SDR and HDR Consumer Display Standards

The below article contains excerpts from The Display Calibration Guide.

SDR Standards

Introduction to SDR Standards

Standards are important because we know what to profile against and what to calibrate to.

However, as we discussed earlier, while SDR standards were created in a way that were achievable for all displays, HDR standards were created for displays that have not yet been created. Hence the need for tone-mapping, whether static or dynamic.

There are many strategies to calibrate both SDR and HDR, each balancing aspects the calibrator deems important. When the display isn’t quite behaving as expected, certain image quality aspects must be sacrificed to improve others that are more critical.

While this balancing act is as much art as it is science, we will try and discuss how, when and what to look out for when doing this balancing to ensure we keep errors and artefacts either below the perceptible threshold or choose the deviation where it won’t be as objectionable to the viewers.

Should you always calibrate to standard (for SDR)?

When it comes to SDR calibration, the answer is absolutely yes, as all displays on the market – with the exception of some single-chip DLP projectors – can achieve the full standard.

However, please note that while flat panels are normally calibrated to output 100nits of brightness at the minimum for SDR, projectors might only be calibrated to half that. This is normally adequate due to projection happening in a dark room with the lights off. Since our eyes’ sensitivity increases in the dark, and we are using a much bigger screen normally than with a flat panel, this is adequate.

I don’t quite count the very cheap LED projectors that are on the market in this camp however. Their quality varies greatly and many do not have adequate white or colour brightness or adequate picture controls to achieve reference SDR calibration.

We will cover this same question for HDR calibration, where the answer isn’t as clear-cut.

PAL / SECAM / NTSC / REC.601

PAL, SECAM and NTSC are the standards of standard- definition broadcast TV and DVD discs.

Sony has a full list of countries with their standards at the following address: https://www.sony.com/electronics/ support/articles/00006701

However, that list is not quite complete or correct, as some countries used both PAL and SECAM concurrently which is shown on the globe diagram included on Wikipedia.

PAL / SECAM / NTSC were standard definition interlaced signals that were designed for CRT TVs. PAL and SECAM uses a very similar gamut, while NTSC’s analogue gamut allows for wider colours.

Due to the mis-match in frame-rate between these standards and that of firm (24 frames per second or 24 fps), films for PAL / SECAM encoding were sped up from 24fps to 25fps causing movies to have a shorter run-time (and ever-so-slightly higher pitch if not pitch-corrected!), while movies transferred to NTSC used 3:2 pulldown: repeating the first frame 3x, the second frame 2x, the third frame 3x and so on to achieve 30fps.

High-end DVD players were able to reverse the 3:2 pulldown to create a 24fps video signal as well as re-assemble interlaced frames to display movies at 24p or 25p (progressive) for NTSC or PAL DVDs respectively.

The REC.601 standard formalised the digital encoding of PAL and NTSC signals for digital broadcast and storage (DVDs). Of critical note here is that the NTSC gamut for digital signals is not the same as the original NTSC analogue broadcast standard. The original NTSC gamut was much larger than the NTSC gamut formalised in REC.601. We will however deal with REC.601 standard only from here on out.

BT.709

The official standard of HDTV, BT.709 is also called REC.709 and ITU.709 according to the relevant standards body – as the standard was developed in collaboration.

The standard still uses the gamma standard developed for SDR content as part of REC.601.

The table below details the valid resolutions and frame rates for BT.709.

The fractional rates are for compatibility for legacy NTSC pull- down rates. BT.709 has support for both interlaced (framerates with the i designation) as well as progressive signals (the p designation).

The interlaced signals are there for backwards compatibility and for reduction of broadcast bandwidth, especially when initial HDTV broadcasts used the MPEG-2 compression system – the same as used for DVD. However, broadcast TV moved to progressive signals where MPEG-4 compression was implemented as this allowed for a sharp reduction of bitrates without relying on interlacing.

From a calibration point of view, the BT.709 system still uses the D65 whitepoint, but defines the gamut differently than the PAL/NTSC BT.601 digital standard. The gamut is wider than both but matches the PAL standard for greens more closely while matching the NTSC standard more closely for blue and red saturations – having taken the best of both standards.

The exact chromaticity coordinates used are at the end of this article.

SDR BT.2020

While BT.2020 was developed as the gamut for Ultra-High- Definition TV (UHDTV) along with HDR, the standard does include the possibility of using the BT.2020 gamut in an SDR container with a normal gamma encoding. This has two purposes:

1. Backwards compatibility with displays that don’t support HDR
2. Ability to do tone-mapping of HDR content on the source device as opposed to the display by tone-mapping to a standard SDR gamma

SDR BT.2020 is therefore calibrated to an SDR gamma of 2.2, 2.4 or BT.1886 depending on the viewing environment and the source but using the BT2020 chromaticity coordinates (which you’ll find at the end of this article.)

It is important to note here that mapping HDR BT.2020 signals to an SDR BT.2020 signal by simply lowering the luminance of colours – but not the saturation – doesn’t really create the perceptually correct tone for the content. This is why Dolby Vision – for example – employs algorithms that work with psycho-visual principles, not only mathematical. Normally, a reduction of luminance also requires a reduction of saturation for perceptual equivalence. So SDR BT.2020 is useful as a container to do dynamic

Other SDR Standards

1. DCI (Digital Cinema Initiative) includes the P3 gamut and a higher gamma (2.6) than consumer formats prior to it (e.g. Blu Ray.) The only reason you would calibrate to DCI in your home is if you were mastering content for a cinema release or you had a device that is able to tone-map HDR content to this standard. There are no consumer- accessible formats or players for DCI in the home.
2. There are other gamuts that are used on PCs such as sRGB and Adobe RGB. If you are calibrating a display for photo editing, these may be more appropriate.

While the above formats are outside the scope of this guide, you are able to calibrate for them – and in fact any other current or future formats – using the same concepts you learn here. As long as you know the colour coordinates, luminance and gamma / EOTF requirements, you will be able to calibrate to the format.

HDR Standards

Introduction to HDR Standards

Standards are important because we know what to profile against and what to calibrate to.

However, as we discussed earlier, while SDR standards were created in a way that were achievable for all displays, HDR standards were created for displays that have not yet been created. Hence the need for tone-mapping, whether static or dynamic.

There are many strategies to calibrate both SDR and HDR, each balancing aspects the calibrator deems important. When the display isn’t quite behaving as expected, certain image quality aspects must be sacrificed to improve others that are more critical.

While this balancing act is as much art as it is science, we will try and discuss how, when and what to look out for when doing this balancing to ensure we keep errors and artefacts either below the perceptible threshold or choose the deviation where it won’t be as objectionable to the viewers.

Should you always calibrate to standard (for HDR)?

For SDR, the answer to this question was relatively straight-forward. I promised a twist for HDR, however. This is because no consumer display can meet the full standard currently.

Until that happens, there are various ways that we can calibrate a display for HDR dependent on the priorities we have. For example, to reach a display’s maximum luminance, you may decide to forgo ultimate accuracy for the purpose of reaching high light output, however you may do this only for your gaming settings, but not for critical viewing such as movies.

Another prime example of this is Sony versus other display manufacturers. Initially, Sony insisted on calibrating HDR to the letter – but set up even consumer displays as you would a mastering monitor. This is actually not a great way to achieve the director’s intent as it cuts important picture information in certain cases. It is therefore a better idea to tone map both the high and low end. Unfortunately, tone mapping was not part of the original HDR standard.

So this is not black and white and should not be treated as such until once again all consumer displays meet the latest standards to the letter.

BT.2100 – HDR10

HDR10 is the standard developed for UHDTV and it is part of the BT.2100 standard (also referred to as ITU 2100 / REC.2100) along with HLG, which we will get to in a moment.

HDR10 uses the Perceptual Quantiser function for its EOTF – as opposed to gamma like we are used to in SDR content. As noted earlier, PQ is not a relative standard – but absolute. That is, it ultimately requires each luminance level encoded in the signal to be played back at the encoded luminance level exactly.

Unfortunately, as I referred to this earlier, this is an ill-conceived notion (refer to the EOTF section in the previous chapter for more on why this is).

PQ is calibrated using the gamma function of the display but requires a different workflow, which we will look at under calibration.

The BT.2100 standard definition is in the table below. You will find the chromaticity coordinates for BT.2020 at the end of this article.

UHDTV only includes progressive frame rates, so interlaced signals have been dropped from the standard. UHD content is normally encoded in HEVC (h.265) codec, but MPEG-4 (AVC / h.264) and the new VVC (h.266 / MPEG-I Part 3) can also be used.

Since consumer displays are unable to reach the standard fully due to their limited dynamic range – and limited gamut – there are certain strategies that have been developed to mitigate this issue. Let’s look at these in turn:

Static Mapping Strategies:

1. Limited Gamut: the BT2020 gamut is mapped to the display’s native gamut so that colours within the P3 gamut are prioritised. Please note that the P3 gamut is NOT part of the standard, but we can map to BT2020/P3 which is the P3 gamut defined using BT2020 coordinates / encoding. The display will still see BT2020 as the signal and will interpret it as such.
2. Limited Max Luminance: to deal with limited max luminance, the following strategies can be used:
   • The PQ curve is tone-mapped at the top end.
   • The PQ curve is hard-clipped at the maximum the display is capable of (or near it). This is normally done at 1000, 2000 or 4000nits.
   • In addition, the PQ curve can be tone-mapped across the whole range using a multiplier function for displays with very limited luminance (e.g. projectors) by either using tone-mapping at the topfo the range or doing a hard-clip.
3. Limited Min Luminance: the PQ curve was designed with varying black level setting (dependent on the mastering display’s capability). This creates a bit of a mess when trying to emulate this on the playback display and really only looks correct on a display with absolute black (OLED). Displays will generally tone-map the lower-end of the scale, however, this creates issues with lower-luminance displays such as projectors. My recommendation here is to do a hybrid approach whereby we introduce SOME clipping while also trying to tone map the rest. More on this in the calibration section.

Dynamic Mapping Strategies:

Dynamic Tone Mapping will be discussed in a separate article.

BT.2100 – HLG

Hybrid Log Gamma – or HLG for short – is also part of the BT.2100 standard and provides an option for EOTF instead of HDR10’s PQ.

HLG uses a hybrid gamma system whereby the lower portion of the gamma scale includes an SDR signal and the upper range includes the highlight signal above SDR luminance levels.

HLG – while not backwards compatible with HDTV in its strictest sense – is compatible with SDR displays in that an HLG signal will play on older displays – albeit looking darker and with desaturated colour.

However, the main upside to HLG is that it still uses the gamma system with its benefits: gamma is a relative system and can be tailored to the viewing environment.

Another advantage of HLG is the lack of near-black gamma issues and different black floor of the mastered content that is present with PQ.

HLG is calibrated with the gamma function of the display and uses the BT.2020 gamut.

Also, HLG does not need dynamic meta-data due to it being mastered as scene-referred, as opposed to display-referred. I won’t expand on this point more as it would require a quick course in content mastering – not in scope of this guide. If you are interested in this topic, it can be an area of further research.

My humble opinion is that HLG was designed by some very clever engineers who thought long and hard about the standard and came up with an incredibly elegant solution to the problem. While HDR10 required lots of amendments to right its wrongs. I was therefore not fond of HDR10’s mathematical approach to the problem from the get-go as it ignored 70+ years of research and wisdom.

Chromaticity Coordinates

The following tables detail the coordinates and Y (luminance) multiplier values for the different standards. Ultimately, these are embedded in the calibration software but it is a good idea to have these handy in case you need them.

The Y value for white is always 1 (100%) and the rest of the colours are calculated as a fraction of the measured Y value. You can multiply the measured Y value for white with the Y value in these tables for the particular colour to arrive at the Y target. This is what all software do as well and hence why you need the 100% white patch measured before anything else when dealing with the CMS.

REC 709 (HDTV):

• Red: x=0.640 / y=0.330 / Y=0.2126
• Green: x=0.300 / y=0.600 / Y=0.7152
• Blue: x=0.150 / y=0.060 / Y=0.0722 Y
• ellow: x=0.419 / y=0.505 / Y=0.9278
• Cyan: x=0.225 / y=0.329 / Y=0.7874
• Magenta: x=0.321 / y=0.154 / Y=0.2848
• White: x=0.3127 / y=0.3290 / Y=1.0000

SMPTE-C / REC 601 (SDTV):

• Red: x=0.630 / y=0.340 / Y=0.2124
• Green: x=0.310 / y=0.595 / Y=0.7011
• Blue: x=0.155 / y=0.070 / Y=0.0866
• Yellow: x=0.421 / y=0.507 / Y=0.9134
• Cyan: x=0.231 / y=0.326 / Y=0.7876
• Magenta: x=0.314 / y=0.161 / Y=0.2989
• White: x=0.3127 / y=0.3290 / Y=1.0000

EBU (PAL/SECAM):

• Red: x=0.640 / y=0.330 / Y=0.2220
• Green: x=0.290 / y=0.600 / Y=0.7076
• Blue: x=0.150 / y=0.060 / Y=0.0713
• Yellow: x=0.417 / y=0.502 / Y=0.9287
• Cyan: x=0.220 / y=0.329 / Y=0.7780
• Magenta: x=0.327 / y=0.157 / Y=0.2933
• White: x=0.3127 / y=0.3290 / Y=1.0000

Rec. 2020 (UHD):

• Red: x=0.708 / y=0.292 / Y=0.2627
• Green: x=0.170 / y=0.797 / Y=0.678
• Blue: x=0.131 / y=0.046 / Y=0.0593
• Yellow: x=0.446 / y=0.345 / Y=0.9407
• Cyan: x=0.146 / y=0.345 / Y=0.7373
• Magenta: x=0.368 / y=0.147 / Y=0.322
• White: x=0.3127 / y=0.3290 / Y=1.0000

The Display Calibration Guide

If you would like to learn more about displays, and display calibration, you can get The Display calibration Guide here.