report ITU-R and the EBU’s Tech Report . The workflow is also used by Sky for all of their English Premier League coverage and a number of other sports too; and outside of the UK it is used by for the Olympics, for the FIFA World Cup, UEFA for the Euros and US broadcasters CBS, NBC and Fox. It is extremely robust, reliable and repeatable and is also straightforward for operational staff, as critical camera control is performed in exactly the same way as for a conventional HD SDR production.

Workflow improvements

Features added to HDR production equipment since our 2019 FA Cup trials have significantly simplified the workflow.

Back in 2019, we needed to use a mixture of ‘scene-light’ and ‘display-light’ HDR/SDR format conversions, adding complexity and increasing the risk of operational errors:

• ‘Scene-light’ conversions are used for matching HDR and SDR cameras, as they operate by calculating the light falling on a camera sensor. As HDR and SDR cameras have a different ‘look’ (colour saturation and tone), scene-light conversions change the look of signals too.
• ‘Display-light’ conversions, however, attempt to preserve the ‘look’ of an image through the conversion process, by basing their calculations on the light reproduced by a reference HDR or SDR displays.

Although not necessary for the Coronation coverage, we now have HDR slow-motion cameras and 10-bit slow-motion replay servers, which was not the case in 2019 when slow-motion replays were restricted to 8-bit SDR. So, the need to operate and colour match a mixture of HDR and SDR cameras has greatly reduced. Consequently, we seldom need to consider ‘scene-light’ conversions now and almost all format conversions are based on display-light. The one exception is where a specialist camera is required (e.g. a miniature camera in a goal post) which might only be available in SDR. If that’s the case, a scene-light SDR to HDR conversion will still be needed.

HLG HDR is now widely supported in non-linear editing systems which meant that for the Coronation, we could run the on-site post-production facility in HLG HDR, further reducing the need for HDR/SDR format conversion.

HDR to SDR down-mapping has greatly improved too, and often uses the extended signal range above SDR nominal peak-white (10-bit code value 940) to convey some of the highlights from the HDR cameras. That in-turn has led to a reduction in SDR > HDR > SDR ‘round-trip’ losses, which is important when SDR content items such as graphics and archive are included in an HDR programme. We’ll take a closer look at that in the next section.

Important for our audiences, HDR camera ‘painting’ controls, which allow a vision supervisor to craft the artistic look for an event, have also matured. These are discussed in greater detail later too.

HDR to SDR format conversion

Critical to our use of the ‘single-stream’ HDR production workflow, are ±«Óãtv R&D’s HDR to SDR format conversion ‘3D-LUTs’. These are three-dimensional ‘lookup’ tables which map an HDR input RGB triplet, to an SDR output RGB triplet. For 10-bit RGB input signals, you might assume that you would need a lookup table with two to the power thirty entries, but that would, of course, be huge. Typically, the R, G and B signal ranges are divided into 33 or 65 values each, and table entries provided for all combinations of those values. The missing output values are interpolated. So, it’s easy to encode a complex conversion algorithm into the LUT, without needing expensive dedicated hardware.

Unlike some other LUTs, our LUTs use a ‘non-linear’ conversion algorithm ensuring the HDR image and the SDR image, when shown on a reference 100 cd/m² nominal peak luminance SDR display, look subjectively similar. Even when monitoring just the down-mapped SDR camera signal, vision engineers can be confident that they’re creating spectacular HDR images. It’s important to use a non-linear converter, because the brightness of HDR and SDR images viewed in TV production are different. Table 1 of Report ITU-R specifies a nominal HDR Reference White level (the diffuse white) of 203 cd/m² for HDR images (75% HLG), whilst Recommendation ITU-R specifies a nominal peak white level (similar to diffuse white) of 100 cd/m² for SDR production. A non-linear converter can take account of the non-linear response of the human visual system to the different brightness HDR and SDR signals, whereas a simple linear converter cannot. A linear HDR to SDR down-mapper will usually yield subjectively darker mid-tones and shadows to those seen in the HDR image - if you’re crafting the ‘look’ of the down-mapped SDR image, the HDR image may look overly bright and ‘sat-up’ in comparison.

HDR to SDR format conversion has improved enormously in recent years. It is now at the point where the SDR signal derived from an HDR original is significantly better than the SDR signal usually obtained from a conventional SDR camera. Furthermore, when an HD signal is derived from a UHD camera, the quality of the HD is usually better than that obtained from an HD camera. Thus, the HD SDR audience benefits from the UHD HDR production too.

For the Coronation, we required the use of the very latest ±«Óãtv R&D conversion LUTs. The HDR to SDR down-mapping LUT used for the critical camera shading (racking) was our display-light down-mapping LUT 9c. The same down-mapping LUT was applied to the UHD production switcher’s (mixer) output in the final ‘Pres’ (Presentation) OB truck at Canada Gate, to create the HD SDR programme.

SDR to HDR format conversion

Today, and for many years to come, it will be important to be able to include SDR content into an HDR programme. The Coronation was no different, as extensive use was made of archive material throughout . In 2019, we used a technique called ‘up-mapping’ to convert SDR content to HDR. The technique places SDR content into an HDR signal container, and gives a slight boost to the SDR ‘highlights’ so that the SDR content more closely resembles the look of native HDR. The technique works well with carefully prepared drama or movie content, but gives variable results with live cameras and older archive material which can have heavily ‘clipped’ highlights. Those clipped SDR highlights can lead to large, ugly and overly bright regions in the HDR version of the content. They were, however, useful conversions as the ‘up-mapping’ tone-curve could be designed to complement the ‘down-mapping’ tone-curve in the final HDR to SDR conversion on the programme output, thereby reducing the end-to-end ‘round-trip’ losses.

However, for Fox’s 2020 coverage of the US Super Bowl, a different approach was used. Until then, it was common practice for HDR to SDR converters to compress the entire HDR signal range into the SDR signals’ nominal range (10-bit code value 64 to 940). But for the Super Bowl we were asked to produce a down-mapping LUT that placed the compressed HDR highlights into the SDR signal’s headroom, above nominal peak white (10-bit code value 940), in a signal range known as the ‘super-whites’. By doing so, we were able to increase the signal value that the HDR Reference White level (75% HLG) maps to in the SDR signal output, from 86% to 95% SDR. We were a little nervous doing this, as we had previously seen problems with equipment when using the signal range below black (below 10-bit code value 64). However, extensive testing, which we repeated just ahead of the Coronation, showed no issues with either our traditional broadcast or iPlayer playout and distribution chains. For good measure, we actually restrict the SDR signal range to the EBU ‘preferred’ signal range of -5%/+105%, rather than exploit the entire 10-bit signal range.

Not only does this new type of conversion give brighter SDR images with less heavily compressed highlights, but it allows SDR content to be included into an HDR programme using ‘direct-mapping’ (i.e. without applying the risky highlight boost) where the 100% SDR signal level is directly mapped to the HDR Reference White level of 75% HLG.

The figure below shows how we’re able to achieve a ‘round-trip’ SDR highlight loss of just 5%, using the ±«Óãtv’s display-light SDR direct-mapping LUT 3 and down-mapping LUT 9.

Camera painting

Different programme genres and geographical regions have their own preferred ‘looks’. By that I mean some genres, such as football, require a ‘punchy’ colourful image with a great deal of contrast, while others like drama may require a less punchy, more desaturated image. Because of those differences, during the ITU-R standardisation process for HLG, it was difficult to agree the default ‘reference’ look. In the end, what is written into the ITU-R BT.2100 standard is a natural look that’s close to nature in terms of its representation of tones and colour. Some professional cameras even refer to it as the HLG ‘natural’ look. It is not as colourful as the SDR production standard BT.709 nor is it as ‘punchy’, but it does provide a neutral image that can then be ‘painted’ to deliver alternative looks. As the name suggests, ‘painting’ controls within a camera allow the vision supervisors to paint the images to deliver the desired aesthetic.

For such a historic production, it was important that we gave the vision supervisors all of the tools they needed to craft the very best images. We understood from previous trials that, for this type of production, the vision supervisors may not be entirely happy with the default ‘natural’ look that HLG provides. So, we specified that all cameras used for the production had to offer a full set of ‘painting’ controls. Typically, these controls allow adjustment of the brightness of shadow detail, mid-tones and colour saturation. The photo app on your mobile phone probably offers similar adjustments. The adjustments were used to great effect to adapt the varied outdoor lighting conditions on the day of the Coronation, and to ensure that the deep, rich colours of the Kings’ robes were faithfully captured within the abbey.

As is often the case, two radio cameras were added late in the day, and they could not support the full set of camera painting controls. We’ve been working within the EBU to create a custom HDR ‘user gamma’ LUT that gets close to the traditional BT.709 look often preferred in Europe. These can often be loaded into cameras that don’t yet have a full set of painting controls, to achieve a better match to painted cameras than the default BT.2100 HLG look. It’s been included in EBU Members’ supplements to the EBU’s ‘Baseline HDR Camera Painting Controls’ specification, .

Coronation signal routing

For such a huge TV production, it was important to keep signal routing as simple as possible. Each OB truck had its own director and provided a finished UHD HDR programme for their section of the parade or abbey coverage. The Pres truck then mixed between those feeds and the ±«Óãtv’s studio cameras at Canada Gate, to produce the final UHD programme output. As described above, this was then down-mapped in the Pres truck from HDR to SDR using our LUT9c, and converted from UHD 2160p to HD 1080i, to provide the HD SDR programme output.

As a confidence check, the 1080i HD SDR programme output created by the Pres truck was fed back to the other OB trucks to allow them to compare against their locally generated HD SDR signal. This also allowed the vision supervisors in each truck to ensure their pictures were consistent with those of the other OB trucks. The whole set-up worked incredibly well, with the 1080i HD SDR ‘return’ programme feed allowing any format converter configuration errors to be quickly spotted.

A word on HDR camera shading in SDR...

Some in the industry, particularly in North America, have started using linear down-mappers with this workflow, and increasing the peak luminance of the critical SDR shading monitor from the standard 100 cd/m² to 203 cd/m². By doing so the SDR monitoring better matches the brightness of any HDR monitoring. The linear scaling in the down-mapper from the HDR Reference White of 203 cd/m² to the usual SDR white level of 100 cd/m², is complemented by a linear scaling in the display from 100 cd/m² to 203 cd/m². So, the appearance of mid-tones and shadows on the 203 cd/m² SDR display are very similar to those in the HDR original image, without the need for the more complex non-linear down-mapping technique. It is argued that modern consumer TVs show SDR images at a peak luminance that’s closer 203 cd/m² than the 100 cd/m² specified in Recommendation ITU-R BT.2023, so we should do the same in programme production. Two vision supervisors working on the Coronation asked me what the ±«Óãtv thought of the practice.

At first glance increasing the luminance of the critical SDR shading monitor might seem like a sensible move, but we and many in our industry have serious concerns. For a given signal, if you adjust the contrast on a monitor to deliver 203 cd/m² rather than the usual 100 cd/m² you will see more detail in the shadows and, because of the non-linear response of the human visual system, the appearance of mid-tones will change too. As a result, a vision engineer will make different artistic adjustments to a camera, depending on whether they are viewing on a standard 100 cd/m² or 203 cd/m² SDR display.

All in the industry recognise that few (if any) consumer TVs produce a picture ‘out of the box’ that’s close to the image seen in the TV production environment. But we appreciate that TV manufacturers have put a great deal of effort into optimising their different picture modes to deliver images that are liked by the consumer and work well in the home. Those optimisations were, of course, performed assuming a standard SDR signal crafted on a 100 cd/m² BT.2035 display. If content producers now change the nature of the signal they distribute by adopting 203 cd/m² monitoring instead of 100 cd/m² monitoring, it’s hard to predict how those TV picture modes will perform and what will be seen in viewers’ homes. Moreover, if the practice were to become widespread, as viewers ‘channel-hop’ between traditional 100 cd/m² broadcasters and 203 cd/m² broadcasters, the appearance of the SDR images would change and they may find themselves wanting to make picture adjustments on a channel-by-channel basis.

For HLG, we put a great deal of effort into ensuring that the display EOTF (electro-optical transfer function) could deliver subjectively similar images on HDR displays of different peak luminance. But that’s not the case with the older SDR BT.1886 EOTF used in professional SDR monitors. It is for that reason that the ITU-R specify a fixed nominal peak luminance of 100 cd/m² in Recommendation BT.2035 for critical SDR monitoring in production.

The use of 100 cd/m² might seem a little old-fashioned these days, particularly as the practice dates back to the use of cathode ray tube (CRT) displays which would defocus at higher brightness. But in international programme exchange, signal consistency is paramount. For this reason, we at the ±«Óãtv firmly believe that critical SDR production monitoring should remain at 100 cd/m² to ensure consistent signals from different programme providers.

The big day, Saturday, 6 May

Part of me wishes that I could say I was at Canada Gate at the heart of the ±«Óãtv’s operation on the day of the Coronation; just as I had been for the wedding of the Duke and Duchess of Sussex, our first ever live UHD HDR production in May 2018. But the truth is, thanks to all of the preparation, our work was done after we saw the UHD HDR Coronation preview programme go to air on ±«Óãtv iPlayer and ±«Óãtv One HD at 7 pm the day before. The UHD HDR workflow, technical approach and equipment configuration were proven, and any issues that arose the next day would be operational and unlikely ones that I, as an R&D engineer, could assist with. As space within OB trucks is at a premium, and the workflow almost now BAU (business as usual), I left my colleague Simon Thompson to provide cover on the day itself. I was actually very happy to be able to enjoy the spectacular UHD HDR images via ±«Óãtv iPlayer, with friends and family at home.