BBC claims to solve live HDR production puzzle
The BBC says it has found the answer to one of the problems bedevilling the industry’s transition to live HDR production — that of not compromising the standard dynamic range signal.
It has developed a means of ensuring that the SDR content derived from an HDR workflow is consistent in terms of colour and light values, for onward distribution. This ‘scene-light format conversion’ underwent a large-scale test which proved its value at the Royal Wedding last month.
“Simple scene-light format conversion might at first seem like a small step towards completing the HDR production eco-system,” said BBC R&D Principal Technologist, Andrew Cotton. “In fact, the problem of ensuring near identical colours in SDR content derived from an HDR workflow, compared with those from an SDR production workflow, is one that the whole industry has been trying to solve. It has so far proved a block towards the widespread adoption HDR live production.”
The BBC has been trailing live UHD HDR streaming on iPlayer for some time, notably from the York City Knights and Catalans Dragons rugby match in April, working with OB providers to develop live HLG HDR production workflows for BBC use.
According to the BBC, the challenges are much harder for live production than non-live production, which is now well understood. Not only is there just one chance to get things right with live but live production must also handle a whole host of different sources (e.g. graphics, HDR cameras, SDR cameras, pre-recorded inserts, slow-motion replays), all available in different signal formats, and blend them together seamlessly into a single programme.”
That might sound straightforward with HDR format conversion technology now widely available, but in fact it’s fraught with difficulty. The BBC has been working within the ITU-R’s Rapporteur Group to document current best practice for HDR TV production.
Last October their findings were published in report BT.2408 [https://www.itu.int/pub/R-REP-BT.2408] which drew on the experience and expertise of broadcast engineers, colour scientists and colourists from around the world. As well as specifying signal parameters, camera line-up levels and methods for SDR/HDR format conversion, it documents BBC R&D’s findings on monitoring image brightness to ensure comfortable viewing of HDR images in the home, along with its work adapting the HLG ‘system gamma’ to ensure the highest image consistency outside of the reference viewing environment, and in the most recent revision [https://www.itu.int/pub/R-REP-BT.2408-1-2018] its work on signal levels for skin tones.
These were all put into practice for the UHD HDR production (though not transmission) of the Royal Wedding.
For the event, NEP UK configured 76 UHD cameras (including three UHD radio cameras) in wide colour gamut, BT.2100 Hybrid Log-Gamma (HLG) HDR. An important requirement was that the UHD HLG HDR signal had to be converted to conventional SDR BT.709 for onward distribution.
“It needed to look identical to the SDR signal that would be available from a conventional SDR camera,” explains Cotton. “The SDR signal could not be compromised in any way through having been derived from the HLG HDR signal.”
Display light conversion
According to Cotton, that type of conversion has proved particularly difficult to achieve. That’s because, he says, the majority of HDR down-converters on the market, including the BBC’s own licensed down-conversions, are based on ‘display-light’ conversion technology.
“That means the conversions calculate the light emitted by a reference monitor being fed the input signal, and then convert that signal to one that would cause exactly the same light to be emitted by a reference monitor operating in the desired output signal format,” said Cotton.
They may apply some adjustments, for example to make an SDR image appear brighter when shown on an HDR display, but the principle remains the same. By doing so, a ‘display-light’ conversion maintains the artistic ‘look’ of the original production format when converting the signal to the new format.
“Display-light conversions work well for non-live graded programmes, but they are not generally suitable for live,” explained Cotton.
Since live workflows frequently need to convert between SDR and the HDR production formats, whichever is being used, this presents a problem because each format yields a different ‘look’.
“By an accident of its design, SDR images tend to be more colourful than nature,” said Cotton. “That was a useful feature when we viewed them on dim CRT displays, as the eye is less sensitive to colour at low luminance levels.”
A good example of this could be seen at the Royal Wedding where in SDR, the walls of Windsor Castle appeared to be built of a yellowish sandstone, rather than the real paler Clipsham limestone.
“By contrast the HLG HDR images are, by design, remarkably natural in appearance,” said Cotton. “We no longer need that colour boost as HDR images are intended to be viewed on more modern brighter displays, and the way in which the natural ‘look’ of HLG is achieved makes it easier to deliver consistent looking pictures across a range of displays of different peak luminance.”
So, converting between SDR BT.709 and HLG using a conventional display-light technology, that preserves the look of the original format, does not necessarily deliver what’s required for live production.
The BBC says that a display-light conversion of a BT.709 camera to HLG “would look more saturated than a native HLG camera when cutting between them”. Similarly, a display light conversion from an HLG HDR camera to BT.709 “would tend to look less saturated than the SDR signal from a camera pointing at the same scene.”
The differences arise because each format has a different relationship between the light in the scene falling on the camera sensor, and the light emitted by the display – known as the OOTF (opto-optical transfer function). Similar problems are encountered using display-light conversions with other HDR production formats.
What was needed instead reckoned the BBC, are ‘scene-light’ format conversions, where those issues can be avoided.
HDR handling by UHD cameras
To understand the process further, Cotton outlines the HDR handling characteristics of UHD cameras. Per Cotton: Many cameras offer two simultaneous outputs – one HDR and another BT.709. The linear light signal from the camera sensor is considered to be normalised in the range zero to one. In practice other normalisations might be used to achieve the desired signal levels and noise performance.
The linear light signal from the sensor is passed through the HDR camera OETF to convert it to the HDR electrical signal for output. The camera exposure is adjusted for the HDR output, and the SDR signal is derived by applying a fixed gain to the linear scene-light from the sensor, followed by a conventional BT.709 OETF.
SDR cameras are usually adjusted so that, under controlled lighting, the SDR signal hits full-range for a 90% reflectance test chart i.e. a card that reflects 90 percent of the light falling on it. In practice, under varying lighting conditions such as those found at an outside broadcast, the cameras are adjusted for correct exposure of skin tones, but they are roughly equivalent setups.
The ITU-R BT.2408 recommends that the same 90 percent reflectance card should deliver an HLG signal of around 73 percent. By working through the mathematics, it can be found that applying a gain of 4.193 (12.4 dB) to the linear scene light signal before applying the SDR BT.709 OETF, ensures that both SDR and HDR signals are correctly exposed. “In practice other gain adjustments may be included in the camera channels,” said Cotton. “We found that we had to dial-in a gain offset of 9.0 dB on the cameras that we were using to achieve the desired exposures.”
For the Royal Wedding, BBC R&D needed to take the HLG HDR signal and convert it to SDR BT.709 after the UHD production mixer (or switcher). To do this, the RGB HLG signal is passed through an HLG inverse OETF to recreate the linear scene-light signal produced by the camera sensor. The scene-light signal is then passed through the same SDR processing chain found in the camera – a scaling of 4.193, followed by a clipping stage and finally the standard BT.709 OETF.
The HLG to SDR conversion could apply more sophisticated tone-mapping from HDR to SDR and colour management.
The final results from the scene-light conversion were so good, reports the BBC, that its vision engineers were able to shade cameras as normal, using SDR monitoring and the SDR output from the CCUs.
“They knew from our earlier tests that the exposure of the HDR signals would track the iris adjustments made for SDR and deliver spectacular HDR images,” said Cotton. “On the day, they were sufficiently confident with the conversion that it was left to just me and my R&D colleague Simon Thompson to monitor the HDR, and the subsequent down-conversion to SDR BT.709.”
Cotton reports small differences “were just perceptible” when comparing the SDR conversion and native SDR from the camera CCU side-by-side, due to minor differences between the HDR and SDR camera channels.
“However, one was not necessarily better than the other, and the vision supervisors were very happy with the results.”
So much so that the HLG to SDR down-conversion was the signal source for the international 1080i SDR feed, reaching an estimated audience of 1.9 billion viewers worldwide.
The BBC believe this new workflow, “greatly simplifies HDR production without compromising quality” adding; “Now that the solution has been shown to work so well, and on such a large scale, it says “we are hopeful that it will enable a significant increase in HLG HDR television production.”