BBC R&D

Posted by Andrew Cotton on , last updated

Many of today’s High Dynamic Range consumer televisions are capable of showing stunning HDR images. But they offer a huge range of picture adjustments, many of which need to be carefully set to ensure that the TV delivers the very best picture quality. In this blog I discuss the most important of those settings, and I also explain why HDR images can often appear dimmer than standard dynamic range images (SDR) shown on the same TV.

Software Update

Before adjusting your TV we suggest that you check that the device’s software is fully up to date.  The TV manual or manufacturer’s website should tell you how to do this.

HDR Picture Brightness

Many viewers have commented that the HDR pictures seen on their TV at home are dimmer than the equivalent SDR pictures.  The is a good reason for that which I will discuss below, but thanks to the ability of the eye to adapt, it does not necessarily mean that those dimmer HDR pictures will lead to a poor experience.

The peak luminance of a TV display, measured in candelas per square meter (cd/m² or “nits”) is determined by the capabilities of the TV’s display electronics.  If an HDR TV has a fixed peak luminance of, for example 500 cd/m², in standard dynamic range mode white objects within the scene may be displayed at up to that maximum of 500 cd/m² – more than enough for daytime viewing in a bright living room.

However, when that same display is switched into HDR mode, at least two stops of the luminance range must be reserved for those specular highlights (brighter than white) that give the HDR images their sparkle – one stop is a doubling in light output.  So those same white objects might only be shown at around 120 cd/m² when the TV switches to HDR mode, to allow those highlights to be up to four times brighter.

The nominal signal level for white objects, known as “HDR Reference White”, is specified by the ITU-R for HDR TV production, in ITU-R report BT.2408.  A value of 75% signal is recommend for HLG (Hybrid Log-Gamma) – the HDR system used by the BBC. The graph below shows how bright a 75% signal (HDR Reference White) appears on different peak luminance displays.

Chart showing HLG HDR Reference White on Different Displays

To put that in context, a professional SDR display, intended for a dim viewing environment, has a peak luminance of around 100 cd/m².   So, from the graph, you will need an HDR display capable of around 400 cd/m² peak luminance, to show white objects at a similar brightness. Whilst that HDR image may appear spectacular in the dim environment, it may be difficult to view in much brighter environments.

Although no longer sold, a plasma TV is usually well regarded, and has a peak luminance of around 200 cd/m².  They work well in most domestic viewing environments.  So, once more from the graph, for an HDR display to match the brightness of a plasma TV it needs to have a peak luminance of around 1000 cd/m².

The confusion arises because modern HDR TVs often show SDR pictures at substantially higher luminance levels than we have been used to.  It is not uncommon for an HDR TV to show SDR pictures at anything between 300 and 500 cd/m² peak luminance – very bright compared with even a plasma TV of a few years ago.   To achieve that sort of brightness for HDR images, the peak luminance of an HDR TV will need to be in the region of 2000 cd/m².  There are a few models that bright today, but not very many.

It’s important to remember that peak luminance is not the only factor that determines a good HDR display.  Other factors such as colour reproduction and contrast ratio are perhaps more important.  I do most of my viewing in the evening and have a shady living room, so image brightness was not top of my list when I bought an HDR TV.  But if you do most of your critical viewing in a brightly lit room in the daytime, then a display’s peak luminance is certainly something you might like to consider.

The UHD Alliance defines two classes of displays that qualify for their Ultra HD Premium™ logo.  As well as requirements on colour reproduction, the logo requires black level and peak luminance from either a minimum of 0.05 - 1000 cd/m² or 0.0005 – 540 cd/m².  The logo does not, however, guarantee compatibility with the BBC’s UHD iPlayer services.  The full list of iPlayer compatible devices can be found here.

Of course, picture processing can be applied within the TV itself to increase the subjective brightness of HDR images on dimmer displays.  These picture modes are often called “HDR Bright”, “Movie Bright” or sometimes referred to as “Dynamic Contrast”.  In a bright viewing environment they can be very effective, but the processing may compromise the impact of specular highlights.  So they should only be used when necessary.

BBC R&D - High Dynamic Range and Hybrid Log-Gamma

BBC iPlayer - UHD Compatibility on BBC iPlayer


Test Material

The BBC HD Test Card

In order to set-up your TV for HDR viewing it’s important to use a good HLG test image.  When we launched the BBC’s HD Trial service back in 2006, we included an HD Test Card as part of the channel’s promo loop, and instructions on how to use it. We are in the process of producing a similar test image for HLG HDR, but it’s not ready just yet. (We will add an update here when this is done.)  So, in the meantime, a good source of HDR test material is the UHD HDR test loop, available in BBC iPlayer Beta, on UHD compatible devices.  Details of how to enable the iPlayer Beta features can be found here.

BBC R&D - World Cup 2018 in UHD HDR on BBC iPlayer

BBC News - Wimbledon tennis to be screened in 4K HDR

BBC R&D - The Royal Wedding in High Dynamic Range

BBC R&D - Blue Planet II in UHD & HDR on BBC iPlayer


HDMI Settings

If you are accessing the iPlayer UHD test loop using the TV’s built-in iPlayer app, there’s no need to worry about your HDMI settings.

However if you are accessing the iPlayer UHD content via a set-top box or HDMI dongle, it is essential that you ensure that the HDMI input you are using is correctly configured for wide colour gamut (BT.2020/BT.2100) UHD HDR.  If the TV is left in standard colour (BT.709) mode, the HDR images will look pale and washed-out.  We have seen reports that suggest this could be happening. Take care as:

  • Some TVs only support wide colour gamut and UHD HDR on a sub-set of the available HDMI inputs, and surprisingly that subset does not always include HDMI input 1.  So check your TV manual to make sure that you are using a compatible (HDMI 2.0b) input.
  • By default, even on compatible inputs, the UHD HDR capability is often disabled.  So check your TV’s input settings and the manual to ensure that “UHD Deep Colour” (or similar) is enabled for the input you will be using.

Sometimes pressing the “info” button (or similar) on the TV remote control when the HDMI input is active will confirm the correct BT.2020 wide colour gamut input setting, but not always.


HDR Picture Settings

HDR televisions usually only offer the HDR picture adjustments when displaying HDR pictures.  So, it is important to ensure the that the iPlayer HDR test loop is actually playing before you start.  You may not be offered the correct HDR picture settings before then.

The most accurate picture settings mode is usually called “Movie” or “Cinema” mode. So that’s a good place to start, before fine tuning the settings.  It’s also a good idea to reset these settings to the factory default, as most of the settings will then usually be about right.  The reset option is often offered as the last item in the picture settings menu.


Basic Settings

Having reset the picture settings, start by checking the most common picture adjustments.  They may have a slightly different name on your make and model of TV.

Backlight/OLED Brightness

The backlight control adjusts the brightness of the LED backlight, and hence the peak luminance of the display.  OLED TVs offer a similar setting, sometimes called “OLED Brightness”.  Unless you have one of the very brightest HDR TVs on the market, and are watching that in a dark room, set this control to maximum.

Brightness

The picture brightness control (sometimes more accurately called “black level”) applies a luminance offset to the displayed picture.  By doing so, detail in the shadows of the picture are lifted, so that they are just visible above any scattered light reflected by the screen from the viewing environment.  The default setting is usually correct for the majority of viewing environments; but if you are in a very bright room you may need to increase the brightness setting to compensate for the higher levels of reflected light.  Conversely, if you are in a very dark room, you may need to reduce the brightness setting to ensure that the blacks in the image appear as deep as they should be.

Contrast

The contrast control adjusts how much of display’s peak luminance capability (determined by the “LCD Backlight” or “OLED Light” control) is exploited by the signal.  The default setting usually ensures that the nominal peak signal level corresponds to the display’s peak luminance. Increasing the contrast above the default setting may cause specular highlights in the HDR image to be clipped.  Reducing the contrast setting will reduce the overall image brightness.

We usually recommend leaving this control in its default position to get the most out of the HDR display, without distorting the image through clipping.

Colour

This setting determines the colour saturation of the displayed picture.  The default setting in “Movie” or “Cinema” mode is most likely correct.

However, because of the way SDR TV works, SDR pictures tend to appear more colourful than nature.  This increase in colour saturation was useful when TV images were shown on dim CRTs, as the eye is less sensitive to colour at low luminance levels.  So over the years, even as displays have got brighter, we have grown accustomed to saturated SDR pictures.

HLG pictures on the other hand are, by design, a more faithful representation of the light falling on the camera sensor, and are remarkably natural in appearance.   Most viewers grow to appreciate the lifelike colours of HLG HDR.  But if you would prefer to match the colourful SDR pictures, you may wish to increase the colour saturation above the default setting.

Colour Temperature/Colour Tone/Colour Balance

Many televisions have a slightly blue tint, as that can give the impression of subjectively brighter pictures.  In the “Cinema” or “Movie” modes, however, the “Warm” or “Warm 2” setting will usually deliver a more accurate TV picture, where the whites are at roughly D65 (6500 K) – the average midday light in Europe.

BBC R&D - A Major Milestone for HDR TV

BBC R&D - 2014 Ultra-High Definition Trials

BBC R&D - Defining the Future of Television


Advanced Settings

Dynamic Contrast/Contrast Enhancer

These settings apply picture processing to try to enhance the appearance of HDR images in brighter viewing environments.  If you have a sufficiently bright display for your viewing environment, they should not be necessary and should be set to “low” or disabled completely.  In a brighter environment, or with a dim HDR display, they may be more useful.

LED Local Dimming/SMART LED or similar

In order to deliver the high luminance levels necessary for HDR TV, whilst complying with energy efficiency legislation, LCD TV manufacturers adjust the luminance of the backlight dynamically.  Thus, power saved in darker areas of the picture can be used to boost the backlight luminance in brighter parts of the picture. The local dimming control adjusts how aggressively power is redistributed from darker to brighter parts of the picture.  A higher setting may increase the peak luminance of the brightest parts of the image, but at the risk of reducing the luminance of other important parts of the picture.  The default setting is probably best for most content, but do experiment if you are not happy with the overall appearance of your HDR pictures.

HLG Gamma (where available)

The end-to-end opto-optical transfer function (describing the relationship between the light falling on the camera sensor and the light emitted by a display) is a gamma law for both SDR and HLG HDR TV.  That is to say, the light produced by a display is proportional to the light in the scene in front of the camera, raised to a power, known as “gamma”. Written mathematically that is:

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The gamma law compensates for the changes in the response of the human visual system as the eye adapts.  For HLG, it is implemented as part of the display processing.

The adaptation state of the eye is determined by both the viewing environment and the peak luminance of the display.  Most, but not all TV manufacturers, will ship a TV with an appropriate gamma setting for that peak luminance display in a dim viewing environment.  If, however, pictures are to be viewed in a brighter environment, a lower value of display gamma is required.  The same is true of SDR TV, and several manufacturers also allow the SDR gamma to be adjusted.

So, when viewing HDR images in a bright environment, reduce the HLG gamma setting if you can.  Confusingly that usually means sliding the control to the right – the picture should get brighter.  When viewing HDR images in a dark environment, increase the gamma by sliding the control to the left – the picture should get dimmer.

More details of the HLG gamma adjustments can be found in Section 6.2 of ITU-R Report BT.2390.

Noise Reduction

In a modern broadcast chain noise is rarely a problem.   So all noise reduction should be switched off, to keep the pictures as sharp as possible.

Motion Compensated Frame Insertion (MCFI)/Motion Flow/Motion Smoothing or similar

MCFI processing creates additional frames and uses those to help smooth the motion within a scene.  Most TV creatives loathe it, because it gives their TV dramas and documentaries (deliberately shot at 25 frames per second (fps) in the in the film style) a similar look to a soap opera shot at 50 fps.

We usually recommend switching off all MCFI processing.  However, on the very brightest HDR screens, the motion judder associated with 25 fps production becomes more visible, and can annoy some viewers.

The UHD Test Loop that’s currently available in iPlayer Beta is football recorded at 50 fps, so that should not suffer from any motion judder.  If, however, you are one of those people who does find 25 fps motion judder on HDR content annoying, you will need to find some TV drama, a documentary or film on an SDR service (usually all shot at 25 fps); experiment with the MCFI settings to find one that adds just a small amount of judder reduction; write down the settings and copy them across to the MCFI section in your HDR picture mode settings.


No doubt your TV offers a whole host of other settings that I’ve not discussed in this blog.  But hopefully I have covered the main settings and lessons learnt from the BBC’s current UHD Wimbledon and UHD World Cup trials to ensure that you get the very best out of your HDR TV and any future UHD trials.

 
 
 

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UHD Production Training and Skills from the BBC Academy including:

UHD production: What do I need to know?

4K and Ultra-HD: Making Programmes Podcast

What does the future hold for UHD?

This post is part of the Broadcast and Connected Systems section

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