PC Computer Hardware Output Devices Pt 1

PC Computer Hardware Output Devices

Visual Display Unit (VDU) The Screen or Monitor

Till third generation of computer it was not possible to verify whether the program being typed into computer‘s memory is correct or correctly entered. It was possible only when microcomputer was introduced. The micro computer consisting of one visual display unit as essential part on which the user can directly see, whatever he or she typing on keyboard. When key is pressed corresponding character is displayed on VDU. The result of the calculation or manipulation can also be seen directly on visual display unit and that to instantaneously. By addition of this unit you can be interactive with computer, i.e. Can talk to comport by typing query and computer will answer your query.
Visual display unit looks likes a color television and is fabricated as integral part of microcomputer. Sometimes it may be an independent unit and connected to keyboard and CPU by flexible chords. Quite often the VDU houses the floppy disk drive. The home computer can be attched to color television itself but thruogh a interfacing device. Some television have this interfacing device in built with them

VDUs screen is made up Cathode Ray Tube (CRT) due to which it is also called CRT terminal. When the data input is fed through the key board of the computer, electrical signals are generated; the CPU converts these signals into alphanumeric or graphic display. So the output is obtained as optical display. It contains about 20 to 40 rows of horizontal lines with each lines divided into 80 space. Some computers are provided with the facility of changing the modes of operation. In that case the number of columns and rows will change. The mode of dividing the screen to the several small squire grid areas for visual display is known as resolutions. One squire area is meant for displaying one dot. A group of several dots makes up a pixel. Every character is made up for several pixels. When the entire screen is divided into high number of squire areas , then that particular mode are VDU is said have high resolution. With high resolution screen the information more accurately represented.

The screen is coated with dots for phosphors called pixel from inside. The more is number of pixels from inside. The more is number of pixels or dots. In a screen, the better will be capability of computer to display the information. The phosphors can be made to glow red, green or blue by firing electons at them from behind.

The electrons have to be guided to the right spot through the vertical and horizontal deflection systems. This is ensures that the beam lights up the right phosphors to produce the correct image on the screen.

All the times , the screen deals with tiny points of light, grouped together , form letters or numbers . It is moves so fast that you never notice all the steps involved inputting together a screen full of information. To make it even faster, microcomputers often have ROM (Read Only Memory) the monitors can be broadly classified as :

MONOCHROME MONITORS

COLORED MONITORS

MONOCHROME MONITOR

These are generally black and white monitors or green phosphor monitors they do not receive TV signals.

COLOR MONITORS

The colors are the human perception of the different wavelengths of light. It is possible

to create almost all visible colors using two systems of primary colors. On a color monitors, colors are displayed as varying intensities of red, green and blue dots. When red, green and blue are turned on high, white is produced. As the intensities are equally lowered, shades of grey are produced. When all dots are turned off, the base color of the screen appears.

LIQUID CRYSTAL DISPLAY (LCD)

In LCD’s liquid crystalline material is sandwiched between glass or plastic plates. The front plate is transparent and the back plate is reflective. There is a coating a of thin film on the front plate. The coating transparent and conductive. It is selection (segments) are in the shape of desired characters LCD’s don not emit their own light therefore, a light source is to be used. LCD’s simply changed the reflection of available light.

LCD TECHNOLOGY

A liquid crystal display is a thin, light weight display device with no moving parts. It consists of eclectically – controlled light- polarizing liquid trapped in cells between two transparent polarizing sheets. Polarizing axes of the two sheets are aligned in the same direction. Each cell is supplied with electrical contacts that allow an electric field to be applied to the liquid. Before an electric field is applied, the molecules in the liquid are in a relaxed state. Light can pass through the liquid without any effect o its direction of polarization , so entire assembly appears transparent . When an electric field is applied, the molecules in the liquid twist in to an excited state, causing the axis of polarization of the light to rotate. Thus the light has been polarized by passing through the first sheet , then rotated by passing trough the liquid , now has the wrong polarization of penetrate the second sheet . The result is that the activated part of the display appears dark .

TRANSMISSIVE AND REFLECTIVE DISPLAYS

LCD can be used in transmissive and reflective modes. A transmissive LCD is illuminated from one side and viewed from the opposite side. An activated cell therefore appears dark while inactive cells appear bright. This type is high brightness application such pocket television receivers. The lamp is used to illuminate the LCD in such product usually consumes more battery power that the LCD itself.

A reflective LCDs, are used in pocket calculators and digital watches, is viewed by ambient light reflected in a mirror behind the display. This type has lower contrast than the transmissive type, because the ambient light passes twice trough the display before reaching the viewer. The advantage of this type is that no lamp to consume power, so the battery life is long. A small LCD consumes so little power that can run a photovoltaic cell.

COLOR DISPLAYS

The liquid crystal used in LCD rotates all visible wavelength equally, but additional refinements have been added to the basic LCD to produce a color display.

In a color LCD each pixel is divided into three sections, one with red filter, one with green filter and one with blue filter the pixel can be made to appear an arbitrary color by varying the relative brightness of its three colored sections.

THIN FILM TRANSISTOR DISPLAY

An LCD is made with either a passive matrix display grid. The active matrix LCD is known as TFT (thin film transistors) display. The passive matrix has a grid of conductors with pixels located at each intersection in the grid a current is end across two conductors on the grid to control the light of any pixel. An active matrix has transistor located at each pixel intersection, requiring less current to control the luminance of a pixel. For this reason the current is on active matrix display can be switched on and off more frequently, improving the screen refresh time.

Medium sized displays, such as those in monochrome personal organizers and pocket television set, have a passive matrix structure. This type has one set of contacts for each row and column of the display, rather than one for each pixel. However the disadvantage is that only one pixel can be address at any instant. The other pixels have to remember their last state until the control circuit has time to revisit them. This results in reduced contrast and poor response to fast moving images. As the number of pixels increases, this type of displays becomes less and less attractive.

For high resolution color displays such as large LCD monitors, an active matrix system is used. The LCD panel contains, besides the polarizing sheets and cells of liquid crystal, a matrix of transistors. These devices store the electrical state of each pixel on the display while all the other pixel are being updated. This method provides much better brighter, sharper displays.