BRING OUT THE SUNGLASSES
Feb 1, 2001 12:00 PM, By Peter Putman, CTS
LED Displays and Ambient Light Slug It Out LEDs can provideof imaging horsepower that plasma won't ever be
able to touch. The Light Emitting Diode panel is a unique beast, combining
emissive display technology with super-large screen arrays.
IN THE COURSE OF MY RESEARCH AND testing of electronic displays, I come across all kinds of “next best things” and “the last display you'll ever need” products. Many of them never make it past the drawing board. Other designs are interminably stuck in laboratory testing with their manufacturers on an endless quest for funding.
While flat-matrix projectors and monitors tend to dominate the news, there is one alternate display technology that is well past the laboratory stage and in fact has been providing bright, contrasty big-screen images for some time now. The Light Emitting Diode panel is a unique beast, combining emissive display technology with super-large screen arrays and generally working where DLP, LCD and plasma display panels fear to tread.
The concept of creating an image using a matrix of emissive devices isn't new — light bulbs have been arranged in rows and columns to create words and pictures for several decades. Of course, conventional bulbs are impractical for today's electronic display requirements as they dissipate too much heat and their switching times are too slow for video and other fast-refresh image sources.
LEDs, on the other hand, can be cycled on and off at extremely high speeds — certainly fast enough to accommodate a 60Hz video rate. They're also quite small and are available in red, green and blue, the primary building blocks of additive color.
However, their round construction poses a bit of a quandary: How do you design an electronic display with this shape? Is it possible to pack LEDs together tightly enough to create a unified field of pixels without excessive spacing between individual pixels?
GETTING THE LEDS OUT
To solve this mystery, we need to think outside the box and look at another, similar imaging system: the printer's dot screen. If you inspect a magazine photograph with a powerful magnifying glass, you'll notice that the photo is actually made up of thousands of tiny dots. Viewed in extreme close-up, the dots appear to be just that — a collection of dots.
A funny thing happens as you increase the viewing distance. The dots seem to disappear, and all you see is a continuous-tone photograph! At some point, your eyes can no longer resolve the individual dots (pixels), and the construction of the photograph (display) is not apparent — you can only see the reproduced image.
That's exactly how an LED display works. Depending on the manufacturer, LEDs are arranged in clusters, with each cluster containing red, green and blue LEDs. One such cluster forms a pixel (some actually resemble a die with the “5” side showing), and thousands of these pixels are joined in stripes to build a huge electronic display. Stand far enough away from this aggregation of LEDs, and you see a smooth image with little of the pixel elements evident.
Because the pixel pitch (physical size of an RGB LED array) is fairly large compared to flat-matrix displays, the viewing distances must be greater to minimize the apparent structure. Typical LED pixel pitches can range from as small as 10mm to as large as 20mm, which is quite coarse for close-up viewing. In contrast, even the largest plasma display panels have a dot pitch of just over 1mm, while a typical 27-inch TV set has a dot pitch of 0.8mm on center.
The NTSC rule of thumb specifies that for viewing a standard TV, we'd want to be positioned at seven times the screen height so as not to notice the scan lines. That “7-times rule” was based on a 20-inch diagonal TV set. Assume a 0.8mm dot pitch for a well-designed consumer TV set, and we can establish a ratio of 31.75 dots per inch, or 381 dots vertically in a 20-inch diagonal picture (picture height is 12 inches).
Now, let's do some interpolation for a comparable LED display with a dot pitch of 10mm and a specified resolution of 100 lines per meter. To watch 525-line video, we'd need a display measuring about 17 feet tall. Using the NTSC 7-times viewing distance guideline, we'd want to be positioned almost 120 feet away from the display for it to appear “normal!”
Selecting LED arrays with a 4mm dot pitch — the smallest currently available — would result in a display 40% the size, or around 7 feet. Even so, we'd still want to be positioned about 50 feet from the display so the scan lines in an NTSC image are unnoticeable.
As you can see, LED displays are best suited for large venues, such as concert halls, arenas and stadiums. And that's exactly where the majority of them wind up, but there's another reason for these site choices: image brightness.
BRIGHT LITTLE BULBS
Typical brightness specifications for LED walls are several orders of magnitude higher than conventional direct-view or even projection video monitors. Let's assume you'd want to build a videowall using rear-projection cubes that would result in a brightness reading of 25 foot-lamberts, suitable for an indoor atrium mall.
Using some quick math (25×3.42) gives us a reading of 85.5
candelas per square meter. Since one cd/m
In contrast, a typical LED wall can produce from 700 to 1,000 nits, or 10 times the luminance. That works out to 200 to 300 foot-lamberts, which is a bright image in anyone's book. These high levels of brightness make LED displays the most logical choice for imaging under full daylight, which can measure in excess of 1,000 to 2,000 nits indirectly and well over 5,000 nits directly.
Some models of LED walls are capable of over 3,000
cd/m
LED displays are always constructed from blocks of LED pixels, so there's no limit to how large they can be, or what shape they may take. LED walls have been constructed in conventional squares and rectangles, and even long and narrow shapes such as the RavenVision electronic displays at PSINet Stadium in Baltimore, each of which measures about 25 feet tall by 100 feet wide.
As far as signal resolution, the majority of LED walls that I've seen process all signals as progressive-scan VGA (essentially, a 640×480 60Hz signal). This means that all sources are up- or down-converted to a 31.5kHz rate, and I have yet to see a LED display that can achieve higher refresh rates than 60Hz. Is this a drawback? Not really, as we aren't looking for HDTV quality so much as we want super-bright images.
LED walls are staking their own place in the permanent install and staging/rental markets. Three major manufacturers of LED displays are Barco of Kennesaw, Georgia; Lighthouse Technologies of Cary, North Carolina, and Saco Smartvision of White Plains, New York.
Barco. A relative newcomer to the industry, Barco gained entry through their purchase of Dr. Seufert GmbH and that company's LED wall technology. Recently, Barco worked with San Francisco Outdoor TV (formally known as Video Board Ventures) to install a 9'×15' Barco DLite 7 Daylight Display LED videowall at Pier 39 in the Fisherman's Wharf section. San Francisco Outdoor TV features programming on the videowall that is of interest to tourists, including segments on the city's history and unique attractions.
The DLite 7 wall has a fine pixel pitch of 7mm and is specified to have 5,000 nits light output with extremely wide viewing angles. It features built-in intelligence that enables auto-configuration and allows hot swapping of LED tiles without interrupting the display of the pictures. The videowall is mounted approximately 14 feet above the ground, which allows pedestrians to easily view the board from any angle.
Lighthouse Technologies. Lighthouse manufactures a range of walls with varying dot pitch specifications. Their LVP-100 arrays are available in 32-inch diagonal sizes with 100 lines of pixels per meter. These particular panels have a 10mm dot pitch, can be used to build high-resolution walls and other geometric shapes for indoor use, and are rated at 1,000 nits of brightness.
Bigger sizes are available as well — the LVP50 is an outdoor panel with somewhat coarser resolution. These panels measure 63 inches diagonally and have 50 lines per meter of resolution with a 20mm pixel pitch and 6,000 nits brightness. Lighthouse's LVP800 panels measure 56 inches diagonally and offer 83 lines per panel with a 12mm pitch, served up at 5,000 nits.
Of course, you have to take brightness readings with a grain of salt. Any electronic display is capable of putting out more “peak” energy than what you'd probably use under normal circumstances, and LEDs certainly can be run at lower power to extend their life cycles. I've seen at least one LED display in Times Square, New York, have a complete meltdown, attributable to running the panel at excessively high power!
MVI Display in Toronto makes extensive use of Lighthouse LVP833 12mm indoor/outdoor LED walls for staging, with a Snell and Wilcox Supervisor doing the scaling of interlaced and progressive-scan signals to standard 640×480 VGA at 31.5kHz. This signal is then fed to the Lighthouse Interface Processor, a high-quality analog-to-digital converter.
The LIP features a comprehensive menu structure that allows MVI to tailor the output levels, color temperature, gamma and screen sizing. The LIP outputs digital information via parallel connections to a Lighthouse Serializer, which converts the parallel data into a serial form for transmission of data to the LED screens on a single 75-ohm cable over long distances.
How about power requirements? “We are using serious power here,” says producer Phil Cane of MVI. “It has been quite a leap from specifying 10 to 15 amp outlets for a large videowall, to asking for a 200-amp, 3-phase service to run the LED system. Our actual experience has shown that the system does not demand that sort of power 90% of the time. When we are doing indoor jobs, quite often we run the LED system at an incredibly low 400 nits (out of a possible 5,000). Power consumption is usually never more than 15 amps at 120 volts per panel in those applications.”
Cane adds, “We have used temporary power diesel generators on many of our outdoor shows, with no problems. When we are in a studio or theater, the venue is so used to their lighting department asking for 800 amps that our requests seem small.”
Constructing a LED wall has become routine for MVI. “We get four panels out of their cases and align them on the ground, underneath whatever will be used to lift them up — usually chainmotors,” said Cane. “Four bumpers are then mounted onto the panels by dropping them onto the pins that protrude from the top of each panel. Once the bumpers are connected, the 1×4 rows of panels are bolted together with a socket wrench. The first row is then lifted with the chainmotors, and a second set of four panels is brought in. They are lined up underneath the first row, which is dropped back down to rest on the new row. A quick insertion of coupling pins, and we fly the first two completed rows out of the way. The third and fourth rows happen just as quickly.
“As we build, we daisy-chain the data cable, connect the power and address each panel. By using this method, two people can have a 4×4 LED system flown in less than 30 minutes — a far cry from our previous videowall builds.”
Saco Smartvision. These people have been very active with permanent installs of their LED walls. Saco also manufactures a wide variety of LED arrays, from 4 to 15mm dot pitch in two different pixel configurations. Last September, they finished the installation of two stadium LED walls at Ohio State University in Columbus, Ohio, and the University of Arkansas in Fayetteville, Arkansas.
The OSU wall measures 30 feet high by 90 feet wide and employs 30mm Smartvision LED panels. It was constructed as a series of 112 sections or profiles, each measuring 30 feet high by 7 inches wide, weighing 300 pounds each. Each profile was lifted over the back of the stadium and slid into place by a pulley system since the football field playing surface had already been finished and the crane was not permitted on the turf.
Ohio State's new video board has the capability to show live game action, as many as three different games at one time during pre-game, replays and a multitude of statistics. The display features Saco's new 10-bit all-digital video processor and a color palette of over 1.07 billion colors. The length of this LED display is equal to that of a basketball court, three continuous end zones, or 675 27-inch television sets, and it's perched 100 feet high between the towers at the south-end entrance.
As big as the Buckeyes' LED wall is, it's topped by the Razorbacks installation at the renovated Donald W. Reynolds Razorback Stadium — this one measures 30 feet high by 107 feet wide and uses 25mm Smartvision7 LED video panels. The display features Saco's recently introduced SV Pro Digital Processor, a 10-bit all-digital system that provides a color palette of over 1.07 billion colors.
This monster LED wall was constructed as a series of 168 factory assembled and pre-wired sections measuring 3 by 4 feet. As with the OSU wall, the playing surface had already been finished and a crane was not permitted on the field. So, the sections were lifted over the back of the stadium in 6'×4' pieces by two cranes on the parking lot and slid into position on the display face. The center of the screen was installed first, and then two installation crews worked outward to complete the installation.
Saco's LED walls have also found their way into more modest installations, such as the Armani Gallery in Milan. This unique application turns the walls into a dynamic video gallery, featuring short films produced by a variety of artists. In this environment, the closest competitor for a LED display would have been plasma display panels, but the screen sizes chosen at Armani were considerably larger than the biggest available plasma panels. By using LEDs, the designers made a trade-off of coarser resolution for a big, bright, seamless image.
As the pro A/V and staging markets embrace flat-panel imaging, 2001 will be a big growth year for LED displays (which most people had never heard of just a few years ago!). Like plasma panels, they are emissive displays. The big difference between plasma and LED panels is this: If an LED segment burns out, it can easily be replaced.
In contrast, an entire plasma substrate must be swapped out to replace any burned or defective pixels. Plasma offers much higher resolution and more realistic looking video, but LEDs can provide a level of imaging horsepower that plasma won't ever be able to touch.
Look for more seminars and displays of LED walls and panels at INFOCOMM 2001. The next step? More advanced signal processing, possibly to run panels at higher scan rates (wouldn't it be neat to see 1080i and 720p HDTV on a 16:9 LED wall?). If LED pixel pitch sizes can be dropped down to 1mm (that would be difficult technically), they could compete head-to-head with plasma panels. Stay tuned!
Peter Putman owns PHP Communications, Doylestown, Pennsylvania. He is the author of The Toastmasters Guide to Audio/Visual Presentations and reviews large-screen displays and computer/video interfaces.
FOR MORE INFORMATION
BARCO Projection Systems
America
www.barco.com
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Lighthouse Technologies
www.lighthouse-tech.com
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Saco Smartvision
www.sacousa.com
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