? I have 100+ rgb LED's where each rgb LED will be used for multi-color.
Hence every individual LED needs brightness control(hence the PWM).
Depends. This isn't a "display" and the LED's are scattered about(not random
but they are not packed tightly). They may or may not change but they will
not change often(maybe at most a few times a second).
Slight fluxuations in brightness won't be a huge deal and they calibrated
out if necessary but I'd rather not have to do that.
My idea right now is to basically use C mosfets, 1 for each column, and R
mosfets, 1 per row. This gives me the matrix and current capacity. The rows
will be driven by a ring counter and the columns by a mosfet driver that is
controlled by a uP.
I believe that will work and the only problem is coordinating the row's with
the individual PWM(which becomes sorta modulated by the refresh rate). I
think by choosing the refresh rate to be large would be ok but might cause
other problems.
Slight variances are not a big issue. And Again, I'm sure they can be
calibrated out if I have control of individual brightness. (i.e., say one
LED is too bright, I can just reduce the current to it... I'll have a lookup
table for all the LED's for changes)
I don't think it will be a huge problem though. I have to keep a table
anyways because I don't think brightness is proportional to the PWM
frequency?
Are you trying for individual proportional control of each led, or
just on-off control maybe with overall dimming (of all together)?
Do these people source their subsystems out to others?
How long have RGB LEDs been around?
Are they economical yet?
http://www.lsi-industries.com/contentindex.asp?ID=6
http://www.smartvision.com/eng/products/indoor.asp?id=30
Silverstar YG-LED 109 Matrix Panel
http://www.nordiskmusik.se/default.asp?id=17992
http://www.ledsignsupply.com/?gclid=CM3IlMrZm5gCFRwwawod0joUnA
http://www.ecvv.com/product/vp1345904/indoor-full-color-led-display.html
http://www.panstadia.com/vol5/54-028.htm
http://www.ledsmagazine.com/news/3/6/25
Old, but I thought some of the description might help,
like the part about "shouldering" for example.
But if you're using RGB LEDs that shouldn't be trouble, right?
http://digitalcontentproducer.com/mag/avinstall_led_displays_light/
LED DISPLAYS: light up the big screen
Jan 1, 2000 12:00 PM, Peter H. Putman
Previous columns covering large-screen displays mainly focused on
projectors and monitors using emissive (CRT and plasma) technology for
direct-view monitors, and we have 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.
That is not always possible when designing and installing electronic
displays in arenas and stadiums. Outdoor stadiums in particular
present a lighting environment that can be characterized as hostile -
extremely bright sunlight, pronounced shadows, variable color
temperature and plenty of glare.
The new breed of arena builders are not satisfied with simple black-
and-white dot matrix displays for player stats, animated graphics and
advertising/promotions. The typical sports fan (or concert attendee)
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.
How do they get there? Front projection systems are certainly bright
enough to light up 40 foot (12 m) wide screens with long projection
throws but cannot produce enough contrast under daylight levels that
can exceed 20,000 lux. Videowalls can crank up the brightness, but
they do not 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 more than 200 yards (183 m)
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 refresh rate fast
enough to show video. Perhaps this emissive display could also be
constructed in a modular fashion for easy assembly. While we are at
it, let's try to keep the weight down and the footprint small.
A 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 has been assembling some pretty
impressive direct-view displays for indoor and outdoor venues, among
them the Oakland-Alameda County Coliseum, the MCI Center in
Washington, D.C., and PSI Net Stadium in Baltimore, home of the
Baltimore Ravens football team.
The LED display concept is not new. LEDs have been used for crude, low-
resolution signboards for years, and they have also been put to work
in matrix configurations for numeric and character displays. The
reason they work well in large screens has to do with our perception
of image resolution. If you have a magnifying glass handy, hold it
over an advertisement or photograph in this magazine. You will 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 do not see the dots, just the image that
they form.
LED displays work exactly the same way. Viewed up close, they are a
jumble of red, green and blue dots. 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 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 mm. Plasma
displays can have slightly large pixels close to 1 mm, while such
matrix displays as LCDs and DMDs are considerably smaller and are
measured in nanometers.
In contrast, the smallest pixel configuration in a Smartvision display
measures 4 mm, about 225infinity larger than a single pixel in a 0.7
inch (18 mm) diagonal SVGA LCD panel. The largest pixel matrix for an
indoor Smartvision screen is 15 mm, 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 4 mm pitch array is 10 feet (3 mm), while the 30 mm and 40 mm
arrays are specified for a 100 foot (30 m) 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 of an image will you see? SACO claims up to 2,500 candelas/
m superscript 2 for its indoor displays and between 5,000 candelas/m
superscript 2 and 6,000 candelas/m superscript 2 for the outdoor
versions. Ambient light levels indoors are more easily controlled, and
it is easy to obtain high contrast from such a display; there is
little light spilling on the screen surface that will reduce contrast.
Outdoors, however, things are different. As I write this article, it
is raining outside, and the sky is a dark, overcast gray. Even so, a
quick light reading shows that I still have more than 1,760 candelas/m
superscript 2 of daylight to contend with, which would reduce image
contrast to 2.8:1 using a 5,000 candelas/m superscript 2 Smartvision
LED array. So, another trick is used to kick up contrast - small
horizontal louvers that line the top of each Smartvision four-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
is something about the color blue that has also vexed scientists in
laser technology. In a typical Smartvision outdoor pixel matrix, four
individual five-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 is 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.
There is a phenomenon to LED displays, known as shouldering, that will
affect normal viewing. Shouldering is caused by mutual obstruction
among adjacent LEDs, creating noticeable color shifts. For example, if
you are positioned at increasingly acute viewing angles from an
outdoor screen using the 4infinity1 matrix, you will 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, NJ. This 8,000 seat venue
(10,000 for concerts) hosts both minor league hockey and basketball
games, and uses four 79infinity99 (2.1 m infinity 2.7 m) arrays of
Smartvision panels for video replays, advertisements, promotions and
special video graphics. The screens in this arena 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 infinity 16 LED matrix containing 256
red, green and blue LEDs, and there are 99 panels per screen (11
horizontal rows and nine vertical rows). The effective pixel
resolution of each active display is 2,816 infinity 2,304 sets of red,
green and blue LEDs. SACO uses 10 bit 4:2:2 component signal
processing or 1,024 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 nine cameras can be handled through
a Ross RVS210A switcher, and a Pinnacle Deko 500 system provides real-
time video SFX.
About 100 miles (161 km) 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 feet wide by 25 feet high (30 m infinity 7.6 m) with a
viewable area of 969infinity249 (29 m infinity 7.3 m). 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 30 mm outdoor 4infinity1 pixel array and are
actually made up of two complete 489infinity249 (14.6 m infinity 7.3
m) 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 is
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 them
with its own 16:9 widescreen coverage.
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 disc
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 online for special effects. Four DPS
465 frame synchronizers feed each of the four individual screens, and
Panorama aspect ratio converters can be used to resize 4:3 material to
the 16:9 format.
During my visit, the sun moved in and out of cloud cover and
completely illuminated 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 game. No doubt, the louvers helped because
the full daylight levels would have been far in excess of 5,000
candelas/m superscript 2 (low haze, direct sunlight). Of course, both
screens are in the field of view of at least 75% of the spectators, so
there is always one screen that is fully legible.
If you run it as a matrix, you have to use higher
