Output Device

Other Output Devices

An output device is a part of the computer system which is used to show results. Output devices convert information from the electronic form used by the Computer into a form that people can easily work with. Output normally can be produced in two ways; either on a display unit or on a paper. Other kinds of output such as speech output, mechanical output are also being used in certain applications. Printer receives data from the Computer and translates the data into a printed document. The same data could also be displayed on the video display unit. The method of output used depends often on whether the user wants the output to be permanent and on paper or not. Let us discuss some of the common output devices.
Display Devices
One of the most important peripherals in computer is the display device. Graphic display is made up of a series of dots called 'pixels' (picture elements) whose pattern produces the image. Each dot on the screen is defined as a separate unit, which can be directly addressed. Since each dot can be controlled individually there is much greater flexibility in drawing pictures. There are three categories of display screen technology;
1. Cathode Ray Tube (CRT)
2. Liquid Crystal Display (LCD)
3. Projection Displays
CRT Displays
The CRT (Cathode Ray Tube) monitor receives video signals from the computer and displays the video information as dots on the CRT screen. The main unit to the CRT monitor is the CRT itself; it is usually called a picture tube. The CRT is an evacuated glass tube with a fluorescent(phosphor) coating on the inner front surface, called screen. An electron gun at one end (neck) emits an electron beam. This beam is directed towards the screen. When the beam strikes the screen, the phosphor coating on the screen produces Illumination at the spot where the electron beam strikes.
The electron beam is deflected by an electromagnetic deflection in order to produce illumination at various spots on the screen. Horizontal deflection coils deflect the beam in the horizontal direction and the vertical deflection coils deflect the beam in the vertical direction. The illumination caused on the screen exists for a few milliseconds due to the persistence of the phosphor. To create a permanent image on the screen, it is necessary to cause illuminations repeatedly. This is done by scanning the CRT screen with the electron beam.

There are two types of CRT displays:

1. Vector CRT displays in which the electron beam is directed only to the places where the image is to be created.
2. Raster Scan displays in which the image is projected on to the screen by directing the electron beam across each row of picture elements from the top to the bottom of the screen, This type of display provides a high dynamic capability since image is continuously refreshed and it allows for continual user input and output. It offers full colour display at a relatively low cost and is becoming very popular.
Liquid Crystal Displays (LCD)
First introduced in watches and clocks in 1970s, LCD is now applied to display devices of a computer. The major advantage of LCD is the low energy consumption. The CRT is replaced by liquid crystal to produce the image. These also have colour capability but the image quality is relatively poor. First introduced in watched and clocks in1970s, LCD is now applied to display devices of a computer. The major advantage of LCD is the low energy consumption. The CRT is replaced by liquid crystal to produce the image. These also have colour capability but the image quality is relatively poor.
Projection Displays
A large screen upon which images are projected replaces the personal size screen of the previous displays. These are normally used for large group presentation. These systems can be connected to computer and whatever appears on the computer terminal gets enlarged and projected on e large screen. Another popular method is to connect computer to an LCD flat screen and to project the LCD image using Overhead Projector. These are popularly used for seminars, classrooms, marketing presentations, etc.
Terminals
Terminal is a combination of Input unit and Output unit like VDU and keyboard. Terminals have become very popular interactive input and output units. With the help of a terminal data and instructions can directly be sent into the computer. Terminals can be classified into two types, namely hard copy terminals and soft copy terminals. A hard copy terminal provides a printout on paper where a soft copy terminal provides a visual display on screen. A soft copy terminal is also known as a CRT (Cathode Ray Tube) terminal. Terminals are also classified as dumb terminals or intelligent terminals depending upon provision for any intelligence or otherwise as the terminal. Current technology has brought about a low price differential between intelligent and dumb terminals. This encourages use of intelligent terminals.
Printers
Printer is a very common and popular output device. It accepts text and graphic output from a computer and transfers the information to paper, usually to standard size sheet of paper. Printed output provides permanent record and is easy to copy and distribute. Familiar, computer printed documents include pay slips, Familiar, computer printed documents include pay slips, bank statements electricity bills, etc. Different types of printers with different brand names are available in the market. They vary in size, speed, sophistication, and cost. Generally we divide printers into two categories: impact and non-impact.
Impact Printers
Impact printers are printers that create characters by striking a print page with a print hammer (like in typewriter). The impact printers developed for use with computers work in much the same way as typewriters.
1. Dot Matrix Printer
This is also a character printer. In the dot-matrix printer, an arrangement of tiny hammers strikes to produce the desired characters. Each hammer prints a small dot on the paper.
2. Line Printer
It prints a line at a time. In the line printer, raised characters extend the length of the drum. There are as many bands of type as there are printing positions.. Each band contains all the possible characters. The drum rotates rapidly, and one revolution is required to print each line. A fast acting hammer opposite each band strikes the paper against the proper character as its passes. Thus, in one rotation, hammers of several positions may “fire” when the A row appears, several others may strike to imprint D's, etc. At the end of the rotation, the line has been printed.
Non Impact Printers
As their name implies, non-impact printer employ some process other than hammers or similar"percussion type'' mechanisms to form characters on a print page. It uses techniques such as ink spray, heat, Xerography or laser to form printed copy, i.e., characters are not formed by mechanical impact. Usually, non-impact printers are quieter than impact printer. 1. Inkjet Printer
It is a non-impact character printer. It fires a fine jet of dots of quick drying ink onto the paper to form character or dot graphics by using an electrostatic field. When Continuous stream inkjet printers are used, droplets of ink are electronically charged after leaving a nozzle. The droplets are then guided to the proper position on the paper by electrically charged deflection plates. The print quality is good because the character is formed by dozens of tiny dots.
2. Laser Printers
They are page printers, i.e., print a whole page at a time. Laser printers write the desired output image on a copier drum with a light beam that operates under computer control. A difference in a electric charge is created on those parts of the drum surface exposed to the laser beam. These laser-exposed areas attract a toner that attaches itself to the laser generated charges on the drum. The toner is then permanently fused on the paper with heat or pressure.