VIDEOWALLS: stay the course
May 1, 2000 12:00 PM, Peter H. Putman
Videowalls hold a unique position in the world of electronic displays. Theydo not command attention at trade shows like the latest ultraportables,high-brightness large-venue projectors, or super-slim plasma monitors.Videowalls quietly chug along, making slow, steady advancements in displayresolution, mass and imaging engines.
Perhaps that is because it is hard to improve on the original concept of avideowall - a self-contained video display that works under virtually anylighting conditions, is easily transported and can be stacked to createbigger images. There is no complicated projection throw to calculate, andoutboard digital signal processors make it possible to achieve multi-imagescreen effects.
The first videowalls appeared in the 1970s and were simply stacked CRTvideo monitors using rudimentary switchers and distribution amps to send asingle channel of video to individual monitors and clusters of monitors. Itdid not take long for the first effects processors to come along in the1980s, making it possible to compress and expand several channels of videosimultaneously. Projection monitor cubes were introduced later in the1980s, allowing for larger individual screen sizes, brighter images andthinner mullions (the dividers between imaging screens).
Over the years, videowalls have incorporated progressive-scan displays,higher resolution and flat-matrix imaging. They were the first dynamicsigns, long before manufacturers of plasma display panels made theexpression popular. Videowalls are a particular favorite of the retailmarket, driven by such national chains as the Warner Brothers Studio andDisney stores.
Videowalls are also mainstays in other permanent exhibits. Arenas use themas eye-catching exhibits to promote upcoming sporting events and concerts.Airports have them strategically placed to catch the eye of arrivingpassengers and steer them to a particular hotel or restaurant. Command andcontrol centers use them as a space-saving alternative to rear-projectiontechnology, while financial institutions have found videowalls ideal forproviding multiple screens of real-time data.
Even so, videowalls represent a niche market dominated by a few players. Atpresent, the majority of projection cubes (CRT direct-view monitors arebecoming passe) are manufactured by Pioneer (CRT only), Electrosonic (CRTand DLP), Synelec (CRT and DLP), Toshiba (CRT and DLP) and Clarity (LCD andDLP). Newcomers like Mitsubishi are jumping into the game to take advantageof high-resolution DLP imaging chips.
Perhaps the biggest shift has been away from rear-projection CRT imaging toDigital Light Processing. DLP projection cubes have made it possible to cutthe weight of 40 inch (1 m) and 50 inch (1.3 m) rear-projection cubes inhalf while reducing footprints to less than 30 inches (762 mm). Thesechanges are in direct response to the needs of the retail marketplace wherethe size and weight of older CRT videowalls precluded their use in manystores. It is now possible to obtain individual cubes with 800 infinity 600pixel (SVGA) resolution with a 40 inch diagonal picture size, and 50 inchto 67 inch (1.7 m) cubes with 1,024 infinity 768-pixel (XGA) resolution.While projector manufacturers slowly edge up in pixel count, videowallsalready have more than enough pixels for any application, and the reason issimple.
Videowall processors make it possible to spread the pixels (or scan lines)of an incoming signal over several cubes. Consider a common videowallmatrix - two cubes vertically by two cubes horizontally. This 2 infinity 2stack of 50 inch CRT cubes has an effective resolution of 960 lines perpixel top to bottom and more than 1,200 lines per pixel horizontally. Bydividing an incoming signal into fourths (and using progressive-scaninputs), an XGA computer signal can be shown at its full resolution whilean SXGA (1,280 infinity 1,024) will be clipped just a little. A 1,280infinity 720p HDTV signal will appear letterboxed at its full height andwidth with no pixel compression.
The result is a high-resolution image with a diagonal size of 100 inches(2.5 m) that occupies about 15 square ft (1.4 square m) and will producemore than 25 foot-Lamberts of screen brightness at a high contrast (morethan 150:1). That would be hard to equal with a front-projection system ina large, open atrium and impossible in a confined retail store with aislespiled high with merchandise.
Although there have not been many demos of HDTV on videowalls to date, thecombination is a natural. By using a long, rectangular stack of cubes (3infinity 4 is ideal because the effective aspect ratio becomes 9 infinity16), a true 1,920 infinity 1,080 HD signal can be shown without pixelcompression. At the correct viewing distance (about 18 feet or 5.5 m for a3 infinity 4 stack of 40 inch cubes), the illusion is complete.
Oddly enough, most of the retail and arena markets choose to feed plain,old-fashioned NTSC interlaced video to their walls. The scan line artifactsand jaggies so commonly associated with extreme close-up views of NTSC donot seem to matter much because the average viewer spends only a fewminutes watching the programming before moving on.
As a result, the majority of projection cubes sold into these markets aregarden-variety 15.75 kHz interlaced displays with RGBS component inputs,although some manufacturers now incorporate S-Video and even compositeconnections. The RGBS input standard, virtually unused anywhere else withinterlaced monitors, is a relic carried over from early analog video signalprocessors.
The financial and command and control markets are a different matter. Here,the emphasis is on matching computer and workstation displays, soprogressive-scan cubes with VGA and higher resolution are preferred.Viewers may spend several hours a day watching these walls, soprogressive-scan imaging is necessary to reduce eye fatigue and preserveimage detail. Some of the sources feeding these walls are workstations with1,280 infinity 1,024-pixel to 1,600 infinity 1,200-pixel displays.
Two more issues in cube design are brightness and color uniformity. Withsuper-thin mullions and small image detail, seamless transitions betweenindividual cubes are critical in data installations. The goal is to achievean image with uniformity characteristics equivalent to that from a singleprojector, and it is not always easy to pull off.
Brightness and color uniformity mismatches are not always apparent toviewers of walls with fast-moving video content, and we have the currentgeneration of professional film transfer colorists to thank for that. Somecommercials that play on retail walls have such dominant shades of greensand blues that individual cubes could be mismatched by a 1,000 degreesKelvin, and nine out of 10 potential customers would not see anything amiss.
Has videowall technology hit a plateau? In terms of resolution, it probablyhas. Although there is no reason that a 1,280 infinity 1,024 DMD enginecould not be fitted to a projection cube (and that will probably happen inthe coming year), it does not bring any more benefits to the table as partof a wall; there are just not enough high-resolution signal sourcesavailable to take full advantage of all those pixels.
As far as brightness goes, flat-matrix projection cubes already exceed 30foot-Lamberts (about 100 candelas/m square), which is more than enoughillumination for an indoor display under normal room lighting. Imagecontrast on some projection cubes exceeds 200:1, which is sufficient fortypical wall applications.
How about mass? Clever projection mirror designs have made it possible toproduce 50 inch CRT cubes that require less than 30 inches of depth and DLPand LCD cubes with footprints as small as 16 inches (406 mm). Flat-matriximaging has cut weights to less than 100 pounds (45 kg) for some 40 inchdesigns. It is hard to imagine much more can be shaved off at this point intime.
What will the next big advancement be in videowall technology? The guesshere is that plasma display panels (PDPs) will grab the headlines if andwhen engineers figure out a way to solve the contrary demands ofmaintaining structural rigidity of the delicate glass element whilereducing the width of individual PDP frames to a thin mullion.
Unlike videowall projection cubes, plasma displays are undergoing hugeleaps in performance every six months. Although they are still not asbright as an LCD or DLP projection cube (about 50% of the way there),plasma panels already have one big advantage - wider viewing angles (up to160 degrees) with no hot-spot problems. It will not be an easy task, but Iwill bet that a 3 infinity 3 wall of 50 inch plasma panels with 1 inch(25.4mm) mullions, more than one million pixels of resolution perindividual panel, less than 5 inches (127 mm) of depth, and a total weightless than 1,000 pounds (450 kg) will be extremely popular.