Output Units

The major component of computer system is CPU, Input Unit and Output Unit. All the performances of computer revolve around these components. The output unit is used to present soft and hardcopy of information.

The VDU (Visual Display Unit or Monitor) and printer are common output units. There are many categories of display units available for computer. The LED (Light Emitting Diode), LCD (Liquid Crystal Display), DVST (Direct View Storage Tube) etc are text and graphics display output devices. The size of screen is measured from one corner to diagonally opposite corner.


VDU (Visual Display Unit):

VDU or monitor is an output device used to display data and information. It is a CRT (Cathode Ray Tube) in which electron gun emits electrons, which glow when, strike the phosphor-coated screen.

Ferdinand Braun develops the first Cathode Ray tube in 1897. In 1953, CRT for color TV was developed. It has three electron guns for three primary colors-Red, Green and Blue. The cathode ray tube is an evacuated glass tube in which phosphor coated screen is used to display information.



When electron beam strike phosphor coating, it glows and lighted dots are produced. To produce color effect, three types of phosphor- (a) red, (b) green and (c) blue used. There are two types of magnetic coils for horizontal and vertical deflection of electron beam. Generally, Monitor has 80 columns and 25 rows for text display.

Technical terms:

  • Monochrome and color monitor: The monochrome monitor displays two colors- one for background and next for foreground. Only one electron gun is used to emit stream of electrons. The color may be black and white, green and black. In color monitors, three electron guns, each for one primary color are used to produce color effects.
  • Phosphor: The number, characters, symbols, and images are displayed on monitor are patterns of lighted phosphor dots. There are many types of phosphor dots are used for display. In black and white TV screen, P4 phosphor is used. The phosphor is classified as P1, P4, P7etc. These categories of phosphor are produced using Ca, Cd, Zn and some trace of rare earth metals. The ability to emit light up to long time is called persistence of phosphor and it is an important characteristic besides color and considered at the time of quality production of monitors.
  • Shadow Mask: Shadow Mask is a thin metal plate placed between phosphor coating and electron guns of color monitors. It contains tiny holes, through which electrons emitted from three electron guns pass and strike phosphor dots to produce color effects. Sony’s Aperture Grille is used is stead of shadow mask in color CRT.
  • Electron Guns: There are three electron guns used in color monitors each for one primary color. We know that red, green and blue are primary colors and its appropriate proportion produce several color combinations.
  • Pixel or Picture elements: It is a unit of presentation. It is made up of red, green and blue phosphor dots. Pixels are very tiny dots, arranged together in rows and columns, appear continuous when in-groups.
  • Dot Pitch: The measurement of quality of display is resolution and dot-pitch. The distance between centers of two pixels is called dot-pitch and its small value indicates sharpness of images. Its value varies 0.22 mm to 0.39 mm.
  • Resolution: The resolution of display depends on screen size, dot-pitch and display adapter. The display adapter is a circuit board to accept formatted information from CPU and to draw image on monitor. The number of pixels per centimeter that can be plotted horizontally and vertically is called resolution. Color monitors have VGA, SVGA cards to support graphics display and colors.







It stands for Color Graphics Adapter, introduced by IBM in 1981 to support different text and graphics display mode.



It stands for Enhanced Graphics Adapter, introduced in 1984 by IBM. It supports graphics mode 640 by 350 pixels and 16 colors.



It stands for Video Graphics Array, developed by IBM in 1987. It supports graphics mode of 640 by 480 pixels with 2 or 16 colors and 320 by 200 pixels with 256 colors.



Multi-Color Graphics Array graphics mode supports graphics mode of 640 by 480 pixels and 320 by 200 pixels.



Super Video Graphics Array supports 1024 by 768 pixels with 60,000 different colors.


  • Display of Character: A character is a pattern of lighted dots of dot matrix or array of dots. The size of dot matrix may be 5×7, 7×9, or 7×12. A character generator ROM is used to store dot patterns of characters. The display RAM or video RAM or display refresh RAM is used to store ASCII or EBCDIC codes of characters.


(a)    It stands for Liquid Crystal Display.

(b)   More economic, lighter in weight, not bulky but its cost increases just screen size increased by 1 inch.

(c)    Low voltage and low pressure consumption.

(d)   The user should sit just in front to monitor, if he sits with angle, contents cannot be visible.

(e)   The material that behaves like both liquid and solid is called Liquid Crystal and it is sandwiched between horizontal and vertical grids of electrodes. Electrical signals are used to turn pixels ON and OFF.

(f)     It is developed at RCA’s Sarnoff Research center of Princeton in 1963 and first time introduced in clocks in 1970. It is widely used in laptop computers, digital clocks, watches, microwave ovens, CD players etc.



(a) Light Emitting Diode

(b) A matrix of diodes are arranged to form pixel positions in display. Electrical current excites diodes to glow in the form of texts, numbers and images.

(c) Applications:

  • Remote control: It operates at infrared frequencies, so light is not visible to users.
  • Optical Touch Panels
  • Key-boards
  • Electronic type-writers
  • Optical Mousses


A Light Emitting Diode (LED) is a semiconductor device that emits visible light when an electric current passes through it. The chip has two regions separated by junction. The p region is dominated by positive charges, and n region is dominated by negative charge. The junction acts as barrier to the flow of electrons between the p and n region. Only when sufficient voltage is applied to the semiconductor chip, current flow, and electrons cross the junction into the p region.



The light is not particularly bright, but in most LEDs it is monochromatic, occurring a single wave length. The output from a LED can ranges from red (wave length 700 nanometers) to blue violet (400 nanometer). Some LEDs emit infrared energy (830 nanometer), such a device is known as infrared emitting diode (IRED). Nick Holonyak Jr. of the University of Illinois at Urbana Champaign developed the first practical visible spectrum LED in 1962.



Printer is an output device used to produce hard copy information. The price of printer depends on output quality, speed and printing technology employed.

The printer is divided on the two backgrounds:

(a) On the basis of printing method and

(b) On the basis of manufacturing technology employed to develop printer.

The quality of printer depends on following factors:

(a) Speed: The impact printer is slower than non-impact printer. The speed of dot matrix printer depends on number of pins. The expensive printer is faster than non-expensive printers. Generally, color printer is also slow.

(b) Resolution: The sharpness of text and image per inch is called resolution of computer. The quality of output depends on resolution. The high-resolution computer is expensive than low-resolution computers.

(c) Memory: The costly input/ output device has own local memory to hold data at the time of processing. The printer is slower than the processor so, if it has its own memory to hold processed data, the speed can be increased. So, high memory printer is preferred for quality and fast printings.

(d) Color: The color is a part of information. The colorful text and graphics are more attractive than plain text and graphics. So, color printer is also a part of quality printings.

Classification of Printer

Classification of Printer


Impact Printer

Non-Impact printer


The head strikes ink coated ribbon to print information.

Te thermal, chemical or optical techniques are applied to print information.


Noisy, slow and poor quality output.

Noiseless, fast and high quality output.


Example: Dot Matrix, Daisy wheel, Golf ball etc.

Example: Thermal, Laser and Ink-jet Printer


Character, Line and Page printer:


Character printer

Line Printer

Page Printer


It is a slow printer used for low volume printing.

It is faster than character printer and used very large volume printings.

It is used for fast and high quality large volume printings.


100 to 300 character per seconds.

200 to 2000 lines per minutes.

It can print 200 lines per minute.


Example: Dot Matrix, Daisy Wheel, Golf Ball Printer, Cylinder Printer

Example: Chain Printer, band printer and Drum printer

Example: Laser Printer, Ink-Jet Printer, Thermal Printer


Laser Printer: It is a non-impact, high-resolution page printer able to processed entire page at a time. It can print 100-200 characters per seconds or 15000 lines per minutes. Generally, its speed is measured in ppm (page per minute). It prints 8-10 pages per minute. The paper passed from electrically charged wire that stimulates a positive or a negative charge depending on tonal variation. The negatively charged region of paper remains white while positively charged attract carbon particles from toner cartilage and become permanent black in the form of text or graphs.

Color Ink-Jet Printer: It is also non-impact page printer, which head spew out ink onto paper. Inkjets are often used for good-quality color output and domestic use. They are often combined with other devices (fax or scanner for example). An inkjet print head is composed of a series of very small holes or nozzles, behind which can be found a reservoir of ink. Under normal conditions, the ink cannot flow though the nozzles because the gap is very small, and the ink reservoir is kept at a pressure slightly below that of the ambient pressure. Characters are formed when a small controlled amount of ink is forced through voltage-charged deflection plates and onto the paper. By synchronizing this action with the movement of the print head across the paper, text and images can be built up in a way similar to that of a dot matrix printer.

In the case of color printer, head contains four ink cartridges (CYMK, C for Cyan, Y for Yellow, M for Magenta and K for Black color). It prints 50 to 300 characters per seconds (cps) or 4500 lines per minutes.

Head technologies:

  • Thermal-Shock Print Heads: Hewlett Packard uses thermal print heads, for example on their Desk-Jet printers. Each ink nozzle has a heating element built around it and when the ink behind the nozzles is heated, it expands and is forced through the holes in a controlled manner, spraying a mark on the paper.
  • Piezoelectric (or electrostatic) Print Heads: Epson uses piezoelectric print heads, for example in their Stylus printers. A piezoelectric element is built into each nozzle. When an electric charge is applied, it changes shape and acts as a small pump forcing ink out through the nozzle. In general, thermal inkjet print heads are cheaper and simpler to produce, but the heating elements have a relatively short life. Most thermal printers use a combined print head and ink reservoir. When the ink runs out, the print head is also replaced.

Dot Matrix Printer: It is a slow, noisy printer which head contains matrix of pins. The number of pins denotes quality of output. In the market 7, 9, 14, 18 or even 24 pins head printer is available.

A type of printer that produces characters and illustrations by striking pins against an ink ribbon to print closely spaced dots in the appropriate shape. Dot-matrix printers are relatively expensive and do not produce high-quality output. However, they can print to multi-page forms (that is, carbon copies), something laser and ink-jet printers cannot do.

Important characteristics:

(i) Speed: Given in characters per second (cps), the speed can vary from about 50 to over 500 cps. Most dot-matrix printers offer different speeds depending on the quality of print desired.

(ii) Printing Quality: Determined by the number of pins (the mechanisms that print the dots), it can vary from 9 to 24. The best dot-matrix printers (24 pins) can produce near letter-quality type, although you can still see a difference if you look closely.

In addition to these characteristics, you should also consider the noise factor. Compared to laser and ink-jet printers, dot-matrix printers are notorious for making a racket.



Plotters are output devices used to produce single or multiple colored images and drawings. They use ink pen or ink-jet for drawing. Pens of varying colors and widths are used for shadings and line styles. The mostly used pens are wet-ink, ball point and felt-tip types. The 4-pens (cyan, magenta, yellow and black) and 8-pens (multi-color) plotters are available in markets. The Engineers, draftsmen use for machine drawings, landscape drawings, building designs etc. So, it is an important tool for CAD (Computer Aided Designs).

The plotters are classified as:

(a) Drum Plotters

(b) Microgrip Plotters

(c) Flat-bed Plotters and

(d) Ink-Jet Plotters


i. Drum Plotter:

(a) Long cylindrical drum and pen carriage

(b) Paper over drum

(c) Drum rotate back and forth


ii. Microgrip Plotters:

(a) no drum

(b) pinch wheel for back and forth movement

(c) high performance at low cost


iii. Flat-bed Plotter: The paper is placed over flat surface. The pen movements n x and y direction, is controlled by computer.


iv. Ink-jet Plotter:

(a) It is an electrostatic, multicolor, fast and suitable for large drawings.

(b) High resolution

(c) Paper is placed over a drum and ink-jet with different color mounted on a carriage Speaker:



The multimedia system without speaker is incomplete. It amplifies sound output. A good pair of speakers connected to sound card will give you loads of listening pleasure. The audio output of the computer system relayed through the speakers. This enables audible alerts to be given to the user, as well as the playback of other sounds and music. More advanced speaker systems exist, allowing certain programs (such as music creation software and games) to use surround sound capabilities to beneficial effect.



Modems are the most common type of communication processor. The term MODEM is not strange, in the context of modern computer societies. MODEM is an Analog electronic device used to convert digital signal into analog and vice versa. It was developed during 1950 to 1960 to connect mainframe computers. Now, it connects computer to Internet.

Modem has two parts:

(a) MOdulator and

(b) DEModulator.

Modulator converts digital signal into analog and demodulator converts analog signal into digital signal. The digital signal (square wave) is not suitable for long distance communication through bounded media (cable) because signal suffer from strong attenuation, delay distortion and noises, so, modem is used to convert digital signal into analog signal.

Modem is a signal converter. It is also I/O device because it convert input signal into digital and output signal into analog. There are two types of modem in use: internal and external modern.

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