Monitor – Display Screen

A monitor can be considered as a window into the computer’s memory. It allows data entry to be checked by echoing input characters on the screen.

The disadvantage of monitors is that data on the screen is temporary (called softcopy) and they can hold only limited amount of data.

Quality Factor – Resolution

Resolution defines sharpness and clarity of an image and is specified as the number of dots per character. Modern packages are using a lot of graphics and therefore require high-resolution monitors.

Display Adaptors

Adaptors are cards installed in motherboard to control a monitor. The following specifications were evolving over the years:

* Hercules – mono

* Colour Graphics Adaptor (CGA) – low quality

* Enhanced Graphics Adaptor (EGA) – medium quality

* Video Graphics Array (VGA and Super VGA) – high quality

* Extended Graphics Array (XGA) – successor of SVGA, has the power to handle the video and animation requirements of the modern multimedia packages and games

Bit Mapping Technique

For monochrome displays, only one bit of memory is required for each pixel. If it is set to 1, the pixel is illuminated. If the corresponding memory location contains 0, the pixel is left dark.

A binary representation in RAM

The equivalent screen representation

Figure 1.Bitmapped representation of letter T coding in RAM and corresponding image on screen

For colour monitors, more than one bit is required to keep information about a pixel because apart from illuminated or not setting, colour specification is also required. Monitors that are capable of displaying 16 colours only would require 4 bits of memory per pixel (16 = 24).

Figure out how many bits of memory per pixel are required to give a choice of 512 colours?

Thus the required size of visual memory depends on the number of pixels on the screen (resolution factor) and on the number of colours available.

If you are buying a modern monitor which is bigger than 14″, it is desirable to have at least 2 M of memory on the video card.

Types of Monitors

1). Cathode Ray Tube (CRT)

Technology:

A beam of electrons lights up pixels/dots on the screen

Colour is achieved by combining Red/Green/Blue (RGB) of different intensities.

Size:

Size is measured diagonally (corner to corner). Today monitors are available in sizes 14″, 15″, 17″, 21″. But the monitor size doesn’t tell anything about the maximum viewable image size. The bezel in front of every monitor’s CRT diminishes the viewable area by approximately an inch.

When deciding on which monitor to buy, think of which resolution you’ll be working in most frequently. The higher the resolution, the bigger the monitor is required.

Monitors usually display 25 lines 80 characters each in text mode.

Standard Resolutions (in pixels)

VGA

640 x 480

SVGA

800 x 600

XGA

1024 x 768

1280 x 1024

1600 x 1200

Some high-end monitors support these high resolutions. Designed for professional level work,

1600 x 1280

e.g. computer-aided design or desktop publishing

1800 x 1440

Supported by at least one: ViewSonic P815 Mega

An interlaced monitor draws its screen in two passes. First it draws every second line and then fills in the missing lines. Interlacing is noticeable because of flicking screen and can cause headaches.

A non-interlaced monitor draws its screen in one pass.

Another reason for a flicking screen is the frequency with which the monitor redraws its screen – called refresh rate or vertical scan rate. The bottom line should be 75Hz so that flickering effect doesn’t show up.

Ensure that your monitor and graphics card can be synchronised to the same refresh rate.

2). LCD (Liquid Crystal Display)

These displays are known as being used in calculators and watches. They have an advantage of being cheap but a disadvantage is that it is very hard to see what they display in the dark. They are used in laptops because they are flat.

LCDs come as monochrome or colour.

Monochrome LCD images usually appear as dark gray images on a grayish-white background.

Color Liquid Crystal Displays use two basic techniques for producing color: passive matrix and active matrix.

Passive matrix is the less expensive of the two technologies. It offers good contrast and viewing angle suitable for graphical user interfaces but its colors are not particularly sharp and it has slow reaction times. This is the most popular display used in notebook computers today.

It consists of a grid of horizontal and vertical wires. At the intersection of each grid is an LCD element which constitutes a single pixel, either letting light through or blocking it.

Active-matrix technology, also called thin film transistor (TFT), produces color images that are as sharp as traditional CRT displays at 1/8 the space, but the technology is expensive. For multimedia applications with video images the fast response time of TFT active matrix displays is ideal. For color active matrix provides individual color bits for each displays. An active-matrix display uses from one to four transistors to control each pixel.

LCDs do not exaggerate actual viewing areas like regular monitors. For example, a 9.4″ LCD provides 9.4″ diagonal viewing area. This is the equivalent of an 11″ CRT.

Color LCD Comparison Chart

Technology

Contrast Ratio

Response

Viewing Angle

Price

Passive Matrix

20:1

300 ms

Medium

Medium

Active Matrix

60:1

80 ms

High

High

3). Gas Plasma

Gas-plasma display is a type of flat display screen, called a flat-panel display, used in some portable computers. Images on gas-plasma displays generally appear as orange objects on a black background.

Although gas-plasma displays produce very sharp monochrome images, they require much more power than the more common LCD displays.

Technology:

A grid of conductors are sealed between two flat plates of glass; neon and/or argon gas fills the space between the plates.

Comparison Of LCDs and Gas-Plasma Displays:

Type

Readability

Battery Life

Cost

LCD

Lowest

Longest

Lowest

Gas plasma

High

Shortest

High