The Best Thing About 4K & UHD Isn't More Resolution: Sayonara Interlacing, Hello Wider Color Gamut
As some have speculated, the recent push for 4K/UHD may have as much to do with hype as it does with quality. And, as has been stated time and time again: you may not get a huge benefit from 4K in your home, depending on viewing distance and screen size. There are some other factors, however, that make ‘Ultra HD’ technology desirable, regardless of clarity so crisp you can’t even tell how crisp it really is. These factors are the other important goals defined in the ITU-R’s (aptly dubbed) Rec. 2020 spec for 4K/UHD. Namely, they are (larger) color space and (progressive-only) frame rate.
The official spec is properly called ITU-R Recommendation BT.2020, and it suggests a few much-needed updates to the seemingly still-pretty-damn-new but actually-quite-aged Rec. 709 (first approved in the early 90′s) for HDTV. It’s worth pointing out that Geoffrey Morrison highlighted these benefits previously at CNET — it’s also worth highlighting them specifically for filmmakers.
No More Interlacing — Progressive-Only Frame Rates
I don’t know about you guys, but I really hate interlacing. I’m admittedly kind of ‘interlacist.’ To me, interlacing represents one of the originally quite ingenious, now entirely vestigial hold-overs from analog broadcast. Along with non-integer frame rates, interlacing fills a need for video that we actually don’t even have anymore. After all, you’re not reading this, or more importantly watching movies, on a tube. Probably. (Anyone continuing to use CRTs industrially will always be able to PSF-ify and/or cross-convert to their needs). This quote is from a 2004 article by Nick Radlo:
Yves Faroudja founded Faroudja Laboratories, and has spent years inventing ways to improve picture quality in broadcast TV, winning a Lifetime Achievement Emmy in 1998 for his work. “Why are we still talking about using interlace? This is something I’ve been dealing with for decades and I can’t believe we’re still considering it in the 21st century,” he said. “I’m a complete enemy of interlace. There are some things in an interlaced picture that you just can’t de-interlace. When TV began it was a very simple way of reducing bandwidth, but now it is not needed. I’m very depressed when every ten years I see people about to make the same mistakes.”
Let the depression be lifted (again, hopefully). So says Rec. 2020, in a bold, progressive (whoa-hoa!) move towards the future: no more fields. 4K/UHD should be progressive-scan only. Quoth the spec:
Picture temporal characteristics: Frame frequency (Hz) 120, 60, 60/1.001, 50, 30, 30/1.001, 25, 24, 24/1.001. Scan mode: Progressive.
Note the specific division by 1.001 in 60, 30, and 24 — this denotes support for the far more common non-integer 59.94, 29.97, and 23.976 rates we’re accustomed to. Also note support for 120 Hz imaging… in a sense, Doug Trumbull foresaw the benefits of a 120-based future with his digital Showscan technology. The good thing about 120 is that pretty much everything divides into that rate evenly — 30, 60, and yes, 24. This is partially already in play with 120 Hz sets, but media actually delivered in 120 (for whatever reason) means the sets wouldn’t have to perform that great ‘motion smoothing’ trick they do.
The problem with phasing out legacy technology (i.e., shooting true 24 over 23.976 fps) is always backwards compatibility — which creates a bit of a conundrum, because you want to shoot something that, ideally, anybody can eventually watch, even on their aging home system. Conforming true 24 to 23.976 and vice-versa are solutions, but not ideal. So when are digital cinematographers to be finally rid of these redundant fractional frame rates as well? It’s a bit of a vicious cycle, in which cameras and display technology may continue to be stuck for some time. Rec. 2020 is, at least, suggesting a big step in the right direction. Speaking of which…
Larger Color Space — A Way, Way Bigger Triangle
Rec. 2020 identifies D65 as the white point of its color space, located at [0.3127, 0.3290], the same as in Rec. 709. The difference is the available gamut — and the fact that Rec. 709 is limited to 8-bit depth, whereas Rec. 2020 supports 10- and 12-bit depth. Rec. 709 defines its outer color limits at R[0.64, 0.33], G[0.30, 0.60], and B[0.15, 0.06], keeping in mind that “Picture information can be linearly indicated by the tristimulus values of RGB in the range of 0-1.” Rec. 2020, on the other hand, sets its boundaries much, much farther apart, defining R[0.708, 0.292], G[0.170, 0.797], B[0.131, 0.046]. This apparently equates to a 75.8% coverage of the CIE 1931 color space compared to Rec. 709′s 35.9% coverage of it. What all this looks like, when graphed, is this:
One of our commenters pointed something out, in Joe’s recent post on ALEXA’s somewhat-begrudged evolution to 4K: that this – an expanded gamut — is what ARRI should really be concerned with, or even excited about. Geoffrey Morrison generally agrees, though he expressed concern that this would be the most difficult part of Rec. 2020 to achieve across the board, citing potential difficulties (or expenses) in developing display technology. And, of course, there’s the ubiquitous question of backwards-compatibility. Might Rec. 709-shot material display all out-of-whack on a Rec. 2020-compliant UHDTV? Will the propositions of Rec. 2020 be universally embraced, with display manufacturers forming a ‘unified front’ for such improvements? If so, how long will it take for the technology to catch up — while still being affordable?
As Morrison states: “Unfortunately, it’s hard to say what of the current version of Rec. 2020 will survive to future revisions, and when, if ever, any of these revisions may come to fruition.” I certainly hope they that they do. These definitions stand only to improve the palettes of digital filmmakers, or allow them to image in ways they’ve always wanted to. Despite the concerns, I think there’s only one way to upgrade: bite the bullet, accept backwards compatibility risks (where not completely self-defeating), and push onward toward a bigger, brighter, more vividly colored future.
What do you guys think? What negative implications, should these specs take hold, have I missed? What about the benefits?
- Rec. 2020 — ITU-R
- Rec. 2020 — Wikipedia
- Rec. 709 — ITU-R
- Rec. 709 — Wikipedia
- “Ultra HD 4K and beyond: Rec. 2020 glimpses the future of TVs” — CNET
- “‘Super Hi-Vision’ as Next-Generation Television and Its Video Parameters” — InformationDisplay.com
- IBC 2004 Report — EBU.ch