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HDR – You take the High road!

David Wood, DVB chair, Commercial Module, UHDTV

To confuse a Scottish folk song; “You take the ‘High’ road, and you’ll be a commercial success before me!” High Dynamic Range – HDR – is seen by many as the key to success for UHD-1. It will improve the image quality as much again as the jump from HDTV to 2160p resolution, and will shine out at all screen-to-viewer distances. The specification for HDR should be out later this year, ready for UHD-1 broadcasts from 2017. But the path to get here has not been, as a Scots might say, a ‘walk in the Glen’.

DVB has been working on the specification for UHD-1 broadcasting and broadband delivery for several years. The work done in the ITU five years ago brought forth a specification for the parameter values for UHDTV production – Rec. 2020. Just as the specification was leaving the door of the ITU in 2012, came the cry that, in order to cope with the TV sets of the future with higher peak screen brightnesses, a better ‘transfer curve’ was also needed to take advantage of the higher peak brightness.

This would mean images with more detail in the ‘blacks’ and in the ‘whites’ – giving images more ‘sparkle’. Many were the documents and meetings before agreement could finally be reached in the ITU.

There was white smoke for ITU-R BT. 2100 in July 2016. It recommends not one but two HDR systems, termed HLG (Hybrid Log Gamma) and PQ (Perceptual Quantizer). This applies to programme production and exchange, but the DVB delivery standard needs to have a relationship to this.

The DVB discussion on the requirements for broadcasting UHD-1 also took time, but these were agreed in autumn 2015. Now the stage was set for the discussion about which parameter values would fulfil
the requirements.

UHD-1 delivery is relatively complex because different broadcasters and manufacturers have different constraints on the timescale for services, and the time to develop the consumer decoders depends on their complexity. DVB needed to find formulae that would allow the different service timescales and the decoder availability to tie up.

A ‘Phased’ approach was necessary: a Phase 1 for those who needed to start in 2015, Phase 2 CPA (Conformance Point A) for those who wish to start in 2017, and Phase 2 CPB (Conformance Point B) for those who plan to start in 2019. With each new Phase, the option of new features would be available. Phase 1 was essentially just the 2160p system.

Phase 2 CPA would also add HDR. Phase 2 CPB would add the option of High Frame Rate as well. For the moment, let’s focus on CPA.

Phase 2 CPA would be used in two environments. In one, the broadcast would need to be receivable on TV sets without HDR capability as well as sets with it – the HDR system needs to be ‘backwards compatible’ (BC). In the other environment, the service would only be
for sets with HDR – (Non BC or NBC). The spec wanted should include both BC and NBC options.

This has precipitated five proposals for the HDR system. One is the (directly BC) HLG system. The other four make use of the PQ curve, with variations on the way the signal is carried – allowing options of backwards compatibility. The task now for the DVB is to examine the proposals and decide which should be included in the DVB specification. This should be agreed later in the year. It is by no means an easy decision.

Stand by, because IBC will be the forum for many a debate on HDR. The addition of HDR will be a vital step for UHD-1 – we just need that critical decision on the HDR system to be taken. The next few months will be exciting. Would a large Scotch help?