One of the most important output devices in computer system is its screen commonlycalled monitor. It is an output device and displays all the programs and applications towhich you are running in your computer system. Monitor is the visual display unit ofthe computer system. It displays images generated from the video output. Monitor ofthe computer system is used to display images without keeping a permanent recordinside it.Graphic display is made up of a series of dots called ‘pixels’ (picture elements) whosepattern produces images in computer system. Each dot on the screen is defined as aseparate unit which can be addressed separately. Since each dot on the screen can becontrolled separately by virtue of which it gives greater flexibility in drawing pictures.Number of dots per inch (dpi) is called the resolution of the screen and is used torepresent the quality of the computer system. There are three categories of displayscreen technology:1. Normal Cathode Ray Tube (CRT)2. Liquid Crystal Display (LCD) / (TFT LCD)3. Projection Displays

Flat-Panel Display Monitor

The flat-panel display refers to a class of video devices that have reduced volume, weight and power requirementcompare to the CRT. You can hang them on walls or wear them on your wrists. Current uses for flat-panel displaysinclude calculators, videogames, monitors, laptop computer, graphics display.

The flat-panel display are divided into two categories

Emissive Displays - The emissive displays are devices that convert electrical energy into light. Example are plasma panel and LED(Light-Emitting Diodes).

Non-Emissive Displays - The Non-emissive displays use optical effects to convert sunlight or light from some other source into graphics patterns.Example is LCD(Liquid-Crystal Device)

Following are few of the important output devices which are used in Computer Systems

Monitors

Graphic Plotter

Printer

Monitors

Monitor commonly called as Visual Display Unit (VDU) is the main output device of a computer. It forms images from tiny dots, called pixels, that are arranged in a rectangular form. The sharpness of the image depends upon the no. of the pixels.

There are two kinds of viewing screen used for monitors.

Cathode-Ray Tube (CRT)

Flat- Panel Display

Cathode-Ray Tube (CRT) Monitor

In the CRT display is made up of small picture elements called pixels for short.The smaller the pixels, the better the image clarity, or resolution.It takes more than one illuminated pixel to form whole character, such as the letter e in the word help.

A finite number of character can be displayed on a screen at once.The screen can be divided into a series of character boxes - fixed location on the screen where a standard character can be placed.

The most screens are capable of displaying 80 characters of data horizontally and 25 lines vertically. There are somedisadvantage of CRT

Large in Size

High Power consumptionVideo Display unit is the most commonly used output device on most personal computers. Visual display unit is another name for monitor of a computer .There are two basic types of VDUs. First is the typical monitor that you see on a desktop computer which looks like a television screen. This type of monitor uses a large vacuum tube called cathode ray tube. The second type, known as the flat panel display is used primarily with portable computers .There are several types of flat panel monitors but the most common is Liquid crystal display monitor .LCD monitors creates images with a special kind of liquid crystal that is normally transparent but becomes opaque when charged with electricity.

Visual Display units can be compared by the following factors:-SizeRefresh rateResolutionDot Pitch

The physical size of a monitor's display area has an obvious bearing on how well you can see images. The resolution of a visual display unit can be classified by the number of pixels on the screen, expressed as a matrix e.g 640 x 480.The refresh rate is the number of times per second that the electron gun scan every pixel on the screen and is measured in Hertz . Dot Pitch is the distance between the phosphor dots that make up a single Monitor is an output device that resembles the television screen and uses a CathodeRay Tube (CRT) to display information. The monitor is associated with a keyboardfor manual input of characters and displays the information as it is keyed in. It alsodisplays the program or application output. Like the television, monitors are alsoavailable in different sizes.(b) Liquid Crystal Display (LCD)LCD was introduced in the 1970s and is now applied to display terminals also. Itsadvantages like low energy consumption, smaller and lighter have paved its way forusage in portable computers (laptops).

Visual display units (VDUs) are television-like screens that provide the user-interface in the form of display of text, numbers and images. The VDUs may be monochrome or colour. The support of monochrome or colour and clarity of display depend on the type of video monitor and the video adapter installed in the microcomputer. The video display terminal (VDT) consists of a monitor orCRT and a keyboard. The CRT serves as an output device and the keyboard as an input device. Thus VDT is an input/output device. If the terminal is provided with some memory and certain processing capability, it becomes a smart or intelligent terminal. A terminal without processing power is called a dumb.

The monitor is the most common type of output device and is also called Visual Display Unit(VDU).1. Monochrome – This type of display uses only one type of colour or a grayscale 2. Colour – This type of display uses more than one type of colour and is generally called RGB monitors. The RGB stand for Red, Green and Blue and a combination of these colours are used to display the image on the monitor. Colour monitors can display range from 16 to16.7 million colours.

One can find different standards for monitors. They support different colour depths (number of coloursit supports). The most common standards for monitors are:1. VGA (Video Graphics Array) mode appeared in 1987. It offered a resolution of 720x400 in textmode and a resolution of 640 by 480 (640x480) in 16-colour graphics mode. It also offered a resolution of 1024 x 768. The VGA quickly became the baseline display mode for PCs.

2. SVGA (Super Video Graphics Array) is a graphics mode which can display 256 colours atresolutions of 640x200, 640x350 and 640x480. SVGA can also display higher definitions such as800x600 or 1024x768 by using fewer colors.

3. XGA (eXtended Graphics Array). is an IBM display standard introduced in 1990. Today, it is themost common appellation of the 1024 × 768 pixels display resolution, but the official definition isbroader than that. It was not a new and improved replacement for Super VGA, but ratherbecame one particular subset of the broad range of capabilities covered under the "Super VGA"umbrella. The initial version of XGA expanded upon IBM's VGA, adding support for two resolutions:800 × 600 pixels with high color (16 bits per pixel, i.e. 65,536 colors).1024 × 768 pixels with a palette of 256 colors (8 bits per pixel)

The Different Types of MonitorsThe different types of monitors are:Cathode Ray Tube (CRT) – uses the same technology as used in television sets.The CRT uses a vacuum tube in which an electron gun is installed. When electrons fired by the electron gun touch the phosphor layer situated at the internal side of the screen, this will glow. This dot of light is called a Pixel.

Liquid Display Unit / Liquid Crystal Display (LCD) – we usually find this installed in laptops and handheldcomputers. The technology used is different from that of the CRT. LCD uses tiny capsules filled withliquid crystals. When the molecules of the liquid are subjected to an electric field, the liquid crystals alignand light reflects off them.

Without the field, their alignment reverts to its original, nonreflecting arrangement, so the elements appear dark. Combinations of LCD’s are arranged to form patterns of reflected light that spell out numbers, symbols or letters.

TFT (Thin Film Transistor) is a variant of LCD which uses thin film transistor technology to improve image quality.

Summary of Video GlossaryCRT Cathode ray tube (monitor)VDU Video display unit (monitor)LCD Liquid crystal display – as available on hand held games and potables .Pixel The smallest area on a screen, seen closely as a tiny dot 14” 15” 17” 21” This is the length measured in inches (1 inch = 2.54cm) of the screen measured diagonally, that across two corners.

VGA Video graphics array, this provided,1. Introduced analogue monitors to the computer system; that is colour was no longer restricted to fixed colours but allowed different hues and colour blending.2. Graphics resolution of 1024 x 768 pixels3. Allows 256 coloursSVGA Super Video Graphics ArrayAs above but allows 16.7 million colour huesBit map Computer memory that represents a video imageSide NOTE: A terminal is an integration of a VDU with a keyboard. There are two types of terminals, namely INTELLIGENT TERMINLAS (having a CPU) AND DUMB TERMINALS (no CPU)

- Intelligent terminals can do some local processing of data and then communicate with a maincomputer just to relay the results- In a dumb terminal, all processing must be done through the main computing system

Interna tional journal OF INFORMATION TECHNOLOGY AND MANAGEMENT VOL-ii ISSUE 1 FEBRUARY20112249-4510

Scanner

The keyboard can input only text through keys provided in it. If we want to input a picture the keyboard cannot do that. Scanner is an optical device that can input any graphical matter and display it back. The common optical scanner devices are Magnetic Ink Character Recognition (MICR), Optical Mark Reader (OMR) and Optical Character Reader (OCR).

1) Magnetic Ink Character Recognition (MICR) - This is widely used by banks to process large volumes of cheques and drafts . Cheques are put inside the MICR. As they enter the reading unit the cheques pass through the magnetic field which causes the read head to recognise the character of the cheques.

2) Optical Mark Reader (OMR) - This technique is used when students have appeared inobjective type tests and they had to mark their answer by darkening a square or circular space by pencil. These answer sheets are directly fed to a computer for grading where OMR is used.

3) Optical Character Recognition (OCR) - This technique unites the direct reading of any printed character. Suppose you have a set of hand written characters on a piece of paper. You put it inside the scanner of the computer. This pattern is compared with a site of patterns stored inside the computer. Whichever pattern is matched is called a character read . Patterns that cannot be identified are rejected. OCRs are expensive though better the MICR.

Output Devices

Recall the model of an information system :

Once the information system has processed the inputs it has received it must output the results of the processing for the user.

An output device takes data from the computer and converts this into information in a form which is normally understandable by humans.

Many different output devices are available to use with a computer system. Here are descriptions of the most common output devices.

Monitors / Visual Display Units

Almost all computers have a monitor. Monitors are also known as Visual Display Units (VDUs). Most computers use this display as the main output device. There are two different types of display :

Desktop Monitors : These work in much the same way as a television set. They are bulky but fairly cheap to buy. e.g. £150 for a 14" screen.

Liquid Crystal Displays (LCDs) : These displays are completely flat and so can be used in portable computers and other small devices such as calculators. They are more expensive to make than desktop monitors. Currently only the most expensive ones (cost over £2000) are as high quality as desktop monitors. In ten years time LCDs will probably replace bulky desktop monitors.

The three most important features of a screen are its size, the colours it can display and its resolution. There is more information about these features below. They apply to both desktop monitors and LCDs.

Size : How big is the screen ? Typical sizes are 10" or 12" for LCDs and 14", 15" or 21" for desktop monitors. The size is measured along the diagonal from the bottom left hand corner to the top right hand corner of the screen.

Colour : Is the monitor colour or black & white ? Most new desktop computers have colour screens as they are no longer that much more expensive than black & white ones and modern computer applications work better with a colour monitor.

Resolution : An image displayed on the screen is made up of lots of dots called pixels. If you look closely at the screen you may be able to see these pixels. The resolution of the screen is how many pixels there are up and down and from left to right across the screen. A variety of different resolutions are available. For PCs these resolutions have names. e.g. VGA is 640 x 480. This means that there are 640 pixels in each row across the screen and 480 pixels in each column up and down the screen.

SVGA is usually 800 x 600. Displays with lots of pixels are called high resolution. Displays with fewer pixels are called low resolution. High resolution displays can show much more detail than low resolution ones and are required for applications such as Computer Aided Design (CAD) and Multimedia. Here are close-up pictures of a circle, one shown on a high resolution display and one on a low resolution display :

Images displayed on the screen are temporary, i.e. they do not last forever. For this reason they are called Soft Copy.

Printers and Plotters

People often prefer to get printouts of the work they have done rather than to look at it on the screen. Printouts are called Hard Copy. Unlike images on a screen hard copy can provide a permanent record of work and can be looked at away from the computer. Hard copy can be obtained using a printer or a plotter. There are lots of different types of printers and plotters available. They vary in the method they use to produce hard copy and the cost and the quality of the hard copy they produce. Different devices are suitable for different applications. Printers are usually divided into two different categories, (i) impact printers and (ii) non-impact printers.

This is a "family tree" of output devices that can produce hard copy :

Impact Printers

Impact printers produce output by striking a ribbon with a hammer. When the ribbon is hit a mark is made on the paper behind it. If carbon paper is put into the printer then an impact printer can print more than one copy of a document at a time. This feature is often used by companies when they print invoices, bills etc. Because of the hammering action impact printers are noisy when they are printing.

Dot Matrix Printers

Dot matrix printers produce an image by hammering a column of 9 or 24 pins against a ribbon. Characters are produced as the print head (which contains the pins) moves across the page.

Dot matrix printers are cheap to purchase and have low operating costs. Most can run in either draft mode (quick but very low quality) or Near Letter Quality mode (slower but better quality). The disadvantages of dot matrix printers are that the output quality is poor, it takes a long time to print a document and the noise can be annoying.

Daisy Wheel Printers

Daisy wheel printers are rarely used any more. A daisy wheel printer hammers character stamps (like those on typewriters) against a ribbon to produce high quality text output. The character patterns are all arranged around the edge of a wheel. When a letter is to be printed the wheel rotates so that the correct letter is against the ribbon.

A hammer then hits the letter against the ribbon to print it on the paper. Daisy wheel printers are very slow and noisy. It is not possible to print graphics using a daisy wheel printer.

Line Printers

Dot matrix and daisy wheel printers both print only one character at a time. For that reason they are called Character Printers. In contrast a line printer prints a whole line (typically 80 or 132 characters) of text at a time. These printers are very fast. They are expensive to buy and are used when high volumes of output are required. Example applications include printing telephone bills or bank statements.

Non-Impact Printers

Non-impact printers do not produce images by hammering pins or characters against the paper. Because there is no hammering action non-impact printers are very quiet but can not print multiple copies using carbon paper.

Laser Printer Ink Jet Printer

Laser Printers

Laser printers are called Page Printers because they print a whole page at a time. A laser beam is used to draw the image to be printed onto a light sensitive Drum. After the image has been drawn on the drum a fine powder ink called Toner is put onto it. The toner only sticks where the laser has drawn the image. The paper then passes over the drum and the toner is transferred onto the paper.

Laser printers produce very high quality output, are very quiet and very fast. Typical home laser printers can print 8 pages per minute (ppm). Some industrial versions print over 40ppm and can print on both sides of a piece of paper at the same time. The main disadvantage of this type of printer is the high cost. Small laser printers cost from £300 to buy and are more usually about £800. Colour laser printers cost £3000+. Running costs are higher than impact printers but lower than most ink jet printers.

Ink Jet Printers

Ink jet printers are the most popular type of printer for use at home or in a small business. This is because they are relatively cheap to purchase and can print in colour. Ink jet printers generate output by squirting tiny jets of ink at the paper to be printed on. A column of ink jets in the print head moves across the page squirting dots onto the page to make the image. Typically ink jet printers can produce 360 or 720 dots per inch (dpi) but these dots are larger than those that a laser printer makes so the quality although very good is not quite as perfect. Ink-jet printers can work in black and white or colour. Black and white versions cost from £50 and colour ones from about £100. They are slower than laser printers (3ppm) and cost slightly more to run.

Thermal Printers

Thermal printers are no longer used as printers for PCs. They are however still used in some computer control systems, tills and in many fax machines. These printers print on special heat sensitive paper which is expensive. The image is produced from small dots which are made on the paper by heating it up. The quality is reasonable and graphics can be printed.

Plotters

A plotter can be used to produce very large drawings on paper sizes up to A0 (16 times as big as A4). A plotter draws onto the paper using very fine pens. There are two types of plotter. They differ in the way that the pen can be moved about on the piece of paper to draw lines :

Flatbed Plotter : The paper is fixed and the pen moves left and right and up and down across the paper to draw lines.

Drum Plotter : The pen moves up and down on the paper and the paper is moved left and right by rotating a drum on which the paper is placed.

Plotters can automatically change their pens and so can produce colour output. The lines drawn by a plotter are continuous and very accurate. Plotters are very slow but produce high quality output. They are usually used forComputer Aided Design (CAD) and Computer Aided Manufacturing (CAM) applications such as printing out plans for houses or car parts. The quality of the output produced by ink jet printers is

now very good and large format (big) ink-jet printers are steadily replacing plotters for most tasks.

Comparisons

This table compares the main features of the different types of printers and plotters : 

Type Cost Speed Quality Graphics Paper Colour Fonts

Dot Matrix £50 + 200 cps Poor YesContinuous or Sheets

PoorLots with Software

Daisy Wheel

Not available.

20 cps Excellent No Sheets NoChange Wheel

Line Expensive.100s of ppm

Excellent NoContinuous or Sheets

No One

Laser£300-£10000Mainly £800

6 - 40 ppm Excellent Yes SheetsVery Expensive

Lots

Ink Jet£50+b/w£100+colour

3 ppm Excellent Yes Sheets Yes Lots

Thermal Cheap. 3 ppm Good YesRoll of Special Paper

No Lots

Plotter£400 - £4000

Very Slow Excellent YesLarge Sheets e.g. A4-A0

YesDrawn as Graphics

Abbreviations : PPM = pages per minute, CPS = characters per second

Other Output Devices

Sound and Speech

Sound can be output through loudspeakers or headphones. On PCs you also need to install a Sound Card to be able to output sound. If you want you can connect the computer to a hi-fi to get louder, better quality sound than you would if you plugged the loudspeakers straight into the computer. The sound produced by PCs is now better quality than sound that is recorded on a CD.

Sometimes a computer can read out text that has been entered into it by using a computerised or recorded voice. This type of system is known as speech synthesis.

Control Devices

Computers can also be used to control other devices such as switches, lights, pumps and motors. These devices are output devices.

Questions

(1) What is the difference between hard copy and soft copy ?

(2) Why do you think most desktop computers do not currently use LCD screens ?

(3) What is meant by screen resolution ? Why is it important ?

(4)

What is the size of this screen ?

(5) How many pixels are there in total on an SVGA screen ?

(6) What do all impact printers have in common ?

(7) Draw a design for a number 2 to be printed on a 9 pin dot matrix printer.

(8) How much would a colour laser printer cost ?

(9)

For each of the parts below select which type of printer (e.g. dot matrix, laser) is being described.

a) Is often used in fax machines.b) Is used for applications where high volumes of output are produced. e.g. printing bills for millions of customers.c) The best choice for a small company who want to do desktop publishing.d) Now rarely used, prints by rotating a wheel and pushing letters against a ribbon.e) Can print multiple copies on carbon paper and some can produce poor quality colour output.

(10) How does a plotter change the colour that it is drawing in ?

(11) Why do you think that the list of all books currently available is usually stored on microfilm rather than on a computer database ? [ Hint : Think of how many people will want to look at the list and

where they will look at it ]

(C) P. Meakin 1998