Public Address

Public address

School public address system

A public address or "PA" system is an electronic amplification system with a mixer, amplifier and loudspeakers, used to reinforce a given sound (e.g.,a person making a speech, prerecorded music, or message) and distributing the 'sound' to the general public around a building.
Simple PA systems are often used in small venues such as school auditoriums, churches, and small bars.

PA systems with a larger number of speakers are widely used in institutional and commercial buildings, to read announcements or declare states of emergency. Intercom systems, which are often used in schools, also have microphones in each room so that the occupants can reply to the central office.

There is disagreement over when to call these audio systems Sound Reinforcement (SR) systems or PA systems. Some audio engineers distinguish between the two by technology and capability, while others distinguish by intended use (e.g., SR systems are for live music whereas PA systems are usually for reproduction of speech and recorded music in buildings and institutions). This distinction is important in some regions or markets, while in other regions or markets the terms are interchangeable.

Small PA systems

The simplest PA systems consist of a microphone, a modestly-powered mixer-amplifier (which incorporates a mixer and an amplifier in a single cabinet) and one or more loudspeakers. Simple PA systems of this type, often providing 50 to 200 watts of power, are often used in small venues such as school auditoriums, churches, and small bars.

In North America, PA systems are also sometimes referred to as "sound reinforcement systems" or simply "sound systems." In colloquial British English, a PA system installed for public address in a building is sometimes referred to as a "Tannoy" system after the company of that name.

Public Address systems typically consist of input sources, pre-amplifiers and/or signal routers, amplifiers, control and monitoring equipment, and loudspeakers. Input sources refer to the microphones and CD Players that provide a sound input for the system. These input sources are fed into the pre-amplifiers and signal routers that determine the zones that the 'audio signal' is fed to. The preamplified signals are then passed into the amplifiers. Depending on a countries' regulation these amplifiers will amplify the audio signals to 50V, 70V or 100V speaker line level. Control equipment monitors the amplifiers and speaker lines for faults before it reaches the loudspeakers.

Telephone paging systems

Most modern telephone systems, such as PBX and VOIP, use a paging system that acts as a liaison between the telephone and a PA amplifier.

In key telephone systems such as those by Nortel, Toshiba or Avaya, paging equipment is usually built into the telephone system itself, and allows announcements to be paged over the phone speakers themselves, through external speakers or through both external and internal telephone speakers.

In PBX and larger VOIP telephone systems such as those by Nortel, Cisco, Avaya or Siemens, used for larger enterprise applications, paging equipment is not built into the telephone system. Instead the system provider must provide a separate paging controller connected to a trunk port on the actual telephone system. The paging controller is accessed as either an unused directory number or unused central office line. Access to the paging system is provided through a "trunk access" code or a preprogrammed feature button on the telephone set itself.

Many retailers and offices choose to use the telephone system as the sole access point for the paging system, because the equipment is already "paging system" ready. That is, the business does not have to buy a separate intercom or microphone. An additional advantage, is that each telephone can access the paging system, which makes initiating a page much more convenient than having just one microphone. Many schools and other larger institutions are no longer using the large bulky microphone PA systems and have switched to telephone system paging, as it can be accessed from many different points of the school in an emergency.

One disadvantage of telephone paging systems compared to microphone paging systems, is that the noise associated with hanging up the telephone can be heard over the speakers unless the user takes the initiative to press the "switchhook" on the telephone or if the phone is equipped, pressing the Release (RLS) button, which is most commonly found on Nortel telephone systems.

Large venue PA systems

For popular music concerts, a more powerful and more complicated PA System is used to provide live sound reproduction. In a concert setting, there are typically two complete PA systems: the "main" system and the "monitor" system. Each system consists of microphones, a mixing board, sound processing equipment, amplifiers, and speakers. There is disagreement over when to call these audio systems Sound Reinforcement (SR) systems or a Public Address (PA) systems. This distinction is important in some regions or markets, while in other regions or markets the terms are interchangeable.

The "main" system (also known as "Front of House", commonly abbreviated FOH), which provides the amplified sound for the audience, will typically use a number of powerful amplifiers driving a range of large, heavy-duty loudspeakers including low-frequency speaker cabinets called subwoofers, full-range speaker cabinets, and high-range horns. A large club may use amplifiers to provide 1000 to 2000 watts of power to the "main" speakers; an outdoor concert may use 10,000 or more watts.

The "monitor" system reproduces the sounds of the performance and directs them towards the onstage performers (typically using wedge-shaped monitor speaker cabinets), to help them to hear the instruments and vocals. In British English, the monitor system is referred to as the "fold back". The monitor system in a large club may use amplifiers to provide 500 to 1000 watts of power to the "monitor" speakers; at an outdoor concert, there may be several thousand watts of power going to the monitor system.

At a concert in which live sound reproduction is being used, sound engineers and technicians control the mixing boards for the "main" and "monitor" systems, adjusting the tone, levels, and overall volume of the performance.

Acoustic feedback

All PA systems have a potential for feedback, which occurs when sound from the speakers returns to the microphone and is then re-amplified and sent through the speakers again. This generally manifests itself as a sharp, sudden high-volume piercing sound which can damage the loudspeakers' high-frequency horns or tweeters - and audience members' hearing.

Sound engineers take several steps to prevent feedback, including ensuring that microphones are not pointed towards speakers, keeping the onstage volume levels down, and lowering frequency levels where the feedback is occurring, using a graphic equalizer, parametric equalizer a combination of both devices, or a notch filter.

Recent developments

In recent years, a number of technological advances have been made to PA systems.

PA speakers

High-end PA speakers have been made lighter by using neodymium speaker magnets, and horns are often wired using protective circuitry such as light bulbs (which illuminate and absorb excess wattage) or polyswitches that protect the horn from damage in the event of feedback or a dropped microphone. These new approaches to speaker protection are more convenient than the formerly used approach of fuses, because the sound system needs to be turned off to change fuses.

Digital signal processors

Small PA systems for venues such as bars and clubs are now available with features that were formerly only available on professional-level equipment, such as digital reverb effects, graphic equalizers, and, in some models, feedback prevention circuits (which electronically sense and prevent feedback "howls" before they occur). These Digital Signal Processing multi-effect devices offer sound engineers a huge range of sound processing options (reverb, delay, echo, compression, etc.) in a single unit. In previous decades, sound engineers typically had to transport a number of heavy "rack-mounted" cases of analog effect devices.

Amplifiers

A number of PA companies are now making lightweight, portable speaker systems for small venues that route the low-frequency parts of the music (electric bass, bass drum, etc.) to a separately-powered subwoofer. Routing the low-frequency parts of the signal to a separate amplifier and low-frequency subwoofer can substantially improve the bass-response of the system. As well, the clarity of the overall sound reproduction can be enhanced, because low-frequency sounds take a great deal of power to amplify; with only a single amplifier for the entire sound spectrum, the power-hungry low-frequency sounds can take a disproportionate amount of the sound system's power.

Power amplifiers have also become lighter, smaller, more powerful and more efficient. Many power amplifiers now use digital switching transformers, for significant weight and space savings as well as increased efficiency.

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Video Projector

Video projector

Projected image from a video projector in a home cinema.

A video projector takes a video signal and projects the corresponding image on a projection screen using a lens system. All video projectors use a very bright light to project the image, and most modern ones can correct any curves, blurriness, and other inconsistencies through manual settings. Video projectors are widely used for conference room presentations, classroom training, and home theatre applications.

A video projector may also be built into a cabinet with a rear-projection screen (rear-projection TV, or RPTV) to form a single unified display device, now popular for “home theater” applications.

Common display resolutions for a portable projector include SVGA (800×600 pixels), XGA (1024×768 pixels), 720p (1280×720 pixels), and 1080p (1920×1080 pixels).

The cost of a device is not only determined by its resolution, but also by its light output, acoustic noise output, contrast, and other characteristics. While most modern projectors provide sufficient light for a small screen at night or under controlled lighting such as in a basement with no windows, a projector with a higher light output (measured in lumens, abbreviated “lm”) is required for a larger screen or a room with a higher amount of ambient light. A rating of 1000 to 1500 ANSI lumens or lower is suitable for smaller screens with controlled lighting or low ambient light.Buying guide. Between 1500 and 3000 lm is suitable for medium-sized screens with some ambient light or dimmed light. Over 3000 lm is appropriate for very large screens in a large room with no lighting control (for example, a conference room).

Projected image size is important; because the total amount of light does not change, as size increases, brightness decreases. Image sizes are typically measured in linear terms, diagonally, obscuring the fact that larger images require much more light (proportional to the image area, not just the length of a side). Increasing the diagonal measure of the image by 25 % reduces the image brightness by 35 per cent; an increase of 41 per cent reduces brightness by half.

Projection technologies

CRT projector using cathode ray tubes. This typically involves a blue, a green, and a red tube. Minimal maintenance is required (unlike projectors that use expensive lamps which must be periodically replaced after they burn out). This is the oldest system and falling out of favor largely because of the bulky cabinet. However, it does provide the largest screen size for a given cost. This also covers three tube home models, which, while bulky, can be moved
LCD projector using LCD light gates.

This is the simplest system, making it one of the most common and affordable for home theaters and business use. Its most common problem is a visible “screen door” or pixelation effect, although recent advances have minimized this.

DLP projector using Texas Instruments’ DLP technology. This uses one, two, or three microfabricated light valves called digital micromirror devices (DMDs). The single- and double-DMD versions use rotating color wheels in time with the mirror refreshes to modulate color. The most common problem with the single- or two-DMD varieties is a visible “rainbow” which some people perceive when moving their eyes. Systems with 3 DMDs never have this problem. More recent projectors with higher speed (2x or 4x) and otherwise optimised color wheels have minimized this artifact.

LCOS projector using Liquid crystal on silicon.
D-ILA JVC’s Direct-drive Image Light Amplifier based on LCOS technology.

Major manufacturers

3M
Barco
BenQ
Christie
Digital Projection International
EIKI
Epson
Hewlett-Packard
Hitachi
InFocus
Kloss Video
Lenovo
Lumens
Matsushita (Panasonic)
Mitsubishi
NEC
Optoma
Panasonic
Samsung
Sharp
Sony
Texas Instruments (component supplier of DLP technology)
Toshiba
Viewsonic

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Liquid Crystal Display Television

Liquid crystal display television

Liquid crystal display television (LCD TV) is television that uses LCD technology for its visual output. The technology used is generally TFT. In the early 2000s, LCD flat-panels captured a large part of the computer monitor market from traditional CRTs. Continuing advances in LCD TV technology enable it to compete against Plasma flat panels and rear-projection televisions (DLP, LCD, and LCoS) for large-screen HDTV.

Early LCD television had drawbacks relative to traditional visual display technologies. It displayed fast-moving action with "ghosting" and could be viewed best only when looking directly at the screen or from a slight angle. Most of these problems were solved in recent years, and LCD televisions, along with plasma displays, have become more popular worldwide than cathode ray display televisions. The LCD design is also known for being more energy efficient than the CRT design.

For a long time it was widely believed that LCD technology was suited only to smaller sized flat-panel televisions, and could not compete with plasma technology at sizes of 40" or larger. At the time, plasma held the edge in cost and performance. Presently, LCD TV's can offer the same performance with the announcements of seventh-generation panels by major manufacturers such as Samsung, Sony, LG.Philips LCD, and Sharp Corporation.

In October 2004, 40" to 45" televisions were widely available, and Sharp had announced the successful manufacture of a 65" panel.

In March 2005, Samsung announced an 82" LCD panell.
In August 2006, LG.Philips Consumer Electronics announced a 100" LCD television
In January 2007, Sharp displayed a 108" LCD panel branded under the AQUOS brand name at CES in Las Vegas.

Manufacturers have announced plans to invest billions of dollars in LCD production over the next few years, with televisions expected to be a key market. (The other main market for LCD displays is in computer monitors.)

Improvements in LCD technology have narrowed the technological gap with plasmas. The lower weight, falling prices, higher available resolution which is crucial for HDTV, and lower electrical power consumption of LCDs make them competitive against plasma displays in the television set market. As of late 2006, analysts note that LCDs are overtaking plasmas, particularly in the important 40" and above segment where plasma had enjoyed strong dominance a couple of years before.

LCD Technology

LCD technology is based on the properties of polarized light. Two thin, polarized panels sandwich a thin liquid-crystal gel that is divided into individual pixels. An X/Y grid of wires allows each pixel in the array to be activated individually. When an LCD pixel darkens, it polarizes at 90 degrees to the polarizing screens.

This pixel has darkened. The pixel darkens in proportion to the voltage applied to it: for a bright detail, a low voltage is applied to the pixel; for a dark shadow area, a higher voltage is applied. LCDs are not completely opaque to light, however; some light will always go through even the blackest LCD pixels.

Developments in LCD televisions

TVs based on PVA and S-PVA LCD panels deliver a broad angle of view, up to 178 degrees. They also deliver an adequate contrast ratio for viewing bright scenes, as well as dark scenes in bright rooms. The dynamic contrast technique improves contrast when viewing dark scenes in a dark room. Alternatively, some manufacturers produce LCD TVs that throw light on the wall behind it to help make dark scenes look darker. PVA and S-PVA panels generally have difficulty with ghosting when going between different shades of dark colours, however in new televisions this is compensated to some degree using a technique called overdriving.

Moving pictures on a CRT TV do not exhibit any sort of "ghosting" because the CRT's phosphor, charged by the strike of electrons, emits most of the light in a very short time, under 1 ms, compared with the refresh period of e.g. 20 ms (for 50 fps video). In LCDs, each pixel emits light of set intensity for a full period of 20 ms (in this example), plus the time it takes for it to switch to the next state, typically 12 to 25 ms.

The second time (called the "response time") can be shortened by the panel design (for black-to-white transitions), and by using the technique called overdriving (for black-to-gray and gray-to-gray transitions); however this only can go down to as short as the refresh period.
This is usually enough for watching film-based material, where the refresh period is so long (1/24 s, or nearly 42 ms), and jitter is so strong on moving objects that film producers actually almost always try to keep object of interest immobile in the film's frame.

Video material, shot at 50 or 60 frames a second, actually tries to capture the motion. When the eye of a viewer tracks a moving object in video, it doesn't jump to its next predicted position on the screen with every refresh cycle, but it moves smoothly; thus the TV must display the moving object in "correct" places for as long as possible, and erase it from outdated places as quickly as possible.

Although ghosting was a problem when LCD TVs were newer, the manufacturers have been able to shorten response time to 2ms on many computer monitors and around an average of 8 ms for TVs.

There are two emerging techniques to solve this problem. First, the backlight of the LCD panel may be fired during a shorter period of time than the refresh period, preferably as short as possible, and preferably when the pixel has already settled to the intended brightness. This technique resurrects the flicker problem of the CRTs, because the eye is able to sense flicker at the typical 50 or 60 Hz refresh rates.

Another approach is to double the refresh rate of the LCD panel, and reconstruct the intermediate frames using various motion compensation techniques, extensively tested on high-end "100 Hz" CRT televisions in Europe.

The best approach may be a combination of two, possibly allowing the viewer to switch them on or off when viewing video- or film-based material.

Some manufacturers are also experimenting with extending colour reproduction of LCD televisions. Although current LCD panels are able to deliver all sRGB colours using an appropriate combination of backlight's spectrum and optical filters, manufacturers want to display even more colours. One of the approaches is to use a fourth, or even fifth and sixth colour in the optical colour filter array. Another approach is to use two sets of suitably narrowband backlights (e.g. LEDs), with slightly differing colours, in combination with broadband optical filters in the panel, and alternating backlights each consecutive frame.
Fully using the extended colour gamut will naturally require an appropriately captured material and some modifications to the distribution channel. Otherwise, the only use of the extra colours would be to let the viewer boost the colour saturation of the TV picture beyond what was intended by the producer, but avoiding the otherwise unavoidable loss of detail ("burnout") in saturated areas.

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Plasma display

An example of a plasma display

Composition of plasma display panel

A plasma display panel (PDP) is a type of flat panel display now commonly used for large TV displays (typically above 37-inch or 940 mm). Many tiny cells located between two panels of glass hold an inert mixture of noble gases (neon and xenon). The gas in the cells is electrically turned into a plasma which then excites phosphors to emit light.

History

The plasma display was invented at the University of Illinois at Urbana-Champaign by Donald L. Bitzer, H. Gene Slottow, and graduate student Robert Willson in 1964 for the PLATO Computer System. The original monochrome (usually orange or green, sometimes yellow) panels enjoyed a surge of popularity in the early 1970s because the displays were rugged and needed neither memory nor circuitry to refresh the images. A long period of sales decline followed in the late 1970s as semiconductor memory made CRT displays cheaper than plasma displays. Nonetheless, plasma's relatively large screen size and thin profile made the displays attractive for high-profile placement such as lobbies and stock exchanges.

In 1983, IBM introduced a 19-inch (483 mm) orange-on-black monochrome display (model 3290 'information panel') which was able to show four simultaneous IBM 3270 virtual machine (VM) terminal sessions. That factory was transferred in 1987 to startup company Plasmaco, which Dr. Larry F. Weber, one of Dr. Bitzer's students, founded with Stephen Globus, and James Kehoe, who was the IBM plant manager. In 1992, Fujitsu introduced the world's first 21-inch (533 mm) full-color display. It was a hybrid, based upon the plasma display created at the University of Illinois at Urbana-Champaign and NHK STRL, achieving superior brightness. In 1996, Matsushita Electrical Industries (Panasonic) purchased Plasmaco, its color AC technology, and its American factory. In 1997, Pioneer started selling the first plasma television to the public. Current plasma televisions are often seen around the home and are thinner and in greater sizes than their predecessors. Their thin size allows them to compete with other display technology such as projector screens.

Screen sizes have increased since the 21-inch (533 mm) display in 1992. The largest plasma video display in the world was shown at the 2006 Consumer Electronics Show in Las Vegas, Nevada, U.S.A, a 103-inch (261.6 cm) unit manufactured by Matsushita Electrical Industries (Panasonic).

Until quite recently, the superior brightness, faster response time, greater color spectrum, and wider viewing angle of color plasma video displays, when compared with LCD televisions, made them one of the most popular forms of display for HDTV flat panel displays. For a long time it was widely believed that LCD technology was suited only to smaller sized televisions, and could not compete with plasma technology at larger sizes, particularly 40 inches and above.

However, since then, improvements in LCD technology have narrowed the technological gap. The lower weight, falling prices, higher available resolution, which is important for HDTV, and often lower electrical power consumption of LCDs make them competitive against plasma displays in the television set market. As of late 2006, analysts note that LCDs are overtaking plasmas, particularly in the important 40-inch (1.0 m) and above segment where plasma had previously enjoyed strong dominance a couple of years before.

Another industry trend is the consolidation of manufacturers of plasma displays, with around fifty brands available but only five manufacturers.

General characteristics

Plasma displays are bright (1000 lux or higher for the module), have a wide color gamut, and can be produced in fairly large sizes, up to 262 cm (103 inches) diagonally. They have a very low-luminance "dark-room" black level, creating a black some find more desirable for watching movies. The display panel is only about 6 cm (2½ inches) thick, while the total thickness, including electronics, is less than 10 cm (4 inches). Plasma displays use as much power per square meter as a CRT or an AMLCD television.

Power consumption will vary greatly depending on what is watched on it. Bright scenes (say a football game) will draw significantly more power than darker scenes (say a movie scene at night). Nominal measurements indicate 400 watts for a 50-inch screen. Recent models, post 2006, consume between 220 and 310 watts for a 50-inch display when set to cinema mode. Most screens are set to 'shop' mode by default and this draws at least twice the power compared to a more comfortable 'home' setting.

The lifetime of the latest generation of plasma displays is estimated at 60,000 hours (or 27 years at 6 hours of use per day) of actual display time. More precisely, this is the estimated half life of the display, the point where the picture has degraded to half of its original brightness. It is watchable after this point, but is generally considered the end of the functional life of the display.
Competing displays include the CRT, OLED, AMLCD, DLP, SED-tv and field emission flat panel displays. The main advantage of plasma display technology is that a very wide screen can be produced using extremely thin materials. Since each pixel is lit individually, the image is very bright and has a wide viewing angle.

Functional details

The xenon and neon gas in a plasma television is contained in hundreds of thousands of tiny cells positioned between two plates of glass. Long electrodes are also sandwiched between the glass plates, in front of and behind the cells. The address electrodes sit behind the cells, along the rear glass plate. The transparent display electrodes, which are surrounded by an insulating dielectric material and covered by a magnesium oxide protective layer, are mounted in front of the cell, along the front glass plate. Control circuitry charges the electrodes that cross paths at a cell, creating a voltage difference between front and back and causing the gas to ionize and form a plasma; as the gas ions rush to the electrodes and collide, photons are emitted.

In a monochrome plasma panel, the ionizing state can be maintained by applying a low-level voltage between all the horizontal and vertical electrodes – even after the ionizing voltage is removed. To erase a cell all voltage is removed from a pair of electrodes. This type of panel has inherent memory and does not use phosphors. A small amount of nitrogen is added to the neon to increase hysteresis.

In color panels, the back of each cell is coated with a phosphor. The ultraviolet photons emitted by the plasma excite these phosphors to give off colored light. The operation of each cell is thus comparable to that of a fluorescent lamp.

Every pixel is made up of three separate subpixel cells, each with different colored phosphors. One subpixel has a red light phosphor, one subpixel has a green light phosphor and one subpixel has a blue light phosphor. These colors blend together to create the overall color of the pixel, analogous to the "triad" of a shadow-mask CRT. By varying the pulses of current flowing through the different cells thousands of times per second, the control system can increase or decrease the intensity of each subpixel color to create billions of different combinations of red, green and blue. In this way, the control system can produce most of the visible colors. Plasma displays use the same phosphors as CRTs, which accounts for the extremely accurate color reproduction.

Contrast ratio claims

Contrast ratio is the difference between the brightest and darkest parts of an image, measured in discrete steps, at any given moment. Generally, the higher the contrast ratio, the more realistic the image is. Contrast ratios for plasma displays are often advertised as high as 20,000:1. On the surface, this is a significant advantage of plasma over other display technologies. Although there are no industry-wide guidelines for reporting contrast ratio, most manufacturers follow either the ANSI standard or perform a full-on-full-off test.

The ANSI standard uses a checkered test pattern whereby the darkest blacks and the lightest whites are simultaneously measured, yielding the most accurate "real-world" ratings. In contrast, a full-on-full-off test measures the ratio using a pure black screen and a pure white screen, which gives higher values but does not represent a typical viewing scenario. Manufacturers can further artificially improve the reported contrast ratio by increasing the contrast and brightness settings to achieve the highest test values. However, a contrast ratio generated by this method is misleading, as content would be essentially unwatchable at such settings.

Plasma is often cited as having better black levels (and contrast ratios), although both plasma and LCD have their own technological challenges. Each cell on a plasma display has to be precharged before it is due to be illuminated (otherwise the cell would not respond quickly enough) and this precharging means the cells cannot achieve a true black. Some manufacturers have worked hard to reduce the precharge and the associated background glow, to the point where black levels on modern plasmas are starting to rival CRT. With LCD technology, black pixels are generated by a light polarization method and are unable to completely block the underlying backlight.

One shortcoming with plasma technology is that running a display at maximum brightness will significantly reduce the panel's lifespan. For this reason, many owners leave the brightness settings well below maximum, which typically still results in a brighter screen than CRT displays.

Screen burn-in

An example of a plasma display that has suffered severe burn-in from stationary text
With phosphor-based electronic displays (including cathode-ray and plasma displays), the prolonged display of a menu bar or other graphical elements over time can create a permanent ghost-like image of these objects. This is due to the fact that the phosphor compounds which emit the light lose their luminosity with use. As a result, when certain areas of the display are used more frequently than others, over time the lower luminosity areas become visible to the naked eye and the result is called burn-in. While a ghost image is the most noticeable effect, a more common result is that the image quality will continuously and gradually decline as luminosity variations develop over time, resulting in a "muddy" looking picture image.

Plasma displays also exhibit another image retention issue which is sometimes confused with burn-in damage. In this mode, when a group of pixels are run at high brightness (when displaying white, for example) for an extended period of time, a charge build-up in the pixel structure occurs and a ghost image can be seen. However, unlike burn-in, this charge build-up is transient and self corrects after the display has been powered off for a long enough period of time, or after running random broadcast TV type content.

Plasma manufacturers have over time managed to devise ways of reducing the past problems of image retention with solutions with grey pillarboxes, pixel orbiters and image washing routines.



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Plasma Display Advantages

• Slim profile Lighter and less bulky than projection televisions
• Easier to manufacture and cheaper at extremely large screen sizes than LCDs
• Can achieve a true black because pixel can be completely turned off, resulting in better contrast, detail, and naturalness
• Better viewing angles than those of LCD

Plasma Display Explained

A plasma display is made up of many thousands of gas-filled cells that are sandwiched in between two glass plates, two sets of electrodes, dielectric material, and protective layers. The address electrodes are arranged vertically between the rear glass plate and a protective layer. This structure sits behind the cells in the rear of the display, with the protective layer in direct contact with the cells. On the front side of the display there are horizontal display electrodes that sit in between a magnesium-oxide (MgO) protective layer and an insulating dielectric layer.

The MgO layer is in direct contact with the cells and the dielectric layer is in direct contact with the front glass plate. The horizontal and vertical electrodes form a grid from which each individual cell can be accessed. Each individual cell is walled off from surrounding cells so that activity in one cell does not affect another. The cell structure is similar to a honeycomb structure except with rectangular cells.

To illuminate a particular cell, the electrodes that intersect at the cell are charged by control circuitry and electric current flows through the cell, stimulating the gas (typically xenon and neon) atoms inside the cell. These ionized gas atoms, or plasmas, then release ultraviolet photons that interact with a phosphor material on the inside wall of the cell. The phosphor atoms are stimulated and electrons jump to higher energy levels. When these electrons return to is natural state energy is released in the form of visible light. Every pixel on the display is made up of three subpixel cells. One subpixel cell is coated with red phosphor, another is coated with green phosphor, and the third cell is coated with blue phosphor.

Light emitted from the subpixel cells is blended together to create an overall color for the pixel. The control circuitry can manipulate the intensity of light emitted from each cell, and therefore can produce a large spectrum of colors. Light from each cell can be controlled and changed rapidly to produce a high-quality moving picture.

Professional audiovisual industry

The professional audiovisual industry is a multibillion-euro industry, comprised of the manufacturers, dealers, systems integrators, consultants, programmers, presentations professionals and technology managers of audio-visual products and services.

The proliferation of audiovisual communications technologies, including sound, video, lighting, display and projection systems, is evident in every sector of society such as these:

• business
• education
• government
• the military
• healthcare
• retail environments
• worship

The application of audio-visual systems is found in:

• collaborative conferencing
• presentation rooms auditoriums and lecture halls
• command and control centers
• digital signage, and more

Concerts and corporate events are among the most obvious venues where audio-visual equipment is used in a staged environment. Providers of this type of service are known as rental and staging companies, although they may also be served by an in-house technology team (e.g., in a hotel or conference center).

2006 is the fourth consecutive year that significant growth is projected for the industry. Revenue for surveyed North American companies is expected to grow by 40% in 2006, and by 10.7% for European audiovisual companies. The single biggest factor for this increase is the increased demand for networked audiovisual products due to the integration of audiovisual and IT technology. The two leading markets for AV equipment in Europe continue to be business/IT and education.