Computer display

A computer display (also known as a computer monitor, computer screen, or computer video display) is a device that can display signals generated by a computer as images on a screen. (From the Latin verb moneo: to warn, advise.) There are many types of monitors, but they generally conform to display standards. Once an essential component of computer terminals, computer displays have long since become standardised peripherals in their own right.

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, PAL, SECAM; NTSC)). As of this writing (June 2006), 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 through the use of a modulator. Image quality was reduced by the additional steps of composite_video
  >modulator
  >TV_tuner==>composite_video, though it reduced costs of adoption by removing the costs of a specialized monitor from the system's price tag. During the era of these early home computers, television sets were almost exclusively CRT-based.
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.

Performance measurements

The relevant performance measurements of a monitor are:

Luminance
Size
Dot pitch. In general, the lower the dot pitch (e.g. 0.24), the sharper the picture will appear.
Color temperature
Contrast ratio
Interface (DVI or VGA, commonly)
H-sync rate
V-sync rate
Response time
Refresh rate

Display resolutions

A modern CRT display has considerable flexibility: it can usually handle a range of resolutions from 320 by 200 pixels (320×200) up to 2048 by 1536 pixels (2048×1536) or 2304 by 1440 pixels (2304×1440), with unlimited colours and a variety of refresh rates. As of 2005, the highest known maximum native resolution for any type of monitor is 3840 by 2400 pixels (3840×2400) on an LCD screen.

Issues and problems

Screen burn-in has been an issue for a long time with CRT computer monitors and televisions. Commonly, people use screensavers in order to prevent their computer monitors from getting screen burn-in. How this happens is that if an image is displayed on the screen for a long period of time without changing, the screen that is showing will embed itself into the glass. Generally, you will find this phenenomon at older ATM machines. In order to prevent screen burn-in on computer monitors, it is recommended that you use a good screensaver program that rotates often.

The other issue with computer monitors is that some LCD monitors may get "dead pixels" over time. This generally applies to older LCD monitors from the 1990's.

Things on both issues have changed over time and are improving in order to prevent these things from happening.

With exceptions of DLP, most display technologies (especially LCD) have an inherent misregistration of the colour 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).

Display Interfaces

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.

Composite Monitors

Early home computers such as the Apple II and the Commodore 64 used composite monitors.

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.

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 primary disadvantage of TTL monitors was the extremely limited number of colors available due to the low number of digital bits used for video signaling.

TTL Monochrome monitors only made use of 5 out of the 9 pins. One pin was used as as ground, and two pins were used for horizontal/vertical synchronization. The electron gun was controlled by 2 seperate digital signals, a video bit, and an intensity bit to control the brightness of the drawn pixels. Only 4 unique shades were possible; black, dim, medium or bright.

CGA monitors used 4 digital signals to control the 3 electron guns used in color CRTs, in a signalling method known as RGBI, or "Red Green and Blue, plus Intensity". Each of the 3 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 seperate jack.

EGA monitors used 6 digital signals to control the 3 electron guns in a signalling method known as RrGgBb. Unlike CGA, each gun is allocated its own intensity bit. This allowed each of the 3 primary colors to have 4 different states (off, soft, medium, and bright) resulting in a total of 64 possible colors.

Although not supported in the original IBM specification, many vendors of clone graphics adapters have implemented backwards monitor compatibilty and autodetection. 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.

Modern Digital Technology

Analog RGB Monitors

Most modern computer displays can show thousands or millions of different colours in the RGB colour space by varying red, green, and blue signals in continously variable intensities.

Digtial 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

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

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.

Major manufacturers

User interface | Computer graphics | Computing output devices

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