Broadcast & AV

The Case for 4K

Steve Lampen

About ten years ago, I was attending a talk given by the famous cinematographer Vilmos Zsigmond (Close Encounters of the Third Kind, Deer Hunter, Heaven's Gate, Deliverance and on and on). He was recently voted one of the Ten Most Influential Cinematographers in history by the International Cinematographers Guild. Someone asked him when he would switch from film to video. His response was direct: "When 8K video arrives." This might be the first time I ever heard the term '8K' mentioned in public. I barely knew what he was talking about. But now we're almost there. We're working hard on 4K, and 8K is next up to bat. At a standard TV viewing distance, the resolution of 8K is better than the human eye, so this might be a good place to stop for a while and allow the rest of the world to catch up. However, we're deep into getting to 4K first.

"Broadcast" and "Hollywood" 4k

The jump from 3G (2K) to UHD or 4K is not trivial. Yes, to make sure you're confused enough, there are two kinds of 4K. The first is the "broadcast" version, with a resolution of 3840x2160. (You will note this is 2 x 1920.) This has a clock frequency of 10.6921 GHz. The second kind of 4K is "Hollywood 4K" with a resolution of 4096x2160 and a clock of 11.88 GHz. Since 12 GHz covers both clock frequencies, that's our goal, 12 GHz. Only a few years ago, this was considered serious microwave territory, when one would use waveguide to get signals from Point A to Point B. This is one of the reasons that most copper-based 4K equipment uses dual-link (2 cables) or quad-link (4 cables) to carry a 4K signal. Of course, the bandwidth on each cable is less on multiple cables. It's just more complicated to install. What you will really want would be single-link 4K (12 GHz) coax. 

The Nyquist Limit

In all digital signals, the sampling limit (Nyquist limit) is half of the clock. Harry Nyquist discovered in the 1920's, working on high speed telegraphy, that in a digital system, when you pass that half-way point, you begin to generate "artifacts", signals that are not real. So every digital signal puts a hard filter at half the clock. In the past, we have tested cables to the third harmonic of the Nyquist Limit (also called the "occupied bandwidth"). The table below shows this history.




However, while we can go to 12 GHz at 75 ohms, nobody can go to 18 GHz at 75 ohms. (And 8K will require 24 GHz at 75 ohms and even higher.) We know we can make single-link coax out to 12 GHz, but there is no test equipment available (even custom made) that will go to 18 GHz at 75 ohms. (If you're a test gear manufacturer and I speak in error, boy, would we like to talk to you!) Network Analyzers are no problem. Ours can go way past 18 GHz, but that's at 50 ohms. We did talk to a few test equipment manufacturers at the past NAB and they said that 18 GHz was "possible" but they were talking about 5 years of development. And I can only imagine what these custom-built pieces would cost.


And even if we only test these new 4K cables to 12 GHz, that is no walk in the park. One thing we used to be able to do was to look at a reel of cable at 3 GHz, and see the return loss. Return Loss sends a pulse down a cable and looks at the reflections coming back. Those reflections are directly related to the impedance of the cable. The less precise it is at 75 ohms, the more the reflections. So we send a pulse down a finished roll of cable and see what comes back. We actually had a guarantee that the reflections would be no worse than -23 dB (5 MHz to 850 MHz), and -21 dB (from 850 MHz to 3 GHz.) We could easily see 500 ft. But we couldn't see 1,000 ft., so we did the test from each end of the 1000 ft. roll. When we went to 4.5 GHz, that squeaked by at 500 ft. This meant we could barely look at a reel from each end at 4.5 GHz and see all 1,000 ft. But our guarantee improved to -23 dB (5 MHz to 1.6 GHz) and -21 dB (from 1.6 GHz to 4.5 GHz.)

I can guarantee you now that we can't see 500 ft. at 12 GHz. So, instead, we are now depending on the testing we do during the processing of the cable. We have laser micrometers measuring dimensions as the wire goes in the extruder and again as the core comes out. We can also read the capacitance of the cable, even as it is cooling down. Because of all these measurements, we can mathematically predict the impedance, and the return loss, at the same time. Some of these measurements are taken 1,000 times a second. The results are fed to a master computer than controls the extruder. This gives us precision and consistency that no human-run machine could ever do. This also means that longer runs of cable (like 2,000 ft. or 5,000 ft.) can be measured for their entire length. We could only see 500 ft. with the old method. Now we can see every foot.


Future of 4k

And 4K single link coax will really open up a number of options. Hollywood is already firmly attached to 4K. This resolution approaches that for modern 35mm negative film and, in some instances, can surpass it. (8K will definitely pass 35mm film and possibly even 70mm film. That's why Vilmos Zsigmond called out that resolution as mentioned above.) That means that shooting something like a TV series or episodic television in 4096x2160 4K will allow that content to be repurposed for theatrical release at no loss in quality. It will give you extended markets to sell your content over and over. There are a lot of TV properties that shoot in 4K, but then edit and release in HD, since that editing and distribution is relatively cheap compared to working in 4K for the entire chain. And, when 4K editing and distribution drops in price, you can go back and use your original 4K image for those new markets.


Of course, there's more to an image than just resolution. And by the time we're at 8K, we'll probably be in 30bit//36bit/48bit "deep color" and high frame rates (like 120 frames) and the bandwidth of that signal will be just under 100 GHz. Then it would be a great time for some new technology, such as carbon nanotubes or room-temperature super conductors, to arrive. If you hear any of those arriving, would you let me know so I can call my stockbroker. Want to comment? Fill out the card below, or send me an email.