Video display technology page


    In video display markets, the traditional cathode ray tube (CRT) as used in traditional TVs has logn dominted the market. Traditional CRT technology is analogue technology. It takes generaly analogue RGB + sync in, processes it somewhat (possibly some equalizing) and then display it on the screen. Monitors can be classified into following general categories:

    • Studio video monitors: This kind of monitors have fixed scanning rate for the TV standards in the country in which they are used. This kind of monitors usualy feature high quality picture, often high cost, utilitarian case and underscan option. Input is usually composite (i.e., NTSC or PAL) although RGB types are available.
    • Fixed frequency RGB: This kind of monitors are high resolution fixed rate monitors. Inputs are analog RGB using either separate BNC connectors or a 13W3 (Sun) connector. These often have multiple sync options. This kind of monitors are generally used underscanned for computer workstation (e.g., X-windows) applications so that entire frame buffer is visible.
    • Multi-scan or auto-scan: This kind of monitors support multiple resolutions and scan rates or multiple ranges of resolutions and scan rates. The quality and cost of these monitors ranges all over the map. Input is most often analog RGB but some older monitors of this type support a variety of digital (TTL) modes as well. A full complement of user controls permits adjustment of brightness, contrast, position, size, etc. to taste. Circuitry in the monitor identifies the video scan rate automatically and sets up the appropriate circuitry. With more sophisticated (and expensive) designs, the monitor automatically sets the appropriate parameters for user preferences from memory as well. The DB15 high density VGA connector is most common though BNCs may be used or may be present as an auxiliary (and better quality) input.
    • VGA/SVGA/XGA monitors: This kind of monitors are designed to be used with a normal PC grahics cards. This type of monitor is most clearly identified by the fact that is uses DB15 high density VGA connector. The signal input is analog RGB with separate horizonal and vertical sync signals. The basic VGA monitor supported 640x480 resolution (and some other resolutions near it). Newer SVGA/XGA moitors support a set of higher resolutions (800x600, 1024x768 etc.). Modern PC monitors are generally advanced multisync monitors which can take anything from 640x480 at 60 Hz (around 31 kHz scan rate) up to the highest resolution supported by the monitor (1024x768 up to 1600x1200) at wide variety of refresh rates (refresh rated from 50Hz up to over 100 Hz and horizonal rate from 31 KHz up to well over 60 KHz).
    There are new digital display technologies to come.Digital display technology is the result of a convergence between digital television and computing. Digital displays accept a digital video data stream and convert it into the technology specific format necessary to drive a display, be it an LCD, PDP, or projector. Those digital display which accept also analogue signals, have A/D converters in their analogue input ports to convert analogue signal to a digital format that rest of the electronics in the display can use.

    Here is an overview of different video display resolution standards and de-facto standards in use:

    Computer Standard Resolution
    VGA 640 x 480 (4:3)
    SVGA 800 x 600 (4:3)
    XGA 1024 x 768 (4:3)
    WXGA 1280 x 768 (15:9)
    SXGA 1280 x 1024 (5:4)
    SXGA+ 1400 x 1050 (4:3)
    WSXGA 1680 x 1050 (16:10)
    UXGA 1600 x 1200 (4:3)
    UXGAW 1900 x 1200 (1.58:1)
    QXGA 2048 x 1536 (4:3)
    QVGA (quarter VGA) 320 x 240 (4:3)
    Analogue TV Standard Resolution
    PAL 720 x 576
    PAL VHS 320 x 576 (approx.)
    NTSC 640 x 482
    NTSC VHS 320 x 482 (approx.)
    Digital TV Standard Resolution
    NTSC (preferred format) 648 x 486
    D-1 NTSC 720 x 486
    D-1 NTSC (square pixels) 720 x 540
    PAL 720 x 486
    D-1 PAL 720 x 576
    D-1 PAL (square pixels) 768 x 576
    HDTV 1920 x 1080
    Digital Film Standard Resolution
    Academy standard 2048 x 1536

      Monitor adjustment

      • Calibrate Your Monitor - Images on screen look different from the same image in print. Calibrating your monitor provides a screen display that simulates what you would see on paper.    Rate this link

      Monitor databases

      • Monitor Technical Specs and Information - This page has technical information on ALL kinds of monitors including: workstation monitors, Mac monitors, PC monitors, fixed frequency monitors, multisync monitors, multimode monitors, sync on green monitors, composite sync monitors, and separate sync monitors.    Rate this link

      CRT monitor introduction

      Most video monitors today use the traditional CRT, which works on the same scientific principle as a television set. This vacuum tube produces an image when an electron beam strikes the phosphorescent surface inside the monitor. In video display markets, the traditional cathode ray tube (CRT) as used in traditional TVs has long time dominted the market. Traditional CRT technology is analogue technology. It takes generaly analogue RGB + sync in, processes it somewhat (possibly some equalizing) and then display it on the screen. The CRT monitors which take other video formats convert those format to RGB in order to display them on monitor screen. The electron guns generate a beam of electrons in each of those colors to create an overlapping image. CRT monitors work by scanning the screen tube surface with an electronic beam. The refresh rate is the rate at which picture is drawn onto monitor screen.The maximum rate that a monitor can refresh the screen is measured in Hertz (cycles/second) and is called the vertical refresh rate (or vertical scan rate). The horizontal scan rate is the number of times that the monitor can move the electron beam horizontally across the screen, then back to the beginning of the next scan line in one second. The number of pixels displayed (or can be displayed), horizontally and vertically determines the monitor's resolutionCRT monitors do not generally have fixed resolution, this means that the same picture tube can display pictures at different resolutions (if the other electronics in monitor support this). The horizontal resolution is probably the most misleading, as it's thoeritically possible to have just about any horizontal resolution on almost any type of monitor which uses CRT tube. There are certain upper limits set by different technical things. The two things that stop us from going to very high resolutions on given monitor are the phyysical speed at which we can turn an electron gun on/off and the 'dot pitch' of the phosphors on the monitor. For the most part, the second 'limitation' will determine the actual horizontal resolution. If you try to display higher resolution than 'dot pitch' allows, the image would still be displayed - but some pixels wouldn't have a unique phospor triad. A slot mask, also known as a shadow mask, is a metal plate with tons and tons of little holes. These holes keep the electron beams perfectly aligned. More dots closer together create the sharpest image. This measurement is called the dot pitch. This is the limit for monitor resolution.Intensity is a measure over some interval of the electromagnetic spectrum ofthe flow of power that is radiated from, or incident on, a surface (nomitor screen in here).The voltages presented to a CRT monitor control the intensities of thecolor components, but in a nonlinear manner. CRT voltages are not directly proportional to intensity.A conventional CRT has a power-lawresponse to voltage: intensity produced at the face of the display isapproximately the applied voltage, raised to the 2.5 power. The numericalvalue of the exponent of this power function is colloquially known asgamma. This nonlinearity must be compensated in order to achieve correctreproduction of intensity.Through an amazing coincidence, vision's response to intensity iseffectively the inverse of a CRT's nonlinearity. In a video system, linear-light intensity is transformed to a nonlinearvideo signa by gamma correction, which is universally done at the camera.You might have sometimes seen phosphor burned ghost images on some veryold monitor displays and might winder when this kind of things can happent to a monitor. This can happen to a monitor when it is used constantly to display the same bright image for a very long time (for example some info-screen etc.). The risk of screen burn in is minimal as long as the same image is notleft up in a static position for many hours on end. This is the samefor any video source that is up on the screen. If the image is movingall the time, and there are not any stationary items displayed for manyhours on end, there will not be any burnin.If you have a bright image in left on the screen, it can be burned in. This can take from several hours to many hours, depending on the intensity,and the retention factor of the tubes in the set. Both rear screen andCRT type sets can burn in. To fix this, the display tube will have to be changed.

      Flat panel displays

      A variety of technologies are currently competing for use inthe flat panel displays of the future. Among these are advanced LCD,plasma discharge, and field emission displays.

      LCDs are far and away the most successful digital display technology. Driven by the explosive growth of the laptop computer market, these products have matured substantially in size, resolution, and quality over the last few years making them ideal for an extraordinary range of applications.LCD displays work by rotating the polarization through the medium. You polarize the light, then pass it through the crystal. The polarizationis then rotated as a function of applied voltage. Then, there isanother polarizer at output. If polarization has been rotated 90 degreesfrom that of output polarizer, display is dark. If zero degrees (or 180)display is full brightness.A liquid crystal display consists of an array of tiny segments (called pixels) that can be manipulated to present information (either pass or block light). This basic idea is common to all liquid crystal displays, ranging from simple calculators to a full color LCD television. Liquid crystal Displays (LCD's) do not emit light, but they manipulate or reflect it. Thus they can display images using very little power. They reflect ambient light or reflect a light source placed behind the LCD (called backlighting). The optical switching action of LCD's can be observed in three selectable viewing modes: reflective, transflective, and transmissive. LCDs have lagged behind plasma displays in size because they are harder to make. An LCD?s polarised light is highly directional, making it harder to view from the side than a cathode-ray tube (CRT) or plasma display. And the speed at which picture frames are refreshed is slower than a plasma display, causing blurring in some fast action scenes.

      Other competing technologies are plasma panel and electroluminence.

      A plasma panel display is made up of millions of phosphor-coated gas-filled pixel cells. When excited by a voltage, the gas emits UV light that makes the cell?s red, green or blue phosphor coating emit visible light. Plasma panel technology grew out of the hardened world of military and industrial displays, so its expensive, but has some compelling advantages. More importantly, it has evolved into the premiere large format direct view digital display technology, and as such is a front runner for premium HDTV and consumer electronic applications. There have been significant improvements in brightness and picture quality since the first 40-inch-plus plasma screens were launched in the mid-1990s. Proponents say that the technology produces more natural colours and a softer picture than the stark brightness of a uniformly backlit LCD ? making viewing easier for tired eyes. However, PDP screens have a shorter lifetime than an LCD and consume more power.

      Electroluminence has not reached the wide audiences, it is generally used on only special applications. The problem on displays based on electroluminence have traditionally have problems in getting the full color spectrum (problems especially on blue color gnration), and have thus been useful only on applications that need only few colors.

      The tecnologies described above have all native resolution. Native resolution is the pixel count of the display. All flat panel displays (FPDs) operate best at their native resolution. To display a non-native resolution, the panel must interpolate the incoming signal. Each manufacturer interpolates these signals differently and usually has different resolutions they support (not all supported resolution always give a good picture, but they give some kind of picture). Any FPD will work with an external scaler to display a non-support resolution or do scaling possibly better than the panel internal electronics does it.

      Touch screen technology

      A touchscreen is any monitor, based either on LCD (Liquid Crystal Display) or CRT (Cathode Ray Tube) technology, that accepts direct onscreen input. The ability for direct onscreen input is facilitated by an external (for example pen) or or an internal device (touch overlay and controller) that relays the X,Y coordinates of point touched to the computer. Touchscreen technology gives you the power to make your computer react without using a mouse or keyboard. You just press what you see on the screen. Touch screen re ideal for unattended public applications in high traffic environments. They are extremely user-friendly and durable. Touchscreen monitors make use of a range of technologies to detect touch, including capacitive-sensing, sound and light sensors, and pressure on the screen surface. Capacitive touchscreens sense electrical signals to determine the presence and location of your finger as it makes contact with the surface of the touchscreen. Strengths of capacitive technology include a fast response time, durability and a tolerance for surface contamination. Resistive screens use a flexible membrane with a coating of transparent metal oxide and a grid of spacers to locate the touchpoint. Resistive LCD touchscreen monitors rely on a touch overlay, which is composed of a flexible top layer and a rigid bottom layer separated by insulating dots, attached to a touchscreen controller. The inside surface of each of the two layers is coated with a transparent metal oxide coating (ITO) that facilitates a gradient across each layer when voltage is applied. Pressing the flexible top sheet creates electrical contact between the resistive layers, producing a switch closing in the circuit. The control electronics alternate voltage between the layers and pass the resulting X and Y touch coordinates to the touchscreen controller. Resistive touchscreen technology exists in 4-wire, 5-wire, or 8-wire forms. 4-wire resistive technology is restricted to small flatpanels (less than 10 inches). Because of its versatility and cost-effectiveness, resistive touchscreen technology is the touch technology of choice for many markets and applications (like retail point-of-sale (POS), medical monitoring devices, industrial process control, handhelds). The downside of resistive technology is that metal oxide coating and spacers may reduce the picture quality and brightness. Infrared screens generate a grid of light across the face of the screen and check for interruptions to that grid. Surface acoustic wave (SAW) touchscreens send sound waves across your screen surface to look for interruptions caused by touch. Guided acoustic wave runs on principles similar to SAW but sends waves through the screen substrate rather than over the surface. The following are key factors in assessing touchscreen performance: Response time (usually between 8ms and 20ms, more than 25ms may create problems for users), touch contact requirement (measured in milliseconds) and touch resolution (points per inch).

    TV receiver technology

    Majority of TV sets these days sold in Europe are multi-standard and can handleeither PAL or NTSC. In the consumer TV sets sold here in North America, there are very fewmodels that will do both PAL, and NTSC. This feature is seen only in somemodels of high end consumer TV's, and some professional monitors. The available interfaces on the TV receivers vary somewhat in the market areas and TV type. So called "high end" TVs like large wide-screen TVs and projection TVs usually have a good set of connections in the for attaching many different video and audio signals to those. But many basic TVs in USA have only antenna RF input (maybe theprice issues, because more connectors cost few dollars more). Virtually all TVs sold in Europe in the past 20 years have had a goodset of connections as standard either as separate connectors or as a single multipin SCART socket.

    The signal coming from the antenna signal is fed first to a TVtuner module (usually a small metal box inside receiver). This TV tuner module takes the antenna signal in and outputs the wanted channel information at the fixed TV intermediate frequency. Which channel information gets to this intermediate frequency output of tuner module is determined by the tuner tuner controls (usually analogue voltage and frequency channel select signals, but can be digital control using I2C in some new tuner modules). The circuits in the tuner are very critical for their alignment. When working in the tuner area, it is possible to miss-align the circuits. These are very critical and require very specialized calibrationfacilities and the proper technical information to check, and align thetuner front end and operational circuits. Few tens of years ago TV receiver manufacturers stated quite much of how sensitive their receivers were (so how weak antenna signal still gets you useable picture quality). Nowadays manufactuers are not generally providing sensitivity specs. Some sources say that sensitivity has typically dropped considerably in the past 15-20 years, maybe because of there might not be need for very sentivide devices (better antennas, amplified antenna system became popular, proliferation of cable systems) and due to the downward price pressure. The sensitivity difference between different manufacturers have also became smaller, because usually nowadays many TV receiver brands use the same tuner module models from the same TV tuner manufacturers, instead of making their own custon tuners (and even custom models could be based mostly on the same basi technology as other similar tuners on the market).

    After the tuner the intermediate frequency signal from it is demodulated using video and audio demodulators. Video demodulator gives out standard composite video signal. Audio demodulator gives out analogue audio signal (many modern European TVs have also demodulator for extracting digital NICAM sound data which is then fed to NICAM decoder which outputs analogue audio signal). From here on the signals are handles in the TV as any other signal coming from any source (no matter if video signal is coming from external A/V input or tuner+demodulator circuitry inside TV).

    There has been some changes over the years on TV antenna connectors. Two varieties have been in wide use 75 ohm coaxial connection ("F" or IEC 9.5 mm coaxial antenna connectors) and 300 ohm connection (pair of screws). Virtually all televisions manufactured since the mid 70's have had a 75 ohm(coax) "F" connector (or IEC 9.5 mm coaxial connector in Europe) for the antenna connection and no longer have the twoscrew terminals for 300 ohm connections for VHF. However, there was often aseparate set of terminals on the back of the set for UHF, usually the twoscrews intended for 300 ohm.Later on, in the mid-80's when most televisions were "cable ready", theseparate UHF tuner connections were eliminated and the only antenna connection that was provided was the single 75 ohm (coax) "F" connector(or IEC as used in Europe) for everything, VHF, UHF, Cable.

    TV resolution is measured in TV scan lines per unit picture heightusing a test chart (originally).An interlaced 525 line picture with about 483 active lineswill measure a vertical resolution of about 70% of that number(the "Kell factor", due to twitter, scan line overlap, aliasing,etc)or about 330 lines on a test chart. The 4.2 megahertz video bandwidthof the analog NTSC standards shows about 330 lines per unit ofpicture height of horizontal resolution.

    Typical consumer TVs are far from ideal display devices, they are just "good enough" for an average consumer to view TV programs at acceptable picture quality. Standard consumer TV sets have a tolerance of about +- 1% for the geometry. If you measure the diagonal of the screen, and take 1% oft his, you will have the approximate allowable geometry error. On a 27inch screen, this would be about +- 1/4 inch allowable error. This can mean 1/2 inch from one end of the screen to the other. Most setsconsumer sets turn out to be better than 1%. These errors are inherent in the tolerance of the design. Only the very high end professional monitors would have better geometry. They usemore compensation circuitry, and tighter design of the deflection yokeand CRT mask in the construction. Sometimes when comparing consumersets, you will see some that are a bit better than others, even in thesame model of set.

    TV CRT is magnetic field sensitive. If you put something magnetic (like a large not magnically shielded speaker) near the TV, you can get color problems and possibily geometry problems in the TV. Speakers inside a TV set will almost certainly have shielded magnetsand this is essential to avoid those spakers to effect the TV picture.

    There are sometimes questions on few coloured lines (typically 3 or 6) at the top of the black raster when a blank AV channel is selected or 16:9 mode on some 4:3 mode TVs are selected. Those lines are caused by a signal inserted by the video driver IC to monitor the beam current for each gun, to ensure the grey scale is correct. Usually the aim is that those lines are outside visible area of picture tube, but on some TVs they may become visible on some cases.

    The way the TV is powered can vary. Many older TV sets have one side of the AC input connected to the internal circuit board and its components. The cable input is an isolation device from the AC input / TV internal electronics (isolation typically made with two small ceramic capacitors, one for signal and other for cable shield, transformer isolation possible on wome equipment). In USA this type of "live chassis" TVs should have a polarity plug (the neutral prong is wider) to the AC wall outlet. There at least has been devices in European markets that have "live chassis" but not polarised plug. For safety testing in this kind of equipment TV repair technician has to check the cableinput with a leakage tester (it must show less than fraction of milliamprere, some sources say less than 0.05 mA). Most TVs made in the last twenty years have a switch mode power supply with a bridge rectifier across the AC supply, and the restof the circuitry isolated.The old practice of using a half wave circuit with the chassis connectedto one side of the suppply died when supply companies started gettingtetchy about DC components getting onto their nice clean AC mains (few dozen half-wave thyristor power supplies could really screw things) and when the number of audio/video connections in the TVs increased. The TV that is isolated from mains connection, all the signal connector ground can be directly wired to the TV circuitry ground. Typically in modern TV all the inputs/outputs in it ate grounded to the common ground plane in TV chassis.Another antenna cable input construction in this kind of TV is that TV antenna jack has a coil of wire across the pins, and thesignal is coupled to the TV thru this magnetic coupling so there's noelectrical connection. But there has to be a way for static charge tobleed off the antenna and coax, so the TV has a 1 or more meg resistorbetween the antenna jack and the chassis. The switched mode power supply can also have some small filtering capacitors from mains wires to the ground on the circuit.

      Video signal processing

      • TV Comb Filters - A comb filter is needed for the TV to show fine picture detail from standard broadcasts, laserdisk, and other composite sources. It also reduces discolorations in fine picture detail and provides purer color overall. Good comb filters reduce discoloration along vertical edges but unfortunately some comb filters add discoloration along horizontal edges.    Rate this link
      • Composite Video Separation Techniques - National Semiconductors application note    Rate this link

      100Hz TV technology

      There are ingreasingly popular (at least in Europe) TV sets which deliver 100Hz (PAL) through digital buffering. This method increses the picture refreshign rate on TV screen, thus reduces the flickering on TV screen. In PAL you get 25 frames a second consisting of 50 half frames. To double the frequency you could digitally store A and B and repeat them. The AB AB sequence works well if the video material was originally from video camera or does not move much. there's movement and the image was filmed with a video camera (instead of a movie camera) you'll get motion blurr between the half-frames resulting in irritating digital artefacts. So in case the 100Hz TV recognizes movement it switches to AA BB repeat sequence.

      Suppoted video standards

      Different TVs around the world are designed for different video standards (whatever used in the country they ae targeted to) and can have different European TV: All European TVs can do PAL because PAL is the official European TV system. Generally, older TVs are PAL only. Most European TVs also support PAL 60. Newer and more expensive ones tends to be multi-standards (PAL and NTSC). Practically all TVs use IEC style antenna connector. Most European TVs have SCART connector which supports composite video and RGB connections. Many newer TVs support also S-video on SCART. Some TVs have also other type of A/V connectors available. TVs in USA: All TVs in USA can do NTSC because this is the official video broadcasting standard used there. Most TVs cannot handle any other video standard (USA is NTSC only country, you need to buy a special multi-standard TV if you want to view some other format). The antenna connector uses is F-connector. Other A/V-connectors you may find are audio, composite video, S-video and component video (Y/Pb/Pr). The connectors used for those connections are generally RCA connectors (except 4-pin minidin for S-video). Some high-end professional TVs use multipin A/V-connectors (manufacturer specific). Viewing NTSC titles on a European TV: If a TV is PAL only, it will display NTSC pictures in black and white, due to the lack of colour signal. TV that supports PAL 60 can display NTSC video signals (in colour) provided the source is providing PAL60.

    HDTV receiver information

    HD television (HDTV) offers far superior picture quality compared to standard definition television (SDTV), but requires substantially more bandwidth to deliver. HDTV (high-definition TV) encompasses both analog and digital televisions that have a 16:9 aspect ratio and approximately 5 times the resolution of standard TV (double vertical, double horizontal, wider aspect). High definition is generally defined as any video signal that is at least twice the quality of the current 480i (interlaced) analog broadcast signal. The ATSC standard digital TV standard in use in USA includes both standard-definition (SD) and high-definition (HD) digital formats. The notation H/DTV is often used to specifically refer to high-definition digital TV. The advent of high definition has allowed monitors to read images differently, either in standard interlaced format or progressively. Sets that do not have any decoding capabilities but can display the high-resolution image is often labeled as "HD-Ready" a term that describes 80% or more of the Digital TVs on the market. HDTV displays support digital connections such as HDMI (DVI) and IEEE 1394/FireWire, although standardization is not finished. The HDTV ready displays that support analogue HDTV signal input generally use YPrPb signal format to transport the progressive video signal to the display. HDTV in the US is part of the ATSC DTV format. There are 18 approved formats for digital TV broadcasts, but only two (720p/1080i) are proper definition of the term HDTV. The resolution and frame rates of DTV in the US generally correspond to the ATSC recommendations for SD (640x480 and 704x480 at 24p, 30p, 60p, 60i) and HD (1280x720 at 24p, 20p, and 60p; 1920x1080 at 24p, 30p and 60i).

    Video projection systems

    Have you ever needed to share with a group what is on your computer screen? Presentations, demonstrations, interactive learning, and such can be much more effective if your audience can clearly see your information. One great way to achieve this is with a data projector. This is a device that attaches to a desktop computer, laptop, VCR, laser disc player, and more. Then it projects the image onto a screen, wall, or any large flat surface.

    Projection technology takes digital video to the big screen. The only available option for very large screens (80"-120" or larger) is front projection. In the conference room, convention hall, and theater, these products cannot be beat. This market has been earlier served mostly by CRT technology (tree tubes, one for each RGB color).Two technologies have emerged to dominate; a derivative branch of the LCD tree, and an innovative use of micro electromechanical systems (MEMS) technology pioneered by Texas Instruments (referred usually with names DLP and DMT). Both of those techniques need a powerful light source, which ismost often quite expensive (easily 500 USD) special gas discharge lamp.The main reason for the cost is the low volumes in which they are produced, andthe fact that people are willing to pay this for the bulbs.

    Currently, there are three basic types of video projector technology in common use today: CRT, LCD, and DLP. Here is a quick overview of them:

    • CRT (cathode ray tube) projector is built from small CRTs (one for each primary color), coupled with a light magnifying lens, can project a color image onto a large screen in a darkened room. With the proper video processing circuitry, CRT size, and lens combination, a CRT projector can produce excellent high resolution images. The image in a CRT projector is basically scanned with an electron beam (just as in regular tube TV) and is not limited to a fixed pixel field, as are other video projector types. CRT based projectors can produce wery good dark colors (black is black) and good color rendering. The downsides is that the CRT projectors are typically very large in size, has limited brightness (typically much lower than other types) and is typically quite hard to set-up to get good picure (there are three separate images that needs to be matched in size and position, lots of work that takes time).
    • LCD (liquid crystal display) projectors usually contain three separate LCD glass panels, one each for red, green, and blue components of the image signal being fed into the projector. As light passes through the LCD panels, individual pixels ("picture elements") can be opened to allow light through. Opening and closing pixels modulates the light and produces the image that is projected onto the screen. Some very cheap consumer projectors can use only on LCD panel.
    • DLP (Digital Light Processing) projectors have a special imaging IC. DLP chip is a reflective surface consisting of thousands of tiny mirrors. Each mirror represents a single pixel. In a DLP projector light from the projector's lamp is directed onto the surface of the DLP chip. The mirrors move back and forth, directing light into the lens path to turn the pixel 'on' or to a dark surface inside projector to turn pixel 'off'. In most single-chip DLP projectors, color is defined by the physical color wheel which rotates on the picture path (different colors projected sequentilly to screen). Some high-end DLP projectors use three separate DLP ICs, one for each color component. DMT (Digital Mirror Technogy/Device) is an anothername for the same projection technology.
    Both LCD and DLP projectors have their good and bad properties. Both technologies can deliver very sharp and good images. The primary benefit of LCD is that it controls red, green, and blue independently through three separate LCD panels. This means that you can easily adjust brightness and contrast of each color channel individually (to get very good color fidelity). In most single-chip DLP projectors, color is pretty much fixed and defined by the physical color wheel and the color temperature of the lamp. A modern LCD projectors tend to be more light efficient than single-chip DLP projectors, which means that LCD projector gives higher ANSI lumen outputs than DLP projectors with the same wattage lamp.The DLP projectors that used theree DLPs tend to be even more light efficient than their LCD counterparts. Since the typical DLP light engine consists of a single chip rather than three LCD panels, DLP projectors tend to be more compact.

    Pixels became the standard measurement for video projector image resolution because computers send their image data to their monitor screens in this format. Video projectors picked up this format as a measurement of resolution because the majority of them project images that originate on a computer. To get the best image from the projector, the input signal to the projector (usually from a computer) must match the native resolution of the projector. If the resolutions do not much, the video projector needs to do image resolution conversion, which usually degrades the picture quality more or less.In other words, if you have a projector that has a 1024 x 768 (XGA) native resolution, the computer it is hooked up to should have it?s video output set at 1024 x 768 (XGA) resolutio. This way the input to the projector matches the projectors output pixels "dot for dot." Quite often video projectors use different acronyms to describe their maximum resolution and/or resolution they support. The following acronyms may be seen for this:

    • MPEG-1: 352x288
    • VGA: 640x480
    • MPEG-2: 720x576
    • SVGA: 800x600
    • XGA: 1024x768
    • SXGA: 1280x1024
    • UXGA: 1600x1200
    Many projectors can take in wide variety of signal formats.Many projectors have a device built in which converts the television signals to the signal the project can display. Switching between vide sources may require a series of menu options (often displayed) from the projector?s remote control, and the installation of a second signal cable to each of the video sources. Some projectors will "sense" the change in the signal from the active input signal and do the conversion automatically. This will often times cause a flicker, flash, or rolling image on the screen as the projector re-syncs with the new signal. Both of these are distracting and certainly would not be considered technical excellence, as it is not as smooth and seamless as television.

    Most video projectors use normal 4:3 picture format as used in normal TV display and computer display. This kind of projection is very suitable for projecting normal TV picture video signal or computer screen picture. If you are building a home theater, you might want to look at the 16:9 picture format capable video projectors, because the more movie like 16:9 picture format is nowadays supported by DVD and digital TV broadcasts.

    To have smooth end results in presentations where you use multiple video sources, have a single source signal for the projector (preferably RGB+HV). This eliminates the need to change the input source signal via the remote control and thus the on screen menus. Next, add a switcher/scaler between your input devices (computer, VCR etc) and your projector. This device will convert all your input signals to match the native resolution of your projector and provide seamless switching (no on-screen flicker, flash, or image-roll) between your input signals.

    Quite often you need to connect both video projector and a separate monitor to a computer which has only one VGA output. The recommended method is through the use of a 1 in 2 out distribution amplifier (sometimes called VGA splitters). This is a small box that not only replicates the incoming signal on two outgoing ports, but also boosts the signal. Boosting ensures that a signal of sufficient strength reaches the projector even when you use longer than normal cables (longer than standard cable length permits). When connecting VGA sources to video projector, use a high quality cable to ensure picture quality. Poor quality cable will distort the picture, especially at high resolutions.

    For data projection uses many new projectors support also DVI interface. This digital interface gives you good image quality. The downsize of the DVI interface is that the practical cable lengths from signal source to projector are limited to around 5 meters (reliable operation at longer distances possible only with special repeaters).

    The most common problem users experience with notebooks and projectors is simply with delivering a VGA signal to the External Monitor port. This breaks down to two parts: delivering a signal to the port and getting the right signal format for the projector. Current notebooks tend to use controls placed in a special Windows program group, or associated with a function Key. You need to select such setup that the notebook outputs signal to the VGA output connector. Some older notebooks used BIOS setup or special TSR program for this. Check also that the screen mode (resolution and refresh rate) is compatible with your projector.Read your computer and video projector manuals if you plan to connect your notebooks to the projector.

    In some special cases you might need more power than available with one video projector. In this case you can consider stacking two projectors to shoot to the same display surface. With this method, you will get nearly twice the brightness (Light is additive: twice the light means twice as bright.).LCD projectors can be easily stacked, but some work needs to do to align them.You want-to superimpose pixels in as close to perfect registration as possible.Usually you cannot superimpose pixels perfectly ... however, you can achieve satisfactory results for normal viewing distances. (use some suitable software or other test signal source to generate a grid to make alignment easier).

      Projection surfaces

      • Angles of View White Papers - Angles of View is an industry "white paper" published regularly by the Da-Lite Screen Company. The white paper documents describe different details of projection screen surfaces.    Rate this link

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