Visual display unit
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A visual display unit, often called simply a monitor, is a piece of electrical equipment which displays viewable images generated by a computer without producing a permanent record. The word "monitor" is used in other contexts; in particular in television broadcasting, where a television picture is displayed to a high standard. A computer display device is usually either a cathode ray tube or some form of flat panel such as a TFT LCD. The monitor comprises the display device, circuitry to generate a picture from electronic signals sent by the computer, and an enclosure or case. Within the computer, either as an integral part or a plugged-in interface, there is circuitry to convert internal data to a format compatible with a monitor.
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[edit] Screen size
[edit] Diagonal size
The inch size is the diagonal size of the picture tube or LCD panel. With 4:3 CRTs the picture is squarer than 16:10 TFT and so has a larger area for the same diagonal, hence a 17" CRT generally gives about the same area of picture as a 19" TFT.
This method of size measurement dates from the early days of CRT television when round picture tubes were in common use, which only had one dimension that described display size. When rectangular tubes were used, the diagonal measurement of these was equivalent to the round tube's diameter, hence this was used.
A better way to compare CRT and LCD displays is by viewable image size.
[edit] Widescreen area
A widescreen display always has less screen area for a given quoted inch size than a standard 4:3 display, due to basic geometry.
[edit] Imaging technologies
As with television, several different hardware technologies exist for displaying computer-generated output:
- Liquid crystal display (LCD). TFT LCDs are the most popular display device for new computers in the world.
- Passive LCD gives poor contrast and slow response, and other image defects. These were used in some laptops until the mid 1990s.
- TFT Thin Film Transistor LCDs give much better picture quality in several respects. All modern LCD monitors are TFT.
- Cathode ray tube (CRT)
- Standard raster scan computer monitors
- Vector displays, as used on the Vectrex, many scientific and radar applications, and several early arcade machines (notably Asteroids - always implemented using CRT displays due to requirement for a deflection system, though can be emulated on any raster-based display.
- Television receivers were used by most early personal and home computers, connecting composite video to the television set using a modulator. Image quality was reduced by the additional steps of composite video ? modulator ? TV tuner ? composite video.
- Plasma display
- Surface-conduction electron-emitter display (SED)
- Video projector - implemented using LCD, CRT, or other technologies. Recent consumer-level video projectors are almost exclusively LCD based.
- Organic light-emitting diode (OLED) display
- Penetron military aircraft displays
[edit] Cathode ray tube
The CRT or cathode ray tube, is the picture tube of a monitor. The back of the tube has a negatively charged cathode. The electron gun shoots electrons down the tube and onto a charged screen. The screen is coated with a pattern of dots that glow when struck by the electron stream. Each cluster of three dots, one of each color, is one pixel.
The image on the monitor screen is usually made up from at least tens of thousands of such tiny dots glowing on command from the computer. The closer together the pixels are, the sharper the image on screen can be. The distance between pixels on a computer monitor screen is called its dot pitch and is measured in millimeters. Most monitors have a dot pitch of 0.28 mm or less.
There are two electromagnets around the collar of the tube which deflect the electron beam. The beam scans across the top of the monitor from left to right, is then blanked and moved back to the left-hand side slightly below the previous trace (on the next scan line), scans across the second line and so on until the bottom right of the screen is reached. The beam is again blanked, and moved back to the top left to start again. This process draws a complete picture, typically 50 to 100 times a second. The number of times in one second that the electron gun redraws the entire image is called the refresh rate and is measured in hertz (cycles per second). It is common, particularly in lower-priced equipment, for all the odd-numbered lines of an image to be traced, and then all the even-numbered lines; the circuitry of such an interlaced display need be capable of only half the speed of a non-interlaced display. An interlaced display, particularly at a relatively low refresh rate, can appear to some observers to flicker, and may cause eyestrain and nausea.
[edit] Performance measurements
The performance parameters of a monitor are:
- Luminance, measured in candelas per square metre (cd/m²).
- viewable image size, measured diagonally. For CRT the viewable size is one inch (25 mm) smaller than the tube itself.
- Dot pitch. Describes the distance between pixels of the same color in millimetres. In general, the lower the dot pitch (e.g. 0.24 mm, which is also 240 micrometres), the sharper the picture will appear.
- Response time. The amount of time a pixel in an LCD monitor takes to go from active (black) to inactive (white) and back to active (black) again. It is measured in milliseconds (ms). Lower numbers mean faster transitions and therefore fewer visible image artifacts.
- Contrast ratio. The contrast ratio is defined as the ratio of the luminosity of the brightest color (white) to that of the darkest color (black) that the monitor is capable of producing.
- Refresh rate. The number of times in a second that a display is illuminated.
- Power consumption, measured in watts (W).
- Aspect ratio, which is the horizontal size compared to the vertical size, e.g. 4:3 is the standard aspect ratio, so that a screen with a width of 1024 pixels will have a height of 768 pixels. A widescreen display can have an aspect ratio of 16:9, which means a display that is 1024 pixels wide will have a height of 576 pixels.
- Display resolution. The number of distinct pixels in each dimension that can be displayed.
[edit] Comparison
[edit] CRT
- High contrast ratio
- High speed response
- Full range light output level control
- Large size
- Large weight
- Geometric distortion in some CRTs
- Greater power consumption than LCD.
- Prone to moire effect at highest resolution
- Can display natively in almost any resolution
- Intolerant of damp conditions
- Prone to burn in
- Small risk of implosion (due to internal vacuum) if the picture tube glass is broken
[edit] Passive LCD
- Very poor contrast ratio (eg 20:1)
- High visible noise if used in more than 8 colour mode (3 bit colour depth).
- Very slow response (moving images barely viewable)
- Some suffer horizontal & vertical ghosting
- Very small size
- Very low weight
- Very low power consumption
- Uses less power when displaying darker colors
- Lower cost than TFT LCDs.
- Zero geometric distortion
[edit] TFT LCD
Almost all modern LCD monitors are the TFT type.
- Medium contrast ratio
- Response rates vary from one model to another, slower screens will show smearing on moving images
- Very small size
- Very low weight
- Very low power consumption
- Uses less power when displaying darker colors
- Higher cost than Passive LCD or CRT.
- Zero geometric distortion
- LCDs of both types only have one native resolution. Displaying other resolutions requires conversion & interpolation, which often degrades image quality.
[edit] Plasma
- High operating temperature can be painful to touch
- Prone to burn-in
- No geometric distortion
- Highest cost option
- High power consumption
[edit] Penetron
- Only found in military aircraft
- 2 colour display
- See through
- Orders of magnitude more expensive than the other display technologies listed here
[edit] Problems
[edit] Dead pixels
A few LCD monitors are produced with "dead pixels". Due to the desire for affordable monitors, most manufacturers sell monitors with dead pixels. Almost all manufacturers have clauses in their warranties which claim monitors with fewer than some number of dead pixels are not broken and will not be replaced. The dead pixels are usually stuck with the green, red, and/or blue subpixels either individually always stuck on or off.
Like image persistence, this can sometimes be partially or fully reversed by using the same method listed below, however the chance of success is far lower than with a "stuck" pixel. It can also sometimes be repaired by physically flicking the pixel, however it is always a possibility for someone to use too much force and rupture the weak screen internals doing this.
[edit] Stuck pixels
LCD monitors, while lacking phosphor screens and thus immune to phosphor burn-in, have a similar condition known as image persistence, where the pixels of the LCD monitor can "remember" a particular color and become "stuck" and unable to change. Unlike phosphor burn-in, however, image persistence can sometimes be reversed partially or completely.[citation needed] This is accomplished by rapidly displaying varying colors to "wake up" the stuck pixels.
[edit] Phosphor burn-in
Phosphor burn-in is localised aging of the phosphor layer of a CRT screen where it has displayed a static bright image for many years. This results in a faint permanent image on the screen, even when powered off. In severe cases it can even be possible to read some of the text, though this only occurs where the displayed text remained the same for years.
This was once a common phenomenon in single purpose business computers. It can still be an issue with CRT displays when used to display the same image for years at a time, but modern computers aren't normally used this way any more, so the problem is not a significant issue. The issue seems to have become exaggerated in popular opinion. The only systems that suffered the defect were ones displaying the same image for years, and with these the presence of burn-in was not a noticeable effect when in use, since it coincided with the displayed image perfectly. It only became a significant issue in 3 situations:
- when some heavily used monitors were reused at home,
- or re-used for display purposes
- in some high security applications (but only those where the high security data displayed did not change for years at a time).
Screen savers were developed as a means to avoid burn-in, but are redundant for CRTs today, despite their popularity. The problem does not occur with multitasking systems, and powering down the display after a period of non-use is as effective and has additional benefits, such as increasing monitor life and reducing power use.
Phosphor burn-in can be gradually removed on damaged CRT displays by displaying an all white screen with brightness & contrast turned up full. This is a slow procedure, and is usually effective.
[edit] Plasma burn-in
Burn-in has re-emerged as an issue with plasma displays, which are much more vulnerable to this than CRTs. Screen savers with moving images may be used with these to minimise localised burn. Periodic change of the colour scheme in use also helps.
[edit] Black level misadjustment
User misadjustment of black level is common. This alters the colour displayed with most darker colours.
A testcard image may be used to set the image black level correctly. On CRT monitors
- Black level is set with the 'brightness' control
- White level is set with the 'contrast' control
- Sometimes black level can need readjustment after setting white level.
[edit] Glare
Glare is a problem caused by the relationship between lighting and screen, or by using monitors in bright sunlight. LCDs and flat screen CRTs are less prone to this than conventional curved CRTs, and aperture grille CRTs, which are curved on one axis only, are less prone to it than other CRTs curved on both axes.
If the problem persists despite moving the monitor or adjusting lighting, a filter using a mesh of very fine black wires may be placed on the screen to reduce glare and improve contrast. These filters were popular in the late 1980s. They do also reduce light output.
[edit] Color misregistration
With exceptions of DLP, most display technologies, especially LCD, have an inherent misregistration of the color channels, that is, the centres of the red, green, and blue dots do not line up perfectly. Subpixel rendering depends on this misalignment; technologies making use of this include the Apple II from 1976,[1] and more recently Microsoft (ClearType, 1998) and XFree86 (X Rendering Extension).
[edit] Incomplete spectrum
RGB displays produce most of the visible colour spectrum, but not all. This can be a problem where good colour matching to non-RGB images is needed. This issue is common to all monitor technologies with 3 colour channels.
[edit] Display interfaces
[edit] Computer Terminals
Early CRT-based VDUs (Visual Display Units) such as the DEC VT05 without graphics capabilities gained the label glass teletypes, because of the functional similarity to their electromechanical predecessors.
Some historic computers had no modern display, using a teletype, modified electric typewriter, or printer instead.
[edit] Composite signal
Early home computers such as the Apple II and the Commodore 64 used a composite signal output to drive a CRT monitor or TV. This resulted in degraded resolution due to compromises in the broadcast TV standards used. This method is still used with video game consoles. The Commodore monitor had S-Video input to improve resolution.
[edit] Digital monitors
Early digital monitors are sometimes known as TTLs because the voltages on the red, green, and blue inputs are compatible with TTL logic chips. Later digital monitors support LVDS, or TMDS protocols.
[edit] TTL monitors
Monitors used with the MDA, Hercules, CGA, and EGA graphics adapters used in early IBM PC's (Personal Computer) and clones were controlled via TTL logic. Such monitors can usually be identified by a male DB-9 connector used on the video cable. The disadvantage of TTL monitors was the limited number of colors available due to the low number of digital bits used for video signaling.
Modern monochrome monitors use the same 15-pin SVGA connector as standard color monitors. They are capable of displaying 32-bit grayscale at 1024x768 resolution, making them able to interface to with modern computers.
TTL Monochrome monitors only made use of five out of the nine pins. One pin was used as a ground, and two pins were used for horizontal/vertical synchronization. The electron gun was controlled by two separate digital signals, a video bit, and an intensity bit to control the brightness of the drawn pixels. Only four shades were possible; black, dim, medium or bright.
CGA monitors used four digital signals to control the three electron guns used in color CRTs, in a signalling method known as RGBI, or Red Green and Blue, plus Intensity. Each of the three RGB colors can be switched on or off independently. The intensity bit increases the brightness of all guns that are switched on, or if no colors are switched on the intensity bit will switch on all guns at a very low brightness to produce a dark grey. A CGA monitor is only capable of rendering 16 colors. The CGA monitor was not exclusively used by PC based hardware. The Commodore 128 could also utilize CGA monitors. Many CGA monitors were capable of displaying composite video via a separate jack.
EGA monitors used six digital signals to control the three electron guns in a signalling method known as RrGgBb. Unlike CGA, each gun is allocated its own intensity bit. This allowed each of the three primary colors to have four different states (off, soft, medium, and bright) resulting in 64 colors.
Although not supported in the original IBM specification, many vendors of clone graphics adapters have implemented backwards monitor compatibility and auto detection. For example, EGA cards produced by Paradise could operate as an MDA, or CGA adapter if a monochrome or CGA monitor was used in place of an EGA monitor. Many CGA cards were also capable of operating as MDA or Hercules card if a monochrome monitor was used.
[edit] Single colour screens
Display colours other than white were very popular on monochrome monitors in the 1980s. These colours were more comfortable on the eye. This was particularly an issue at the time due to the lower refresh rates in use at the time causing flicker, plus the use of less comfortable colour schemes than used with most of today's software.
Green screens were the most popular colour, with orange displays also available. 'Paper white' was also in use, which was a warm white.
[edit] Modern technology
[edit] Analog RGB monitors
Most modern computer displays can show thousands or millions of different colors in the RGB color space by varying red, green, and blue signals in continuously variable intensities.
[edit] Digital and analog combination
Many monitors have analog signal relay, but some more recent models (mostly LCD screens) support digital input signals. It is a common misconception that all computer monitors are digital. For several years, televisions, composite monitors, and computer displays have been significantly different. However, as TVs have become more versatile, the distinction has blurred.
[edit] Configuration and usage
[edit] Multi-head
Some users use more than one monitor. The displays can operate in multiple modes. One of the most common spreads the entire desktop over all of the monitors, which thus act as one big desktop. The X Window System refers to this as Xinerama.
Terminology:
- Dualhead - Using two monitors
- Triplehead - using three monitors
- Display assembly - multi-head configurations actively managed as a single unit
[edit] Virtual displays
The X Window System provides configuration mechanisms for using a single hardware monitor for rendering multiple virtual displays, as controlled (for example) with the Unix DISPLAY global variable or with the -display command option.
[edit] Additional features
[edit] Power saving
Most modern monitors contain a power saving mode that will switched to if no video input signal is received. Modern operating systems can thus power down a monitor after a specified period of inactivity. This also extends the service life of the monitor.
Some monitors will also switch themselves completely off after a time period on standby.
Most modern laptops provide a method of gradually dimming the screen after periods of inactivity, or when the battery is in use. This is to extend battery life, and reduce wear.
[edit] Directional screen
Narrow viewing angle screens are used in some security conscious applications.
[edit] Touch screen
These monitors use touching of the screen as an input method. Items can be selected or moved with a finger, and finger gestures may be used to convey commands. This does however mean the screen needs frequent cleaning due to image degradation from fingerprints.
[edit] Major manufacturers
- Acer
- Apple Inc.
- BenQ
- Eizo
- Hewlett-Packard
- HannStar Display Corporation
- Iiyama Corporation
- LaCie
- LG Electronics
- NEC Display Solutions
- Philips
- Samsung
- Sharp
- Sony
- ViewSonic
- Westinghouse
[edit] See also
- 3D display - a computer display capable of conveying three-dimensional images to the viewer.
- Color calibration - used to calibrate a computer monitor or display.
- Computer display standard
- Screenless - computing without a display
- Video
- CRT
- LCD
- TFT LCD
- Plasma display
[edit] References
[edit] External links
- Troubleshooting and repair
- General SVGA monitor diagram
- Free online tools for calibrating monitors
- Adjust your Computer Monitor
- Repairing a notebook display - a step by step procedure with replacing the CCFL of a notebook
- A free program for measuring pixels on your computer monitor (works with MS Windows)
- Monitor buying guide - CNET reviews
