Also see Plasma Videowalls

Videowalls hold a unique position in the world of electronic displays. They do not command attention at trade shows like the latest ultra portables, high-brightness large-venue projectors, or super-slim plasma monitors. Videowalls quietly chug along, making slow, steady advancements in display resolution, mass and imaging engines.

Perhaps that is because it is hard to improve on the original concept of a videowall - a self-contained video display that works under virtually any lighting conditions, is easily transported and can be stacked to create bigger images. There is no complicated projection throw to calculate, and outboard digital signal processors make it possible to achieve multi-image screen effects.

The first videowalls appeared in the 1970s and were simply stacked CRT video monitors using rudimentary switchers and distribution amps to send a single channel of video to individual monitors and clusters of monitors. It did not take long for the first effects processors to come along in the 1980s, making it possible to compress and expand several channels of video simultaneously. Projection monitor cubes were introduced later in the 1980s, allowing for larger individual screen sizes, brighter images and thinner 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 dynamic signs, long before manufacturers of plasma display panels made the expression popular. Videowalls are a particular favorite of the retail market, driven by such national chains as
the Warner Brothers Studio and Disney stores.

Videowalls are also mainstays in other permanent exhibits. Arenas use them as eye-catching exhibits to promote upcoming sporting events and concerts. Airports have them strategically placed to catch the eye of arriving passengers and steer them to a particular hotel or restaurant. Command and control centers use them as a space-saving alternative to rear-projection technology, while financial institutions have found videowalls ideal for providing multiple screens of real-time data.

Even so, videowalls represent a niche market dominated by a few players. At present, the majority of projection cubes (CRT direct-view monitors are becoming passe) are manufactured by Pioneer (CRT only), Electrosonic (CRT and DLP), Synelec (CRT and DLP), Toshiba (CRT and DLP) and Clarity (LCD and DLP). Newcomers like Mitsubishi are jumping into the game to take advantage of high-resolution DLP imaging chips.

Perhaps the biggest shift has been away from rear-projection CRT imaging to Digital Light Processing. DLP projection cubes have made it possible to cut the weight of 40 inch (1 m) and 50 inch (1.3 m) rear-projection cubes in half while reducing footprints to less than 30 inches (762 mm). These changes are in direct response to the needs of the retail marketplace where the size and weight of older CRT videowalls precluded their use in many stores. It is now possible to obtain individual cubes with 800 infinity 600 pixel (SVGA) resolution with a 40 inch diagonal picture size, and 50 inch to 67 inch (1.7 m) cubes with 1,024 infinity 768-pixel (XGA) resolution. While projector manufacturers slowly edge up in pixel count, videowalls already have more than enough pixels for any application, and the reason is simple.

Videowall processors make it possible to spread the pixels (or scan lines) of an incoming signal over several cubes. Consider a common videowall matrix - two cubes vertically by two cubes horizontally. This 2 infinity 2 stack of 50 inch CRT cubes has an effective resolution of 960 lines per pixel top to bottom and more than 1,200 lines per pixel horizontally. By dividing an incoming signal into fourths (and using progressive-scan inputs), an XGA computer signal can be shown at its full resolution while an SXGA (1,280 infinity 1,024) will be clipped just a little. A 1,280 infinity 720p HDTV signal will appear letterboxed at its full height and width 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 produce more than 25 foot-Lamberts of screen brightness at a high contrast (more than 150:1). That would be hard to equal with a front-projection system in a large, open atrium and impossible in a confined retail store with aisles piled high with merchandise.

Although there have not been many demos of HDTV on videowalls to date, the combination is a natural. By using a long, rectangular stack of cubes (3 infinity 4 is ideal because the effective aspect ratio becomes 9 infinity 16), a true 1,920 infinity 1,080 HD signal can be shown without pixel compression. At the correct viewing distance (about 18 feet or 5.5 m for a 3 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 artifacts and jaggies so commonly associated with extreme close-up views of NTSC do not seem to matter much because the average viewer spends only a few minutes watching the programming before moving on.

As a result, the majority of projection cubes sold into these markets are garden-variety 15.75 kHz interlaced displays with RGBS component inputs, although some manufacturers now incorporate S-Video and even composite connections. The RGBS input standard, virtually unused anywhere else with interlaced monitors, is a relic carried over from early analog video signal processors.

The financial and command and control markets are a different matter. Here, the emphasis is on matching computer and workstation displays, so progressive-scan cubes with VGA and higher resolution are preferred. Viewers may spend several hours a day watching these walls, so progressive-scan imaging is necessary to reduce eye fatigue and preserve image detail. Some of the sources feeding these walls are workstations with 1,280 infinity 1,024-pixel to 1,600 infinity 1,200-pixel displays.

Two more issues in cube design are brightness and color uniformity. With super-thin mullions and small image detail, seamless transitions between individual cubes are critical in data installations. The goal is to achieve an image with uniformity characteristics equivalent to that from a single projector, and it is not always easy to pull off.

Brightness and color uniformity mismatches are not always apparent to viewers of walls with fast-moving video content, and we have the current generation of professional film transfer colorists to thank for that. Some commercials that play on retail walls have such dominant shades of greens and blues that individual cubes could be mismatched by a 1,000 degrees Kelvin, and nine out of 10 potential customers would not see anything amiss.

Has videowall technology hit a plateau? In terms of resolution, it probably has. Although there is no reason that a 1,280 infinity 1,024 DMD engine could not be fitted to a projection cube (and that will probably happen in the coming year), it does not bring any more benefits to the table as part of a wall; there are just not enough high-resolution signal sources available to take full advantage of all those pixels.

As far as brightness goes, flat-matrix projection cubes already exceed 30 foot-Lamberts (about 100 candelas/m square), which is more than enough illumination for an indoor display under normal room lighting. Image contrast on some projection cubes exceeds 200:1, which is sufficient for typical wall applications.

How about mass? Clever projection mirror designs have made it possible to produce 50 inch CRT cubes that require less than 30 inches of depth and DLP and LCD cubes with footprints as small as 16 inches (406 mm). Flat-matrix imaging has cut weights to less than 100 pounds (45 kg) for some 40 inch designs. It is hard to imagine much more can be shaved off at this point in time.

What will the next big advancement be in videowall technology? The guess here is that plasma display panels (PDPs) will grab the headlines if and when engineers figure out a way to solve the contrary demands of maintaining structural rigidity of the delicate glass element while reducing the width of individual PDP frames to a thin mullion.

Unlike videowall projection cubes, plasma displays are undergoing huge leaps in performance every six months. Although they are still not as bright as an LCD or DLP projection cube (about 50% of the way there), plasma panels already have one big advantage - wider viewing angles (up to 160 degrees) with no hot-spot problems. It will not be an easy task, but I will 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 per individual panel, less than 5 inches (127 mm) of depth, and a total weight less than 1,000 pounds (450 kg) will be extremely popular.

May 1, 2000 12:00 PM
Peter H. Putman