SVGE Analysis: Delivering on the rich promise of HDR at Sony Technology Day
When HD overtook SD, many viewers noticed the change in screen shape more than the higher resolution. With Ultra HD, most viewers sit too far from the set to really notice the higher resolution (or need to get a much bigger set). However, judging from comments at CES and other trade shows, the quality of the image on the latest High Dynamic Range UHD displays really does set them apart from HD screens. If broadcasters are to persuade viewers to adopt UHD, then the brighter, more vivid and lifelike colours of HDR will play a big part.
“HDR is one of the key issues that people are wrestling with at the moment,” Peter Sykes, Strategic Technology Development Manager, Sony Europe, told attendees at a Sony Technology Day held at Pinewood Studios (which it is holding instead of taking part at BVE2016).
Although it is now possible to deliver a complete HDR production, being able to view it in the home is still a problem. There are currently two main delivery standards being put forward that could do the trick, but only one of them is really suitable for general live broadcast.
They are SMPTE’s ST2084, which was adopted as a standard last year, and Hybrid Log-Gamma, which has jointly been developed by the BBC and NHK.
Sony has been involved in a few live trials of HDR, both for the MotoGP, using its new 2/3-inch HDC-4300 4K/UHD camera (which can be used with S-Log for HDR capture), and a trial with Sky Deutschland at the German Super Cup Final in Wolfsburg (vs Bayern Munich), where Sony was involved in both the production and the final display, which is where they tested ST2084 against HLG.
“At the stadium we set up one single camera chain, using the HDC-4300 with its Camera Control Unit and a BPU-4000 base band processor unit — to take the data from the camera and produce the Ultra HD output,” said Sykes. These fed S-Log3 into a prototype convertor that output the HDR signals: ST2084 or HLG, with BT2020 or Rec.709 colour spaces. These were monitored on a BVM-X300 display and recorded on a Sony PWS-4400 server.
The signal was then output via SDI on four BNCs, for contribution and encoding, using HEVC Main 10 at 25Mbps. That was uplinked via SES Astra to Sky’s Munich HQ, where the various HDR, SDR and colour space variants could be viewed. “This was a game that happened at night, which was a bit unfortunate, but we managed to get some good pictures of the warm up before the game, when it was still sunny,” he added.
Sky was particularly interested in how the signal could cope with both HDR and SDR viewing, given that almost all of its subscribers still have SDR sets. “So, performance of an HDR signal into a standard dynamic range set is critical for the broadcaster. It is no good having great HDR if, for everybody watching in SDR, their TVs can’t handle the signal.”
The initial reaction from Sky Deutschland was that “both of the systems worked fantastically well in HDR mode,” when viewed on a UHD HDR screen. “The Hybrid Log-Gamma system was determined to offer very good compatibility with standard dynamic range systems,” said Sykes.
Although the transmission used for the test didn’t contain metadata, and the TV sets couldn’t automatically switch into ST2084, this will be possible, using the ST2086 metadata-signalling standard. How this will be done for HLG isn’t yet finalised, but HLG is a variable gamma setting and fully compatible with SDR sets, whereas ST2084 is a fixed value setting and doesn’t offer any compatibility with SDR.
HLG will also work a lot better with a typical broadcast, where promos can push the live shot into a corner box as the presentation details what’s coming next, with multiple sources on screen at one time, “where if you did have metadata running through the system it would get very complicated. That’s one of the big benefits that’s being proposed by the developers of the Hybrid Log-Gamma system,” said Sykes.
HLG is already standardised as ARIB STD-B67 by the Association of Radio Industries and Businesses in Japan — where the JSAT pay-TV operator SkyPerfect is offering HDR UHD transmissions using HLG to its subscribers via the JCSAT-3A satellite. However, it will also face an ITU standardisation meeting in the next few weeks where it will be put forward for adoption. Sony is supporting both ST2084 and HLG.
“Lots of broadcasters are evaluating [HDR] at the moment. There aren’t widespread services yet, but end-to-end HDR workflow is what many customers are looking to achieve,” said Sykes.
HDR-capable cameras have been in use for some years. Sony’s first HDR camera was the F65, introduced in 2011, with its PMW-F55 launched a year later. The F55 has been used to shoot Marco Polo for Netflix, and Amazon’s Mozart in the Jungle, both of which are available for streaming in HDR. Amazon and Netflix have also shot other shows in HDR, while YouTube has promised to provide HDR playback through its TV app later this year using Google’s VP9-Profile 2 codec.
Of course, it is simpler for a streaming service to send an HDR video direct to a single user than for broadcasters to transmit combined HDR and SDR images to millions of viewers. First viewers need an HDR display, but there are now quite a few of these available from Sony (which showed its new 75-inch HDR display at the Tech Day), while other companies (such as LG and Panasonic) launched HDR displays recently at CES in Las Vegas.
These new sets can often stretch the current 8-bit transmitted image to give a slightly extended dynamic range look, “but it’s not a perfect representation. We can never recover what has been lost” in the transmission path. Sony’s 75-inch HDR TV uses a variable LED panel, using X-tended Dynamic Range Pro, that analyses the picture and uses localised dimming and boosting to lower the blacks and raise the highlights by modulating the LED backlight, and it produced very vibrant, realistic images when playing back recordings of the MotoGP.
The problem, of course, is that “the range of luminance in the real world is huge,” from direct sunlight at 109 candelas per square metre (or nits) to starlight at 10-6 cd/m2.
“The typical range that the eye can resolve is in the order of magnitude of 105, with a fixed position of the iris,” although the iris can widen or narrow to cope with changing lighting. Today’s cameras can offer dynamic range of 14-15 stops, which is similar to what the eye can resolve at a fixed position of the iris. The problem, so far, has been in how to display that level of dynamic range at home.
“We use a standard dynamic range system where the capability of the transmission system, from the broadcaster to the home, doesn’t match what can be captured,” with an order of magnitude of 103, explained Sykes.
Sony’s current cameras can acquire 16-bit linear Raw images, or 10-bit S-Log (which offers similar dynamic range but at lower bitrates), to deliver good highlight handling together with good shadow detail. “Today’s transmission system doesn’t match this capability. We have, typically, an 8-bit transmission system, which is limited to 100 cd/m2.”
HDR is not just about luminance, but also colour. Within the chromaticity curve that denotes all the colours that people can perceive, current HD (Rec. 709) can only display a small portion of that, while UHD’s Rec. 2020 colour gamut can display a lot more, “although it still doesn’t match the performance of the human visual system.”
Sony now offers grading monitors that can be used to colour correct HDR images. “Until recently, this was quite a challenge,” before the release about 18 months ago of its BVM-X300 OLED monitor, which can operate in HDR and SDR, at up to 4090×2160, and has a wide colour gamut. “It doesn’t cover the whole of the 2020 colour space, but it gets closer than any other monitor,” he claimed.
“The pictures that you can get using it really are quite incredible.” It has a peak brightness of 1,000 nits (which is equivalent to an extra 3.5 stops of dynamic range).
He recommends a Raw or XAVC-based workflow to produce the production master in XAVC and S-Log, “and we recommend the use of S-Log3 in the camera for live production as well, with an HDR-HDR conversion at the point at which the customer wants to start to deliver the pictures to the viewer at home.”
Disney’s movie, Tomorrowland, released last year, was shot on the F65 and mastered in HDR. “If you want the highest quality, you can set your camera to shoot Raw, with a wide colour gamut and a great dynamic range,” and there are now several post production systems that can handle HDR, notably Blackmagic Design’s Da Vinci Resolve system, which Sony had set up in its demo suite.
Blackmagic is one of 50 Sony partners for HDR Raw production, where it is also working with companies like FilmLight and Colorfront. About 75 companies are working with Sony in its XAVC Alliance programme, including HDR (in XAVC S-Log3), which is more suitable for productions that don’t have the data capacity required for Raw.