Computer display: Wikis

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A 19" LG flat-panel LCD monitor.

A monitor or display (sometimes called a visual display unit) is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure. The display device in modern monitors is typically a thin film transistor liquid crystal display (TFT-LCD), while older monitors use a cathode ray tube (CRT).

Screen size

For any rectangular section on a round tube, the diagonal measurement is also the diameter of the tube
The area of displays with identical diagonal measurements can vary substantially

The size of a display is usually given as the distance between two opposite screen corners. One problem with this method is that it does not distinguish between the aspect ratios of monitors with identical diagonal sizes, despite the fact that the area of a given diagonal span decreases as it becomes less square. For example, a 4:3 21-inch (53.3 cm) monitor has an area of about 211 sq in (1,361 cm2), while a 16:9 21-inch widescreen has about 188 sq in (1,213 cm2).

This method of measurement is from the first types of CRT television, when round picture tubes were in common use. Being circular, they only needed to use their diameter to describe their tube size. When round tubes were used to display rectangular images, the diagonal measurement was equivalent to the round tube's diameter. This method continued even when cathode ray tubes were manufactured as rounded rectangles.

Another problematic practice was the direct measurement of a monitor's imaging element as its quoted size in publicity and advertising materials. Especially on CRT displays, a substantial portion of the imaging element is concealed behind the case's bezel or shroud in order to hide areas outside the monitor's safe area due to overscan. Seen as deceptive, widespread consumer objection and lawsuits eventually forced most manufacturers to instead measure viewable size.

Performance measurements

The performance of a monitor is measured by the following parameters:

  • Luminance is measured in candelas per square meter.
  • Viewable image size is measured diagonally. For CRTs, the viewable size is typically one inch (25 mm) smaller than the tube itself.
  • Aspect ratios is the ratio of the horizontal length to the vertical length. 4:3 is the standard aspect ratio, for example, so that a screen with a width of 1024 pixels will have a height of 768 pixels. If a widescreen display has an aspect ratio of 16:9, a display that is 1024 pixels wide will have a height of 576 pixels.
  • Display resolution is the number of distinct pixels in each dimension that can be displayed. Maximum resolution is limited by dot pitch.
  • Dot pitch is the distance between subpixels of the same color in millimeters. In general, the smaller the dot pitch, the sharper the picture will appear.
  • Refresh rate is the number of times in a second that a display is illuminated. Maximum refresh rate is limited by response time.
  • Response time is the time a pixel in a monitor takes to go from active (black) to inactive (white) and back to active (black) again, measured in milliseconds. Lower numbers mean faster transitions and therefore fewer visible image artifacts.
  • Contrast ratio is the ratio of the luminosity of the brightest color (white) to that of the darkest color (black) that the monitor is capable of producing.
  • Power consumption is measured in watts.
  • Viewing angle is the maximum angle at which images on the monitor can be viewed, without excessive degradation to the image. It is measured in degrees horizontally and vertically.
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Comparison

CRT

Pros:

  • High dynamic range (up to around 15,000:1 [1],) excellent color, wide gamut and low black level.
  • Can display natively in almost any resolution and refresh rate
  • No input lag
  • Sub-millisecond response times
  • Near zero color, saturation, contrast or brightness distortion. Excellent viewing angle.

Cons:

  • Large size and weight, especially for bigger screens (a 20" unit weighs about 50lbs or 22kg)
  • High power consumption
  • Geometric distortion caused by variable beam travel distances
  • Older CRTs are prone to screen burn-in
  • Produces noticeable flicker at low refresh rates

LCD

Pros:

  • Very compact and light
  • Low power consumption
  • No geometric distortion
  • Rugged
  • Little or no flicker depending on backlight technology

Cons:

  • Limited viewing angle, causing color, saturation, contrast and brightness to vary, even within the intended viewing angle, by variations in posture.
  • Bleeding and uneven backlighting in some monitors, causing brightness distortion, especially toward the edges.
  • Slow response times, which cause smearing and ghosting artifacts. Modern LCDs have response times of 8ms or less.
  • Only one native resolution. Displaying other resolutions requires a video scaler, which degrades image quality at lower resolutions.
  • Fixed bit depth, many cheaper LCDs are incapable of truecolor.
  • Input lag
  • Dead pixels are possible during manufacturing

Plasma

Pros:

  • Compact and light.
  • High contrast ratios (10,000:1 or greater,) excellent color, wide gamut and low black level.
  • High speed response.
  • Near zero color, saturation, contrast or brightness distortion. Excellent viewing angle.
  • No geometric distortion.
  • Highly scalable, with less weight gain per increase in size (from less than 30 inches wide to the world's largest at 150 inches).

Cons:

  • Large pixel pitch, meaning either low resolution or a large screen.
  • Noticeable flicker when viewed at close range
  • High operating temperature and power consumption
  • Only has one native resolution. Displaying other resolutions requires a video scaler, which degrades image quality at lower resolutions.
  • Fixed bit depth
  • Input lag
  • Older PDPs are prone to burn-in
  • Dead pixels are possible during manufacturing

Problems

Phosphor burn-in

Phosphor burn-in is localized 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 turned 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 anymore, so the problem is not a significant issue. 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 three 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 unnecessary for CRTs today, despite their popularity.

Phosphor burn-in can be gradually removed on damaged CRT displays by displaying an all-white screen with brightness and contrast turned up full. This is a slow procedure, but is usually effective.[citation needed]

Plasma burn-in

Burn-in re-emerged as an issue with early plasma displays, which are more vulnerable to this than CRTs. Screen savers with moving images may be used with these to minimize localized burn. Periodic change of the color scheme in use also helps.

Glare

Glare is a problem caused by the relationship between lighting and screen, or by using monitors in bright sunlight. Matte finish LCDs and flat screen CRTs are less prone to reflected glare than conventional curved CRTs or glossy LCDs, and aperture grille CRTs, which are curved on one axis only and 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[citation needed]. They do also reduce light output.

A filter above will only work against reflective glare; direct glare (such as sunlight) will completely wash out most monitors' internal lighting, and can only be dealt with by use of a hood or transreflective LCD.

Color misregistration

With exceptions of correctly aligned video projectors and stacked LEDs, most display technologies, especially LCD, have an inherent misregistration of the color channels, that is, the centers of the red, green, and blue dots do not line up perfectly. Sub-pixel 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).

Incomplete spectrum

RGB displays produce most of the visible color spectrum, but not all. This can be a problem where good color matching to non-RGB images is needed. This issue is common to all monitor technologies with 3 color channels.

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.

Some historic computers had no screen display, using a teletype, modified electric typewriter, or printer instead.

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.

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 display.

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 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 signaling 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 signaling 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.

Single color screens

Display colors other than white were popular on monochrome monitors in the 1980s. These colors 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 color schemes than used with most of today's software.

Green screens were the most popular color, with amber displays also available. 'Paper white' was also in use, which was a warm white.

Modern technology

Analog monitors

Most modern computer displays can show the various colors of the RGB color space by changing red, green, and blue analog video signals in continuously variable intensities. These have been almost exclusively progressive scan since the middle 1980s. While many early plasma and liquid crystal displays have exclusively analog connections, all signals in such monitors pass through a completely digital section prior to display.

While many similar connectors (13W3, BNC, etc…) were used on other platforms, the IBM PC and compatible systems long ago standardized on the VGA connector.

Digital and analog combination

The first popular external digital monitor connectors, such as DVI-I and the various breakout connectors based on it, included both analog signals compatible with VGA and digital signals compatible with new flat-screen displays in the same connector.

Digital monitors

Newer connectors are being made which have digital only video signals. Many of these, such as HDMI and DisplayPort, also feature integrated audio and data connections. One less popular feature most of these connectors share are DRM encrypted signals.

Configuration and usage

Multiple monitors

More than one monitor can be attached to the same device. Each display can operate in two basic configurations:

  • The simpler of the two is mirroring (sometimes cloning,) in which at least two displays are showing the same image. It is commonly used for presentations. Hardware with only one video output can be tricked into doing this with an external splitter device, commonly built into many video projectors as a pass through connection.
  • The more sophisticated of the two, extension allows each monitor to display a different image, so as to form a contiguous area of arbitrary shape. This requires software support and extra hardware, and may be locked out on "low end" products by crippleware.
  • Primitive software is incapable of recognizing multiple displays, so spanning must be used, in which case a very large virtual display is created, and then pieces are split into multiple video outputs for separate monitors. Hardware with only one video output can be made to do this with an expensive external splitter device, this is most often used for very large composite displays made from many smaller monitors placed edge to edge.

Multiple video sources

Multiple devices can be connected to the same monitor using a video switch. In the case of computers, this usually takes the form of a "Keyboard Video Mouse switch" (KVM) switch, which is designed to switch all of the user interface devices for a workstation between different computers at once.

Virtual displays

Screenshot of a user's 'workspaces' (virtual desktops) on Ubuntu GNU/Linux.

Much software and video hardware supports the ability to create additional, virtual pieces of desktop, commonly known as workspaces. Spaces is Apple's implementation of virtual displays.

Additional features

Power saving

Most modern monitors will switch to a power-saving mode if no video-input signal is received. This allows modern operating systems to turn off a monitor after a specified period of inactivity. This also extends the monitor's service life.

Some monitors will also switch themselves off after a time period on standby.

Most modern laptops provide a method of screen dimming after periods of inactivity or when the battery is in use. This extends battery life and reduces wear.

Integrated accessories

Many monitors have other accessories (or connections for them) integrated. This places standard ports within easy reach and eliminates the need for another separate hub, camera, microphone, or set of speakers.

Glossy screen

Some displays, especially newer LCD monitors, replace the traditional anti-glare matte finish with a glossy one. This increases saturation and sharpness but reflections from lights and windows are very visible.

Directional screen

Narrow viewing angle screens are used in some security conscious applications.

Autopolyscopic screen

A directional screen which generates 3D images without headgear, distortion or eyestrain.

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. The screen will need frequent cleaning due to image degradation from fingerprints.

Tablet screens

A combination of a monitor with a graphics tablet. Such devices are typically unresponsive to touch without the use of one or more special tools' pressure. Newer models however are now able to detect touch from any pressure and often have the ability to detect tilt and rotation as well.

Major manufacturers

See also

References


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