A QUESTION OF BALANCE

ARTICLE

Three Cheers for the
Red, Green and Blue
LED Displays Light Up the Big Screen

by Peter H. Putman, CTS

In previous columns, our discussions of large-screen displays have largely focused on projectors and monitors using emissive (CRT and plasma) technology for direct-view monitors, and we've concentrated on transmissive (LCD) and reflective (LCD and DLP) imaging for high-brightness large screen displays.

In many of these applications, the ambient lighting environment is controlled to maximize image contrast and legibility. But that isn't always possible when designing and installing electronic displays in arenas and stadiums. Outdoor stadiums in particular present a lighting environment that can only be characterized as "hostile" - extremely bright sunlight, pronounced shadows, variable color temperature, and lots of reflected light (read: glare).

The new breed of arena builders aren't satisfied with simple black-and-white dot matrix displays for player stats, animated graphics, and advertising/promotions. The typical sports fan (or concertgoer) is used to a steady diet of video and fancy visual effects, not to mention instant replay. For them, full-color, full motion video is the only way to go.

But how to get there? Front projection systems are certainly bright enough to light up 20', 30', and 40' wide screens with long projection throws, but can't produce enough contrast under daylight levels that can exceed 20,000 lux. Videowalls can crank up the brightness, but don't provide wide enough viewing angles and their rear-screen surfaces are easily washed out in direct sunlight.

The answer is to construct an emissive display that delivers a point source of light bright enough to be seen over 200 yards away under any kind of lighting. This ideal display would be visible from viewing angles up to 160 degrees, provide full-color imaging (at least 8-bit processing per color channel) and have a fast enough refresh rate to show video.

If we aren't being too unreasonable, perhaps this emissive display could also be constructed in a modular fashion for easy assembly. While we're at it, let's try to keep both the weight down and the footprint small. (Darn, there goes the light bulb matrix display idea...)

Pipe dream? Not really. Using nothing more than the garden-variety light emitting diode (LED), a switching matrix and a lot of wiring, SACO Smartvision of Montreal, Quebec has been assembling some pretty impressive direct-view displays for both indoor and outdoor venues, among them the Oakland-Alameda County Coliseum, the MCI Center in Washington, DC, and PSI Net Stadium in Baltimore, home of the Baltimore Ravens football team.

The concept of an LED display isn't new. LEDs have been used for crude, low-resolution signboards for years, and have also been put to work in matrix configurations for numeric and character displays. The reason they work well in very large screens has to do with our perception of image resolution.

If you have a magnifying glass handy, hold it over an advertisement of photograph in this magazine. You'll see thousands of colored dots or pixels, which make up the 'screen' of the photograph. At close range, these dots are quite noticeable. But at normal viewing distances, you don't see the dots anymore - just the image that they form.

LED displays work exactly the same way. Viewed up close, they are just a jumble of red, green, and blue dots. But as you back away from the display, the dots become less noticeable. Eventually, your eyes stop paying attention to the display structure and concentrate instead on the images being formed. Depending on how coarse or fine the matrix of LEDs, this distance will vary from tens of feet to hundreds of feet.

SACO's Smartvision screens are manufactured in a variety of screen resolutions, primarily determined by the effective pixel pitch. In a CRT display, that pitch is determined by the spot size of the electron gun as it traces a raster, and is typically less than 1 millimeter (mm). Plasma displays can have slightly large pixels close to 1 mm, while matrix displays such as LCDs and DMDs are considerably smaller and are measured in nanometers.

In contrast, the smallest pixel configuration in a Smartvision display measures 4mm - about 225 times larger than a single pixel in a .7" diagonal SVGA LCD panel! The largest pixel matrix for an indoor Smartvision screens is 15mm, while the largest available for an outdoor screen is 40 mm. Obviously, these screens were designed for viewing at long distances. SACO's recommended minimum viewing distance for a 4mm pitch array is 10 feet, while the 30mm and 40mm arrays are specified for a 100 foot viewing distance.

Using custom interfaces and driver boards, the individual pixel matrixes are driven in a progressive-scan configuration. This allows the display of both line-doubled video and computer graphics at a 60 Hz refresh rate.

How bright an image will you see? SACO claims up to 2,500 candelas per square meter (cd/m²) for their indoor displays and between 5,000 and 6,000 cd/m² for the outdoor versions. As I mentioned earlier, ambient light levels indoors are more easily controlled and it's easy to obtain high contrast from such a display - there is little light spilling on the screen surface that will reduce contrast.

Outdoors is another matter. As I write this, it's raining hard outside and the sky is a dark, overcast gray. Even so, a quick light reading shows that I still have over 1760 cd/m² of daylight to contend with, which would reduce image contrast to 2.8:1 using a 5,000 cd/m² Smartvision LED array. So, another 'trick' is used to kick up contrast - small horizontal louvers that line the top of each Smartvision 4-pixel array. This louver can reduce stray light levels by a factor of eight or more, boosting image contrast by a corresponding amount.

The secret to making these displays work was the discovery of a bright blue LED, a process that has stumped engineers for many years. (There's something about the color blue that has also vexed scientists in laser technology.) In a typical Smartvision outdoor pixel matrix, four individual 5-LED arrays contain eight red LEDs, eight green LEDs, and four blue LEDs. The red and green LEDs surround the blue LEDs and the effect looks like four dice rolled to show 'fives".

SACO claims 150,000 hours for each LED with a brightness fall-off of only 15% after 100,000 hours of operation. It's possible to get even more illumination from LEDs, but at an accelerated aging cycle. Maximum viewing angles are specified at 170 degrees and 90 degrees vertically (+30 and -60).

There is a phenomenon to LED displays known as shouldering that will affect normal viewing. Shouldering is caused by mutual obstruction among adjacent LEDs, causing noticeable color shifts. For example, if you are positioned at increasingly acute viewing angles from an outdoor screen using the 4x1 matrix, you'll notice more red and green in the image and less blue as many of the blue LEDs are partially blocked from view.

I traveled to two different arenas to check out a couple of Smartvision screens recently, both installed by Professional Products of Maryland. The first installation is part of a scoreboard in the brand-new Sovereign Bank Arena in Trenton, New Jersey. This 8,000 seat venue (10,000 for concerts) hosts both minor league hockey and basketball games, and uses four 7' x 9' arrays of Smartvision panels for video replays, advertisements, promotions, and special video graphics. The screens in this arena are bright, to say the least - in fact, they are the brightest thing you see in the arena, even more so than the spotlights on the ice.

Each panel is made up of a 16 x 16 LED matrix containing 256 red, green, and blue LEDs, and there are 99 panels per screen (11 horizontal rows and 9 vertical rows). The effective pixel resolution of each active display is 2816 x 2304 sets of red, green, and blue LEDs. SACO uses 10-bit 4:2:2 component signal processing, or 1024 colors per red, green, and blue channel. The interface is all digital and conforms to SMPTE 259M and CCIR-601, using single coaxial cables for signal distribution.

Laura Black, the technical services coordinator for Sovereign Bank Arena, uses a variety of formats to feed video and graphics to the hanging video board, including Sony DVCAM and VHS videotape playback. A Media 100 workstation is used for editing and special graphics effects, and a variety of Videotek DDRs and framestores are available for replays and still shots. Up to 9 cameras can be handled through a Ross RVS210A switcher, and a Pinnacle Deko 500 system provides real-time video SFX.

About 100 miles to the south, the staff at the Baltimore Ravens facility have configured a unique Smartvision screen into their own proprietary game-day video system they call "Raven Image". The pair of LED displays sit at opposite ends of PSINet Stadium and measure 100' wide by 25' high, with a viewable area of 96' x 24'. Unlike other stadiums where electronic displays are mounted high about the nosebleed seats, these two screens sit nicely between the first and second levels of the stadium, providing a more natural sightline.

Both screens use the 30mm outdoor 4x1 pixel array, and are actually made up of two complete 48' x 24' screens that are precisely aligned to provide a 4:1 panoramic image. As a result, there are four remote controls setting up and calibrating the two screens. All video and graphics originate in a sophisticated production studio that takes 15 people to operate during a game. All images are captured, edited, and manipulated as 16:9 525-line video in the studio, then effectively 'cropped' by the long, narrow Smartvision screen.

According to producer/director Marcia Kapustin, no other professional sports team uses such an unusual production and display format, which she considers ideal for the perspective of a football game. What's even more interesting is that Raven Image runs continuously during a game, just like a network broadcast. In fact, the Raven Image crew will sometimes take camera feeds from CBS NFL telecasts and mix 'em with their own 16:9 widescreen coverage. (No commercials, though - they appear on moving signboards.)

The control room at PSINet Stadium includes a full-bore Sony DVS-7250 digital switcher, with a raft of Ikegami cameras set up with 16:9 monitors for acquisition and three Tektronix PDR200 Profile disk recorders/players with 12 channels of video for instant replay and video segments. Up to 10 cameras can be sourced, and several Type DeKo and Pinnacle DVExtreme boxes are on-line for special effects. Four DPS 465 frame synchronizers feed each of the four individual screens, and Panorama aspect ratio converters can be used to re-size 4:3 material to the 16:9 format.

During my visit, the sun was moving in and out of cloud cover, and was completely illuminating the west screen. Despite this much ambient light, there was enough contrast (about 10:1) in the image to clearly see the clips of an earlier Buffalo Bills - Ravens game. No doubt the louvers helped, as the full daylight levels would have been far in excess of 5,000 cd/m² (low haze, direct sunlight). Of course, both screens are in the field of view of at least 75% of the spectators, so there's always one screen that is fully legible.

Copyright ©1999 Peter H. Putman / Primedia Intertec
This article appears in the January 2000 issue of Sound and Video Contractor.


						ARTICLE
					Three Cheers for the Red, Green and Blue LED Displays Light Up the Big Screen


					by Peter H. Putman, CTS In previous columns, our discussions of large-screen displays have largely focused on projectors and monitors using emissive (CRT and plasma) technology for direct-view monitors, and we've concentrated on transmissive (LCD) and reflective (LCD and DLP) imaging for high-brightness large screen displays. In many of these applications, the ambient lighting environment is controlled to maximize image contrast and legibility. But that isn't always possible when designing and installing electronic displays in arenas and stadiums. Outdoor stadiums in particular present a lighting environment that can only be characterized as "hostile" - extremely bright sunlight, pronounced shadows, variable color temperature, and lots of reflected light (read: glare). The new breed of arena builders aren't satisfied with simple black-and-white dot matrix displays for player stats, animated graphics, and advertising/promotions. The typical sports fan (or concertgoer) is used to a steady diet of video and fancy visual effects, not to mention instant replay. For them, full-color, full motion video is the only way to go. But how to get there? Front projection systems are certainly bright enough to light up 20', 30', and 40' wide screens with long projection throws, but can't produce enough contrast under daylight levels that can exceed 20,000 lux. Videowalls can crank up the brightness, but don't provide wide enough viewing angles and their rear-screen surfaces are easily washed out in direct sunlight. The answer is to construct an emissive display that delivers a point source of light bright enough to be seen over 200 yards away under any kind of lighting. This ideal display would be visible from viewing angles up to 160 degrees, provide full-color imaging (at least 8-bit processing per color channel) and have a fast enough refresh rate to show video. If we aren't being too unreasonable, perhaps this emissive display could also be constructed in a modular fashion for easy assembly. While we're at it, let's try to keep both the weight down and the footprint small. (Darn, there goes the light bulb matrix display idea...) Pipe dream? Not really. Using nothing more than the garden-variety light emitting diode (LED), a switching matrix and a lot of wiring, SACO Smartvision of Montreal, Quebec has been assembling some pretty impressive direct-view displays for both indoor and outdoor venues, among them the Oakland-Alameda County Coliseum, the MCI Center in Washington, DC, and PSI Net Stadium in Baltimore, home of the Baltimore Ravens football team. The concept of an LED display isn't new. LEDs have been used for crude, low-resolution signboards for years, and have also been put to work in matrix configurations for numeric and character displays. The reason they work well in very large screens has to do with our perception of image resolution. If you have a magnifying glass handy, hold it over an advertisement of photograph in this magazine. You'll see thousands of colored dots or pixels, which make up the 'screen' of the photograph. At close range, these dots are quite noticeable. But at normal viewing distances, you don't see the dots anymore - just the image that they form. LED displays work exactly the same way. Viewed up close, they are just a jumble of red, green, and blue dots. But as you back away from the display, the dots become less noticeable. Eventually, your eyes stop paying attention to the display structure and concentrate instead on the images being formed. Depending on how coarse or fine the matrix of LEDs, this distance will vary from tens of feet to hundreds of feet. SACO's Smartvision screens are manufactured in a variety of screen resolutions, primarily determined by the effective pixel pitch. In a CRT display, that pitch is determined by the spot size of the electron gun as it traces a raster, and is typically less than 1 millimeter (mm). Plasma displays can have slightly large pixels close to 1 mm, while matrix displays such as LCDs and DMDs are considerably smaller and are measured in nanometers. In contrast, the smallest pixel configuration in a Smartvision display measures 4mm - about 225 times larger than a single pixel in a .7" diagonal SVGA LCD panel! The largest pixel matrix for an indoor Smartvision screens is 15mm, while the largest available for an outdoor screen is 40 mm. Obviously, these screens were designed for viewing at long distances. SACO's recommended minimum viewing distance for a 4mm pitch array is 10 feet, while the 30mm and 40mm arrays are specified for a 100 foot viewing distance. Using custom interfaces and driver boards, the individual pixel matrixes are driven in a progressive-scan configuration. This allows the display of both line-doubled video and computer graphics at a 60 Hz refresh rate. How bright an image will you see? SACO claims up to 2,500 candelas per square meter (cd/m²) for their indoor displays and between 5,000 and 6,000 cd/m² for the outdoor versions. As I mentioned earlier, ambient light levels indoors are more easily controlled and it's easy to obtain high contrast from such a display - there is little light spilling on the screen surface that will reduce contrast. Outdoors is another matter. As I write this, it's raining hard outside and the sky is a dark, overcast gray. Even so, a quick light reading shows that I still have over 1760 cd/m² of daylight to contend with, which would reduce image contrast to 2.8:1 using a 5,000 cd/m² Smartvision LED array. So, another 'trick' is used to kick up contrast - small horizontal louvers that line the top of each Smartvision 4-pixel array. This louver can reduce stray light levels by a factor of eight or more, boosting image contrast by a corresponding amount. The secret to making these displays work was the discovery of a bright blue LED, a process that has stumped engineers for many years. (There's something about the color blue that has also vexed scientists in laser technology.) In a typical Smartvision outdoor pixel matrix, four individual 5-LED arrays contain eight red LEDs, eight green LEDs, and four blue LEDs. The red and green LEDs surround the blue LEDs and the effect looks like four dice rolled to show 'fives". SACO claims 150,000 hours for each LED with a brightness fall-off of only 15% after 100,000 hours of operation. It's possible to get even more illumination from LEDs, but at an accelerated aging cycle. Maximum viewing angles are specified at 170 degrees and 90 degrees vertically (+30 and -60). There is a phenomenon to LED displays known as shouldering that will affect normal viewing. Shouldering is caused by mutual obstruction among adjacent LEDs, causing noticeable color shifts. For example, if you are positioned at increasingly acute viewing angles from an outdoor screen using the 4x1 matrix, you'll notice more red and green in the image and less blue as many of the blue LEDs are partially blocked from view. I traveled to two different arenas to check out a couple of Smartvision screens recently, both installed by Professional Products of Maryland. The first installation is part of a scoreboard in the brand-new Sovereign Bank Arena in Trenton, New Jersey. This 8,000 seat venue (10,000 for concerts) hosts both minor league hockey and basketball games, and uses four 7' x 9' arrays of Smartvision panels for video replays, advertisements, promotions, and special video graphics. The screens in this arena are bright, to say the least - in fact, they are the brightest thing you see in the arena, even more so than the spotlights on the ice. Each panel is made up of a 16 x 16 LED matrix containing 256 red, green, and blue LEDs, and there are 99 panels per screen (11 horizontal rows and 9 vertical rows). The effective pixel resolution of each active display is 2816 x 2304 sets of red, green, and blue LEDs. SACO uses 10-bit 4:2:2 component signal processing, or 1024 colors per red, green, and blue channel. The interface is all digital and conforms to SMPTE 259M and CCIR-601, using single coaxial cables for signal distribution. Laura Black, the technical services coordinator for Sovereign Bank Arena, uses a variety of formats to feed video and graphics to the hanging video board, including Sony DVCAM and VHS videotape playback. A Media 100 workstation is used for editing and special graphics effects, and a variety of Videotek DDRs and framestores are available for replays and still shots. Up to 9 cameras can be handled through a Ross RVS210A switcher, and a Pinnacle Deko 500 system provides real-time video SFX. About 100 miles to the south, the staff at the Baltimore Ravens facility have configured a unique Smartvision screen into their own proprietary game-day video system they call "Raven Image". The pair of LED displays sit at opposite ends of PSINet Stadium and measure 100' wide by 25' high, with a viewable area of 96' x 24'. Unlike other stadiums where electronic displays are mounted high about the nosebleed seats, these two screens sit nicely between the first and second levels of the stadium, providing a more natural sightline. Both screens use the 30mm outdoor 4x1 pixel array, and are actually made up of two complete 48' x 24' screens that are precisely aligned to provide a 4:1 panoramic image. As a result, there are four remote controls setting up and calibrating the two screens. All video and graphics originate in a sophisticated production studio that takes 15 people to operate during a game. All images are captured, edited, and manipulated as 16:9 525-line video in the studio, then effectively 'cropped' by the long, narrow Smartvision screen. According to producer/director Marcia Kapustin, no other professional sports team uses such an unusual production and display format, which she considers ideal for the perspective of a football game. What's even more interesting is that Raven Image runs continuously during a game, just like a network broadcast. In fact, the Raven Image crew will sometimes take camera feeds from CBS NFL telecasts and mix 'em with their own 16:9 widescreen coverage. (No commercials, though - they appear on moving signboards.) The control room at PSINet Stadium includes a full-bore Sony DVS-7250 digital switcher, with a raft of Ikegami cameras set up with 16:9 monitors for acquisition and three Tektronix PDR200 Profile disk recorders/players with 12 channels of video for instant replay and video segments. Up to 10 cameras can be sourced, and several Type DeKo and Pinnacle DVExtreme boxes are on-line for special effects. Four DPS 465 frame synchronizers feed each of the four individual screens, and Panorama aspect ratio converters can be used to re-size 4:3 material to the 16:9 format. During my visit, the sun was moving in and out of cloud cover, and was completely illuminating the west screen. Despite this much ambient light, there was enough contrast (about 10:1) in the image to clearly see the clips of an earlier Buffalo Bills - Ravens game. No doubt the louvers helped, as the full daylight levels would have been far in excess of 5,000 cd/m² (low haze, direct sunlight). Of course, both screens are in the field of view of at least 75% of the spectators, so there's always one screen that is fully legible. Copyright ©1999 Peter H. Putman / Primedia Intertec This article appears in the January 2000 issue of Sound and Video Contractor.