Computer display
From Wikipedia, the free encyclopedia
A computer display monitor, usually called simply a monitor when the meaning is clear from the context, is a piece of electrical equipment which displays viewable images generated by a computer without producing a permanent record.
The display device is usually either a cathode ray tube or some form of flat panel such as a TFT LCD display. 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.
The word monitor is used in other contexts; in particular in television broadcasting, where a television picture is displayed to a high standard.
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[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. 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 .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] Imaging technologies
As with television, several different hardware technologies exist for displaying computer-generated output:
- Liquid crystal display (LCD). (LCD-based monitors can receive television and computer protocols (SVGA, DVI, PAL, SECAM, NTSC). LCD displays are the most popular display device for new computers in North America.
- Cathode ray tube (CRT)
- Vector displays, as used on the Vectrex, many scientific and radar applications, and several early arcade machines (notably Asteroids (game) - 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
[edit] Performance measurements
The performance parameters of a monitor are:
- Luminance
- Size
- Dot pitch. In general, the lower the dot pitch (e.g. 0.24), the sharper the picture will rate.
- V-sync rate
- Response time
- Refresh rate
[edit] Display resolutions
A modern CRT display has considerable flexibility: it can usually handle a range of resolutions from 320 by 200 up to 2560 by 2048 pixels. The record for highest known resolution CRT was held by Data-Ray Corp. Model DR120.
[edit] Problems
Screen burn-in, where a static image left on the screen for a long time embeds the image into the phosphor that coats the screen, is an issue with CRT computer monitors and televisions. The result of phosphor burn-in are "ghostly" images of the static object visible even when the screen has changed, or is even off. 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 "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. This is accomplished by rapidly displaying varying colors to "wake up" the stuck pixels. Screensavers using moving images, prevent both of these conditions from happening by constantly changing the display. Newer monitors are more resistant to burn-in, but it can still occur if static images are left displayed for long periods of time.
It is possible for an LCD monitor to have "dead pixels", even when first purchased. The dead pixel does not change color or brightness when instructed to do so. Like image persistance, this can sometimes be partially or fully reversed by using the same method listed above, however the chance of success is far lower than with a "stuck" pixel.
With exceptions of DLP, most display technologies (especially LCD) have an inherent misregistration of the color planes, 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] 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.
[edit] Composite monitors
Early home computers such as the Apple II and the Commodore 64 used composite monitors. However, they are now used with video game consoles.
[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 Personal Computers 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.
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 unique 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 unique 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 possible 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 a 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] 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.
A monitor may also clone another monitor.
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] 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
[edit] Major manufacturers
- Acer
- AOC / TPV - Manufacture Monitors for major PC brands
- Apple Inc.
- BenQ
- Dell, Inc.
- Hewlett-Packard
- Eizo
- HannsStar Display Corporation
- Iiyama Corporation
- LaCie
- LG Electronics
- NEC Display Solutions
- Philips
- Samsung
- Sharp
- Sony
- ViewSonic
- Westinghouse
