What is a Computer display?

What is a Computer display?

Computer display

A cable connects the monitor to a video adapter (video card) that is installed in an expansion slot on the computer’s motherboard. This system converts signals into text and pictures and displays them on a TV-like screen (the monitor).

The computer sends a signal to the video adapter, telling it what character, image, or graphic to display. The video adapter converts that signal to a set of instructions that tell the display device (monitor) how to draw the image on the screen.

It is important that the monitor have a TCO Certification.

Cathode ray tube
The CRT, or cathode ray tube, is the picture tube of your monitor. Although it is a large vacuum tube, it is shaped more like a bottle. The tube tapers near the back where there is a negatively charged cathode, or electron gun. The electron gun shoots electrons at the back of the positively charged screen, which is coated with a phosphorous chemical. This excites the phosphors causing them to glow as individual dots called pixels (picture elements). The image you see on the monitor's screen is made up of thousands of tiny dots (pixels). If you have ever seen a child's LiteBrite toy, then you have a good idea of the concept. The distance between the pixels has a lot to do with the quality of the image. If the distance between pixels on a monitor screen 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. (See sidebar.) Most modern monitors have a monitor with a dot pitch of .28 mm or less.

Note: From an environmental point of view, the monitor is the most difficult computer peripheral to dispose of because of the lead it contains.

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.

Imaging technologies

19" inch (48 cm) CRT computer monitorAs 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). 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 using a modulator. Image quality was reduced by the additional steps of composite video → modulator → TV tuner → composite video, though it reduced costs of adoption because one did not have to buy a specialized monitor.
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
During the era of early home computers, television sets were almost exclusively CRT-based.

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 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 up to 2560 by 2040 pixels.

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 without changing, the screen that is showing will embed itself into the glass. Generally, you will find this phenomenon 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 1990s.

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 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).

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. However, they are now used with video game consoles.

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

IBM PC with green monochrome displayMonitors 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 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.

Modern technology

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

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.

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
Apple Computer
BenQ
Dell, Inc.
Eizo
Iiyama Corporation
LaCie
LG Electronics
NEC Display Solutions
Philips
Samsung
Sony
ViewSonic