Special Report: 3D Video in Pro AV
In part one of a two-part special report, our video expert details how 3D video works and which technologies work best depending on need.
THE BEST WAYS TO SHOW 3D
What’s the best means of delivering the 3D effect? As AV pros know, there are a number of display technologies in use todayfrom direct-view displays, such as monitors and televisions, to projection displays, such as front and rear projectors.
We can further break down display categories by imaging method used. The first group is emissive displays, where a burst of light energy is viewed directly. The emissive category includes plasma display panels, cathode-ray tubes (CRTs), organic light-emitting diodes (OLEDs), conventional light-emitting diodes (LEDs), and lasers.
Of these technologies, only plasma is in widespread use today. CRTs are being phased out and OLEDs are still a few years away from mass production. LEDs, such as those used in large-scale digital signage, are not practical to use in 3D applications. Lasers can create the required parallax and are capable of fast switching times, but isolating the left and right eye information is difficult.
Transmissive displays, also known as light-shuttering displays, include LCDs of all shapes and sizes, including large TFT LCDs used in monitors and televisions and smaller high-temperature polysilicon LCDs used in front projectors.
Finally, there are reflective displays, including liquid crystal on silicon (LCoS) and DLP. These technologies are popular for showing 3D movies.
For active-shutter 3D applications, emissive displays such as plasma are generally the best choice. They can switch quickly enough between states to keep up with the rapid refresh rates required for active-shutter 3D viewing. The limited number of 3D tests using OLEDs show they too can keep up without modification. Emissive displays always have the widest viewing angles, as well as consistent black levels, brightness, contrast, and color of gray over all viewing angles.
Transmissive displays, specifically LCDs, require the liquid crystals to move from one position to another in a short period of time in order to show video at rates of 60 frames per second or faster. This is not easy to do; many LCD monitors and TVs exhibit motion-blur problems, and techniques such as black-frame insertion are used to minimize the effect. Current evidence shows that frame rates of at least 240 Hz are required to show fast motion in 3D with a minimal level of blur—a tall order for current LCD technology.
That said, large LCD monitors have one advantage: They are much brighter than plasma, which is helpful for overcoming light loss caused by wearing 3D polarized glasses. LCDs have also been equipped with circular-polarized microfilters for direct-passive 3D viewing through inexpensive glasses, a solution favored by 3D content producers.
Projectors that use 3LCD technology face similar limitations and (for now) are better suited to showing passive 3D images through two stacked and precisely aligned projectors that are equipped with linear or circular polarizers.
When it comes to reflective displays, DLP currently has the upper hand. DLP can switch its tiny mirrors thousands of times per second with no motion lag or smearing, making it the preferred choice for 3D digital cinema. Variations on the same chip are used in single-chip and three-chip projectors for home, business, education, and large venue applications. DLP technology is also found in rear-projection TVs and projection cubes.
LCoS panels can be used for 3D, but their switching times are similar to those of LCDs and they have a harder time showing active-shutter 3D. Like LCDs, LCoS projectors are better suited to passive 3D applications, operating in aligned stacks and equipped with linear or circular polarizers.
Whichever way you plan to build a 3D display system, your work is far from done. You also need to ensure that the application (visualization, engineering, education) actually benefits from 3D imagery. and that the content and signaling infrastructure working in the background matches the display technology you’ve chosen.
Fortunately, there are industry standards groups coalescing around ways of delivering 3D content using methods such as frame-packed, side-by-side, and top-bottom encoding. But best of all, there are, indeed, pro AV applications of 3D technology available today. Stay tuned.