| The above descriptions, examples and
pictures apply to video wall applications typically used in public display
venues, command and control rooms, operation centers, situation rooms,
traffic monitoring rooms, emergency operation centers, etc. There are many
other applications for video walls and it is beyond the scope of this
article to identify and describe them all.
Which Display Device to Use?
We are asked this question daily. When selecting a
display device technology to build a matrix of displays to create a "Large
Video Wall", designers are faced with basically 2 different display
technologies, Video Wall rear projection cubes or Direct View LCD/Plasma
screens. Direct
View LCD or Plasma displays: On the
outset it makes logical and fiscal sense to just go down to the local home
electronics reseller (Costco, Best Buy, Etc.) and buy enough inexpensive LCD
displays to create a large display matrix and hang them on the wall, Video
Wall done... Not so fast. There are some long
and short term problems with this method: 1)
Direct view LCD's/Plasma displays each have a border or bezel around each
one and when you tile these together to create an array, you create gaps in
the overall image that creates what the industry referrers to as
mullions and
transoms which is fine
for windows in a building but causes problems when viewing images or data on
a video wall. The human brain does not like to have information distorted in
this way, (its hard to look at). Doing this
destroys the aspect ratio of the images as shown below with various shapes:
Using video wall cubes with their ~1mm mullions you
would see something more like this with the same data:
Actual installations come out looking like this:
Although video processors that drive these walls can
compensate for the width of the mullions, you are losing data in the gap of
the mullion and although may be acceptable for some images, it is not for
numerical data or mapping functions. This is a major compromise in the
overall looks of the project when all is said and done and the end user will
have to decide if they can live with this flaw.
2) What is not as obvious is that consumer grade LCD's
and plasma displays are made, sold and discontinued from production in a
very short life cycles, generally between 1 year and 3 months. This is
because of a cost and competitive marketplace that demands that manufactures
are constantly churning the product mix to have the latest and greatest
being offered. This works fine if you have an application with a single
display that fails in 2 years for example, no problem, buy a new one, and
toss the old one. It will cost less to buy a new unit, than fix the old one.
Manufactures know this and so there is no motivation for them to provide
service parts and cost effective support for these devices long term, and so
they don't. Even with long warranties, generally there is a stipulation that
if the failed item is not repairable, then they have the option to replace
with a similar item. This doesn't work when that
failed LCD is in a tightly spaced array because inevitably the replacement
unit will not be the same physical size, nor have the same optical
characteristics (colors wont match). This will require replacement of the
entire array if 1 unit fails with new product which can wipe out any savings
by using this technology instead of video wall cubes which cost more day one
but are made for this purpose. Even if you accept the above risks, because
of budget cycles there may be monies available day 1 to do it right with
video wall cubes but there wont be down the line to do a complete
replacement. Generally there is some reasonable funds available for repairs,
which is the case with video wall cubes. 3) The
human eye is extremely critical of very small color temperature, brightness
and hue difference between closely spaced displays placed in this fashion.
Since direct view LCD's were never made for this application they generally
don't have the necessary color and geometry adjustment in them to match the
neighboring display perfectly. This may be more of an issue over time as
each device ages and therefore its color output changes, making the problem
worse with age. 4) Since direct view displays
were never designed for this function, cooling and their long term life
expectancy can be an issue as well (and may void any warranty).
5) Image retention issues, Plasma displays will
prematurely burn into their phosphor any image displayed that is static in
very short time periods, rendering the display destroyed and not covered
under warranty. LCD's all suffer from some form of TIR or temporary
image retention with static images. The term for this varies but it is a
function of the technology and manufactures exclude this as a failure, but
the end user wont because it is visible but not necessarily permanent.
6) If a unit fails and assuming a repair can be done, it
wont be in the field. These devices are made to be repaired at a service
facility not at the customers site. Which brings up how do you pack it to be
shipped for repair? Its not a simple put it in a box and send it because of
its size, shape and fragility. Manufactures shipping packaging is very
specialized and difficult to obtain as most of it is made overseas. And even
it when using manufactures packing materials damage is rather common.
7) Direct view display devices when mounted in an
array are generally installed starting with the bottom row to the top row,
and when one fails on anything but the bottom row, all units above it need
to be removed to get access to the failed unit. We are aware of no front
access mounting system that maintains small gaps between individual displays
and allows for removal of any 1 display only. 8)
The weight of an array of direct view display devices in aggregate is more
than most commercial building walls are able to support safely. An example
would be a 4x3 array of 57" LCD's with mounting hardware would weigh
approximately 1620 Lbs. Most commercial building walls were never meant to
have this kind of weight being cantilevered off of them and if the wall were
to fail it could be lethal especially in an earthquake area. The cost to
retrofit the building walls will have to be considered in any design such as
this, as without this it would be considered negligent.
9) All the above issues don't generally arise when these
devices are used for their intended design application which is for
occasional television use. They were not designed and don't function well
when being used in a group as a video wall array. There are "Commercial"
versions of LCD's available which help to mitigate "some" of the above
problems, but they are not optimal and will cost many times what the
consumer counterparts do and will always be another form of compromise as
the actual LCD panel itself comes from the same factory as the consumer
model. So at its core suffers from the same planned obsolescence issues
mentioned above.
Video Wall Cube devices:
Rear projection video wall cubes,
although they cost more day 1, are designed and purpose built for this
function and address the above issues, and their designs have been refined
over many years. Certain tier 1 manufactures of these devices design and
build them with high image quality day 1 and adjustments for the long term.
Field serviceability is a part of the overall design
and can easily be performed in the field by dealer or end user
personnel. Making spare parts available for time horizons in the 7 year
range is quite common with many going longer. In
summary, these systems are made for incorporation into an overall
installation design that if done correctly will look good on day 1 and for
years to come. Selection of a qualified dealer to do the design and
installation, or support the end user in doing the installation is
recommended. In a traditional rear projection
video wall cube, the light source comes from a specially designed
short arc lamp
similar to These. The light then passes
through a
DLP or LCD apparatus that filters out the white light and only shows the
colors that are in the image that is to be displayed. After a series of
lenses and mirrors the image is shown on the back side (rear projection)
screen that you look at the front of. In
addition to short arc lamp illuminated Video Wall cubes, there are LED
illuminated Video Wall Cubes. Using LED's for illumination alleviates the
need for color wheels in the light path and adds significant cost to the
display. Although doing this theoretically reduces the cost to operate (no
purchasing of lamps), the brightness will fade over the life of the LED
illumination module. These devices have only been commercially available
since mid 2010 and their long term reliability and cost savings over a long
service life is as yet undetermined.
Narrow Mullion LCD displays.
Introduced in early 2010, narrow mullion LCD's are
commercial grade direct view displays purpose built for use in creating
video wall matrices. This scheme is a hybrid between typical LCD's and Video
Wall Cubes, although the mullions are not as small as cubes can provide
(~1mm), they are small enough for some applications where some mullion can
be tolerated. As of early 2011 mullions on 46" is 7.3mm and on 55" displays
it is 5.7mm. Below are 2 examples of narrow
mullion LCD's:
46" LCD running at 1366x768
with 7.3mm mullions above, this overall array is 80.76" wide x 45.66" high
55" LCD running 1920x1080 with
5.7mm mullions above, this overall array is 96" wide x 53.8" high.
Although narrow mullion LCD's are an attractive solution
they still suffer a number of problems stated above for long term support
and longevity. |
