April 4, 2023 - When using an LED video wall for in/on-camera backgrounds, virtual production or extended reality there are many variables to consider. It is important to think about the type of content and colors on screen, shooting distance and angles, lighting, reflections and more.
When deciding on a particular LED video wall, many people focus on pixel pitch—the physical distance between the centers of adjacent LED pixels—because the finer the pixel pitch the closer a person (or camera) can be to the video wall without noticing individual pixel outlines. This is an important consideration, depending on the size of the studio and director's preferred filming style and the clarity of the on-screen content.
But another specification that should not be overlooked is scan ratio. Unlike a static background, LED video walls are dynamic with millions of constantly changing and flashing LED lights. LED driver integrated circuits (ICs) control Pulse-Width Modulation (PWM) for lines of pixels as well as pixel refresh rate, telling them when and how long to be lit. The more lines of pixels one driver IC controls, the slower it is to process. Even though it all happens very quickly, it simply takes longer to illuminate 30 lines than 8. The number of lines the driver IC controls is called the scan ratio—or sometimes called scan rate or simply, scans.
In technical jargon, a display designed with a lower scan ratio has more time available to increase the LED refresh rate and/or increase the number of PWM bits—either of which will result in reduced camera artifacts, especially for low luminance content. Figure 1 at right shows an instantaneous snapshot of a 1:16 scan ratio display with only one line (out of every 16) of the LED cabinet's pixels lit. Figure 2 shows the same LED cabinet as the eye would see it. Because the scans happen so quickly, the human eye and mind piece it together.
Cameras also have shutter speeds. As much as it may appear that video cameras are recording constantly, they are like super-fast flip books, letting in light with high-speed shutter pulses that capture periods in time. When the camera shutter opens its sensors capture the LED pulses. The more lines of LEDs the driver IC has to cover, the longer it takes and the camera sensors can pick it up. For dark content where the LED refresh rate effectively decreases, some sections of the camera sensor might catch four LED pulses while other sections might catch only three pulses, resulting in artifacts that look like dark bands. These band artifacts disrupt the desired visual effect and remind the audience that what they're seeing in the background is not, in fact, real. They are not viewing aliens on a distant, red-skied planet with three moons, but a studio set in Hollywood.
To reduce these kinds of artifacts LED video walls need lower scan ratios with driver ICs that scan fewer lines, and therefore leave fewer opportunities for camera sensors to catch LED pulse variations.
We're Doing It with Planar CarbonLight CLI VX Series
It's becoming increasingly common to use LED video walls for cinematic virtual production, where it's arguably most important not to show camera artifacts. This growing market is evolving, and just as directors learn new strategies and tactics for virtually transporting audiences in movies, TV and corporate or educational video, technology needs to adjust as well. Scan ratio is one of the things that is increasingly important that didn't necessarily seem to be in the early days, so we're improving it.
Planar has been on the forefront of display technology for 40 years, pushing and expanding the boundaries of possibility to deliver visual experiences that exceed current needs. To continue this trend and deliver a camera-ready product that is even better for in/on camera, we've upgraded the camera-ready Planar® CarbonLight CLI VX 1.9 to feature a high-performance 1:8 scan ratio. Along with boosting its IC drivers so that they scan only 8 lines, shortening the time that the LEDs are powered off. We've also increased the brightness to 1,500 nits.
With these changes we expect the Planar CarbonLight CLI VX 1.9 to be an even more viable option for moviemakers who are conscious of camera artifacts and want more vibrant background content for their shots. As time goes along and on/in camera LED becomes even more prominent, we expect to discover more ways that we can upgrade our LED video displays solutions to increase their effectiveness for this unique market.