Ultra HD broadcasts demand higher frame rates
Now that people have begun in earnest to experiment with extremely high-resolution Ultra HD production techniques and workflows, the realities and unique requirements of this bandwidth-intensive, high frame rate format are beginning to take shape. Ultra high definition (currently 4K resolution) displays around 8 million pixels, compared to the 2 million in today’s 1080 HD TV pictures — for four times the detail.
Speaking with 3D Focus, Andy Quested, Chief Technologist HD & 3D at BBC Technology, said that the Ultra HDTV formats (4K and 8K) will require the use of much higher frame rates than the commonly used 24/25fps (for feature films) or 50/60fps (for television broadcast), in order to avoid “image smearing.”
“Frame rates of 24/25 will always exist but will require much more attention to the rules of 35mm movie shooting,” Quested said. “At 50/60 [fps] though, the increased temporal resolution accentuates the smearing caused by movement when the shutter is open, but make the shutter too short, and the low frame rate causes multiple images during some motion (look at what looks like a multi-arm bowler in short shutter cricket). At somewhere around 100+ fps, your eyes cease to see separate images even during high movement with short shutter.”
There is standards work being completed in the UK to address the issue: the ITU-R BT2020 has two UHD formats at 7 680 x 4 320 and 3 840 x 2 160 (16:9) at 24/1.001, 24, 25, 30/1.001, 50, 60/1.001, 60 and 120 progressive frame a second. The International Television Union has put together a technical paper asking the general production community to explore whether 120fps would work as a viable worldwide standard.
“There are thoughts it needs to be higher or there needs to be 50Hz multiples of either 100 or 150fps,” Quested said. “Tests are under way, and results may take a while to resolve into standards.”
Quested said more testing is needed to know what works and what doesn’t. This includes real-world tests, such as at the upcoming Wimbledon tennis championships, where the EBU and Sony will collaborate and experiment with different options.
When asked whether shooting in multiple frame rates like Showscan Digital or displaying certain scenes in higher frame rates (such as for 70mm IMAX presentations) was a good option, Quested was a bit cautious about its viability for television broadcasts.
“[It] would not work on TV,” he said. “It takes time for displays and receivers to change (5 to 6 seconds), and that assumes we can do it at the broadcast end.”
For theaters, assuming the projector runs at the same speed all the time but plays frames at different repeat rates, Quested said it “would work for non-live, but for live, you would have to run the chain at the same speed, and there is no saving for repeated frames. Also, we can use Long GoP, so the higher the frame rate, the more efficient. At the moment, production GoPs are around 12-15 (but we could go to 20- 22) based on a 25fps system. If we were to run at 120fps, we could run a GoP of over 100-110 and at 300fps around 280-290.”
Additional 4K production tests will be conducted next month at the 2013 FIFA Confederations Cup in Brazil. As announced at the NAB Show in Las Vegas, Sony is helping broadcast vehicle provider Telegenic build what is being called the world’s first 4K-capable OB van. Naturally, the production vehicle will include a 4K-ready Sony MVS-8000 production switcher, a set of PVM-X300 4K professional monitors and Sony’s F65 and F55 4K cameras.
UK pay-TV broadcaster Sky is now producing all of its 3D nature documentaries in 4K for cinemas, with some in 5K resolution for IMAX presentations.
Chris Johns, chief engineer for Sky’s Central Architecture Group, said the industry has also only recently agreed on a standard for High Efficiency Video Coding compression, required to distribute 4KTV via broadband and over-the-air transmission to the home.
“A lot of the challenges [with 4K] are like the early days of HD,” Johns said. “The cameras are drama cameras, so they have a very short depth of field, which is not what we need in sports. If you remember the early days of HD sports, the encoders were not very good with movement; it used to go into a blur and then snap back into detail as the camera panned and stopped —and we are seeing the same thing in 4K.”
In the U.S., most of the 4K testing (by CBS Sports, Fox Sports and MLB Network) has thus far been done on high-quality slow-motion replays for live sports production.