What technology is best suited for control room video walls??: Projection screens vs flat screens

David Griffiths, Market Development Manager, Control Rooms de Christie EMEA, defends in this article the suitability of rear projection screens based on DLP technology for the configuration of video wall systems in operations centers and control rooms in contrast to the growing trend to use flat LCD screens.

There are few things as impressive as a multi-screen video wall. Y, however, The impressive appearance of the video wall takes a backseat to its incredible power to display a large amount of data. This is an essential requirement for operations center and control room applications., which includes telecommunications, emergency operations, service management, process and transportation control, etc. Given the sensitivity of most of these applications, Choosing an appropriate display technology is essential.

For its proven performance and continued reliability, rear projection based on Digital Light Processing technology (DLP) has established itself as the most widespread option. Yet, direct view flat screens are beginning to be considered as an alternative. Part of their appeal lies in the relatively low cost of these commercial screens thanks to the extensive consumer television market.. Nevertheless, in addition to taking into account the low initial cost, We must not forget other aspects related to reliability and performance and that, in some cases, can end up increasing the final cost of ownership.

Performance

The performance requirements are much more demanding on each of the screens that make up a video wall than on a screen for individual use.. To achieve a completely uniform image it will be necessary, not only that each of the mosaic images that make up the matrix have the same light output, contrast and color than the surrounding quadrants, but that the image itself preserves in itself, and individually, that uniformity and in those same aspects. Besides, the large size that these video walls usually have, coupled with the infinite number of possible viewing locations for the control room technician, require each of the screens to maintain these high levels of uniformity both individually and together for a wide range of viewing angles.

If we choose a suitable screen material, Achieving the necessary uniformity and viewing angles is no problem with a Christie projection screen based on Digital Light Processing technology.. Something that is not always possible with the flat screen, which will always depend on the technology used. Specifically, with liquid crystal or LCD screens, the most popular flat screen technology, The viewing angle poses many problems. LCDs larger than 32” (in diagonal) They usually have a viewing angle of 176 degrees (±88 degrees) both horizontally and vertically; but its real performance is far from acceptable.

In a relatively recent report by DisplayMate Technologies published by Insight Media, Describes the results of a test carried out with eight LCD HD televisions from five of the most prominent consumer electronics manufacturers, including high-end models. Some of the findings of this study are described in detail here. Result: all LCD HD TVs experienced visible and, in some cases, significant changes in brightness, contrast and/or color depending on the viewing angle (see figure 1).

With one exception, All LCD screens tested showed a dubious color shift, even within a narrow range of ±15 degrees. The only screen that was saved from this alteration included an S-IPS panel, Known to undergo less chromatic shift under normal television viewing conditions. Even so, this panel demonstrated, at the widest viewing angles, a significant drop in contrast and color saturation levels. Besides, S-IPS panels tend to experience significant color shifts if viewed from a relatively high diagonal angle, something that, although it does not have to be a problem in the case of a television, yes it is when it comes to the top screens of a video wall.

LCD screens can also have uniformity problems even when the viewing angle is frontal.. Complaints from owners of HD televisions about image errors as well-known as the “cloud-shaped spots” effect are common., the lack of homogeneity in the backlighting or the appearance of vertical bands in gray tones. This does not mean that they are present on all LCD screens., but they are so frequent that they have every chance of ending up causing serious problems in a video wall made up of a large number of panels.. Even if a video wall was configured with screens carefully chosen so as not to present any of these unwanted effects in principle, they would end up developing them over time. As if that were not enough, very rarely is a function or control included to adjust the uniformity of the screen image.

What can be stated with absolute certainty is that over time the screen will lose luminosity.. Screen color temperature (the color of white and all levels of gray) and the RGB primary color dots themselves can also change (see figure 2). Unfortunately, it is most likely that these changes will not appear at exactly the same rate on all screens., so over time the video wall can end up looking like a wall full of patches. Although most screens have controls that allow the technician to compensate for these changes, It can be said that there are practically no integrated solutions that coordinate these settings across an entire array of screens., Therefore, the maintenance of the video wall becomes a laborious task that requires a lot of labor..

On the other hand, Christie projection screens based on DLP technology and designed for control room and operations center applications, They have been specifically designed to maintain a constant level of brightness and uniform colors as the life of the lamps expires.. It's more, with the help of communication channels that have been set between the projectors, These screens are capable of coordinating the settings made throughout the matrix, allowing to achieve a level of brightness, a constant color temperature and set of primary colors in each of the projectors that make up the matrix and that can easily remain stable over time.

Configuration

One of the most important handicaps of flat panel video walls is the relatively large size of the studs or frames that frame the screens. (see figure 3). If you want to achieve the visual effect of a uniform image without interruptions and guarantee maximum continuity of information across all the screens of the video wall, The separation between screens must be less than one millimeter. Projection screens meet this condition without problems. However, Current LCD panels have frames that, in the best of cases, measure 7 mm (a figure that projection screens improved many years ago). In fact, The frame width of most commercial LCDs measures well over 25 mm. This means that the separation between screens many times exceeds 50 mm. Even so, a separation of only 7 mm is still insufficient for most current command and control applications as interruptions in the display of image characteristics can negatively affect its readability.

Also, size should be taken into account, the aspect ratio and pixel formats of the most common flat screens. Nowadays, screens of more than 100 inches (in diagonal); however, The large screens that you usually find on the market only reach 70”, The most economical models being those that are closest to 50”. Due to High Definition TV regulations, Today's 32" flat screens″ (in diagonal) and the largest ones have a practically universal aspect ratio of 16:9 (on the contrary, aspect ratio is not available 4:3 although it continues to be common for computer data).

Regarding Christie Rear Projection Screens, are available in both 4:3 as in 16:9. With a variety of screen sizes ranging from 50″ at 100″ in diagonal, There is no doubt that fewer screens are needed to configure a wall with a certain number of screens and a specific shape, than one composed of flat screens available on the market.

The two common pixel formats for flat screens are 1366 x 768 y 1920 x 1080, the latter being (known as 1080p) the more widespread of the two. However, This format does not help to perceive the video wall images of an operations center and control room more clearly.. Let's take the aforementioned 70” screen as an example., This is a 1080p model with a pixel pitch of 0,8 mm. In order to appreciate the resolution of an image of these characteristics, the spectator (assuming you have absolute vision) should be located less than 2,7 meters away from the screen. The stations that the technicians have in the control rooms are usually much further away, with the consequent waste of image resolution. This incompatibility becomes even more evident in the case of small 1080p screens. But this is not the only problem, since the image generators have to execute a much greater total number of pixels on the video wall than is actually necessary, resulting in lower image refresh rates or the need for much more expensive generators.

Reliability

Christie DLP-based rear projection displays are specifically designed for 24/7 operation with documented mean times between failures exceeding 63.000 hours. On the other hand, Most flat screens are designed to be used as televisions., some devices that, in general, not used for more than eight hours at a time. The continued use of 24 hours of the day of these flat screens would subject them to an excessive thermal load well above the technical specifications of the device.

In the case of LCD panels, the heat source mainly comes from the backlight, which not only subjects the components to thermal stress, but to the panel itself, generating, ahead of time, backlight failure if design specifications are exceeded. The probability of this type of failure does not have to be lower if a newer backlight system based on light-emitting diodes is used. (LED). In fact, one of the reasons that have slowed down, more than expected, The advancement of LED backlighting technology has been the challenge of thermal management.

Normally, Any failure in the backlight implies a systematic replacement of the entire panel. Repair or replacement of the backlight itself (in the hypothetical case that it were possible) es, most of the time, practically impossible if the screen is not removed from the video wall. On the contrary, When the lamp on a Christie DLP projector breaks, it can be quickly and easily replaced without affecting the video wall setup.. Besides, some specific models with dual lamp systems can operate with a single lamp, automatically switching to the secondary lamp the moment the main one breaks down without causing the slightest disruption. Christie Entero™ models based on LED lighting are even better in this regard as the LEDs have an estimated lifespan of 50.000 hours and no “lamp” needs to be replaced for more than five years of uninterrupted operation.

Another of the most common problems with flat screens is image retention., phenomenon that is known (mistakenly) such as the “screen burn” effect or formation of a static image. The term has its origins in the era when cathode ray screens were common., although it continues to be a widespread problem among plasma screens (PDP). Like cathode ray screens, Plasma screens use phosphoric compounds, and depending on the degree of use, they lose, little by little, brightness. Consequently, When static images of high contrast are displayed, a differential drop in light output occurs between one pixel and another, which results in the formation of a fixed residual image with inverted colors.. This problem means that plasma screens are ruled out as an option for any video wall application that requires uninterrupted display of data..

But it turns out that LCD screens also have, to some extent, an image retention, although the product of a different mechanism and that, although not common in most low duty cycle applications, can become a serious problem under continued operation when viewing high-contrast still image elements. LCD screen manufacturers warn in their user manuals about image retention, but guarantees rarely cover the appearance of these effects.

Cost of ownership

When comparing and choosing a technology for the video wall, One of the most important aspects to consider is the cost of ownership, which depends on various factors. The first of all is the initial acquisition cost. Then there is the energy cost of the video wall throughout its useful life and the electrical supply necessary for adequate cooling of the device.. This last cost is especially significant if the HVAC system has to be subjected to a greater load. (heating, ventilation and air conditioning) from the operations center, but it can also include the cost of updating a system in case the old one is insufficient to meet the needs.

Nor should we forget the maintenance costs of the video wall itself.. On the one hand we have inevitable costs, How to repair hardware failures, y, for another, those expenses (not only economic, but of time) that derive from the fine-tuning of the display system so that it can offer without interruptions the image quality necessary for the application. In the case of the video wall, This mainly translates into maintaining uniform brightness and colors throughout the video wall.. Requirements that are much easier to satisfy in systems that have been specifically designed from the beginning for these applications than in improvised solutions based on flat screens initially designed for other purposes..

Finally, Another variable to take into account is the useful life of the video wall.. Experience with current installations suggests that the practical life of a flat panel-based video wall subjected to uninterrupted operation may be as little as two years, point from which performance degrades to the point where replacement of practically all of them becomes necessary., not to say all, video wall panels. Besides, it is not possible to replace only some of the panels, instead of the entire wall, given that, due to continuous changes in product models, It would be impossible to find identical replacement panels that fit perfectly as they are not available on the market..

Conclusion

The low cost of many of the flat screens on the market make them an attractive option as an alternative to rear projection for the creation of video walls in control rooms and operations centers. However, Most flat panel displays are not designed to withstand the demands of continuous operation and neither plasma nor LCD technology have been specifically adapted to meet the demanding performance requirements of matrix display solutions.. On the contrary, Christie rear projection screens, based on DLP technology, with its proven performance and continued reliability, They remain the best option.

David Griffiths

Market Development Manager, Control Rooms Christie EMEA