Mark Hill 4K UHD analysis: it’s not just about the number of pixels, is it?

If you are reading this article, writes Mark Hill, you may already be part of a project that has moved well beyond the ‘if’ question for 4k. Indeed, there is no point in trying to suggest that 4k will not happen. It’s happening already. The first wave of TVs are in the shops and professionals are notching up ‘proof of content’ outings with each passing week.

With the pop-up shop that was 3D still in mind though, a more relevant question is: will 4k stop happening, or follow-through to become something worthwhile and profitable for those involved?

Of course, 3D rumbles on in movie theatres and may yet make a return to TV screens in the next decade, or so. What was an amber light of 3D TV — cost of production — quickly became the red light that killed it. Arguably, it could reasonably have been predicted that this would become so. 4k has a number of amber lights already shining, however the same degree of inevitability that one or more of these will turn to red does not yet seem to exist.

So, given that it’s not a matter of ‘if, what about the ‘when’? The history of TV might be viewed as an ongoing series of rolling cycles of improvement: black and white to colour; analogue to digital; SD to HD. What we can say for sure is that the trend is for such cycles to become shorter and, of course, we don’t have to wait until the one is complete before starting the next!

Counting channels currently available over air in Europe might indicate something like a 70/30 percent SD/HD split. Counting hours of content captured/produced in these formats might, however, give completely the opposite ratio! Taking the average of 50/50 from these two counts, would seem more than sufficient justification to start preparing the ground technically for 4k as the next cycle, plus the same advantages exist for using 4k technology to oversample the capture and viewing of HD as existed for the early use of HD kit in what was once a mainstream, SD-only world.

Content-wise, early 4k channels are likely to be orientated around movies and (fortunately for us!) sport. With a massive library of finished content already in the can (much of it literally), and with film makers having so far given a thumbs down to frame rates higher than 24, the former represents the quickest and easiest win for new services.

Sport is clearly the next most appealing target, but also the most demanding technically. Consumer expectations for immediacy and quality is highest with this genre and so any standards must provide for live and pre-recorded content and give proper support for interoperability around high-quality and timely post production. It is difficult to have a movie or sport service without interstitials content, therefore our 4k ecosystem must support production and integration of high volumes of this short-form content too.

Towards settled standards

In Europe, the root of future standards for 4k TV is ITU recommendation ITU-R BT.2020. This short document summarises key preferred technical parameters for both 4k and 8k TV (known as UHD-1 and UHD-2, respectively). While the Japanese broadcaster, NHK, is striding purposefully straight for 8k, and is intending to deliver it into the home in time for the 2020 Olympics, it seems most unlikely that European broadcasters will follow suit. Instead the vibe in Europe is that an incremental move to 4k would be more appropriate as there are sufficient challenges here to keep the industry on its toes for the remainder of this decade.

For 4k (and subtlety different to the 4k of the D-cinema/film industry) BT.2020 defines the TV frame as 3840 (H) x 2160 (V) pixels. No surprises there and a sensible (integer) times two in both axes from where we are with current HD.

The first real point of debate between interest groups comes with frame rate. High(er) Frame Rate (HFR) could be the key ingredient to the ‘wow’ factor of UHD, even more so than pixels, as the latter is not as readily appreciated. When choosing frame rate, we need to deceive the brain in two respects. Firstly, we should choose a high enough rate to avoid flicker. Secondly, we could also take the opportunity to choose a rate high enough to eliminate motion blur.

For the former, we are not that far off already at 60 Frame Per Second (FPS), as research indicates that 80 FPS and up would be totally safe. For us to be safe from the negative effects of motion blur however could require as high as 300 FPS, depending on the type of motion — tracked or un-tracked — the brain is being challenged to perceive.

Supporting such high frame rates (and these for x1 speed pictures!) has major implications for sport, where use of slow- and super-slow motion footage is also common. More frames per second means more data generated, more bandwidth used in storage and transmission systems and, as a result, more time taken to process video and get it from A to B.

BT. 2020 proposes the figure of 120 FPS as a compromise to the motion blur debate. For Europe, below that, a figure of 100 FPS might have been expected, but this is absent from the current recommendation. Existing, well-trodden, frame rates based around 24, 25 and 30 also feature in the recommendation.

Bizarrely, so too do the frame rates dreamt up to fix an issue with the 60-year old US NTSC analogue colour television system — will we never learn! The presence in the recommendation of the lower order, current crop of frame rates will suggest to many that a first step into UHD can be made without this contribution to the wow factor.

It should be noted that debate regarding refresh rates of consumer displays should be kept well separated from that concerning the capture/production system frame rate. Display refresh rate (already well above 100 Hz on many models of TV), does nothing to fix a lack of dynamic resolution at the point of content capture.

Colourimetry and signal bit depth

BT.2020 proposes an expanded colour gamut over the current standard for HD (Rec. 709), but retains the same white point (D65) definition as for HD. Content produced to Rec. 709 colourimetry — perhaps some legacy HD inserts into your new masterpiece, or a sports programme shot in HD shown in its entirety on your UHD channel — will be backwards compatible.

Of course, this compatibility also leaves the door somewhat open for Rec. 709 colourimetry to be used for new UHD content. BT.2020 seeks to encourage improvement on today’s defacto use of 10-bit video signals in TV production, with defined parameters for 12-bit working. Note that this addition scarcely begins to address the developing agenda for High(er) Dynamic Range (HDR) working in TV, another potential contributor to the wow factor.

So, having started this piece by stating that 4k is not just about the pixels, the BT.2020 recommendation has signalled the way for a ‘stage 0’ implementation of UHD at 4k to feature this aspect as essentially the only necessary change. Will we be able to do better before the first channel launches? Watch this space!