From Wikipedia, the free encyclopedia
A computer display is an interface between the computer and the operator. Although there are other interfaces (such as a printer) the main link to the operator is usually a CRT or TFT monitor. To connect the computer's output to the monitor, the video adapter converts the computers instructions to a form that tells the monitor what to display.
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, or electron gun. The electron gun shoots electrons down the tube and onto a positively charged screen. The screen is coated with a pattern of red, green and blue phosphor dots that will glow when struck by the electron stream. Each cluster of three dots is one pixel (picture element).
The image on the monitor screen is made up from thousands of such tiny dots glowing on command from the computer. If the distance between pixels is too great, the picture will appear fuzzy, or grainy. 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 (yokes) around the collar of the tube, which bend the beam of electrons. The beam scans (is bent) across the monitor from left to right and top to bottom to create, or draw the image, line by line. 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 (Hz). If the scanning beam hits each line of pixels, in succession, on each pass, then the monitor is known as a non-interlaced monitor. The electron beam on an interlaced monitor scans the odd numbered lines on one pass, and then scans the even lines on the second pass. Interlaced Monitors are typically harder to look at, and have been attributed to eyestrain and nausea.
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
The performance parameters of a monitor are:
- 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
- Main article: Display resolution
A modern CRT display has considerable flexibility: it can usually handle a range of resolutions from 320 by 200 up to 2560 by 2040 pixels.
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, used to be an issue with CRT computer monitors and televisions. Screensavers, using moving images, prevent this happening. This problem is now found only at older ATM machines.
The other issue with computer monitors is that some monitors may have dead pixels, even when first purchased. The dead pixel does not glow when commanded to do so.
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 , and more recently Microsoft (ClearType, 1998) and XFree86 (X Rendering Extension).
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.
Early home computers such as the Apple II and the Commodore 64 used composite monitors. However, they are now used with video game consoles.
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.
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 place of an EGA monitor. Many CGA cards were also capable of operating as MDA or Hercules card if a monochrome monitor was used.
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.
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.
Configuration and usage
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.
- Dualhead - Using two monitors
- Triplehead - using three monitors
- Display assembly - multi-head configurations actively managed as a single unit
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.
- Color calibration - used to calibrate a computer monitor or display.
- Computer display standard
- Screenless - computing without a display
- AOC / TPV - Manufacture Monitors for major PC brands
- Apple Computer
- Dell, Inc.
- HannsStar Display Corporation
- Iiyama Corporation
- LG Electronics
- NEC Display Solutions