Types of output devices of the computer system

In the previous session we studied about the types of devices used in computer systems. Out of which in this blog we are going to study in detail about the types of input devices.Output is data that has been processed into useful information. It can be displayed or viewed on a monitor, printed on a printer, or listened through speakers or a headset. Generally, there are two basic categories of output: the output, which can be readily understood and used by the humans, and which is stored on secondary storage devices so that the data can be used as input for further processing. The output,which can be easily understood and used by human beings, are of the following two forms.

1. Hard Copy: The physical form of output is known as hard copy. In general, it refers to the recorded information copied from a computer onto paper or some other durable surface such as microfilm. Hard copy output is permanent and a relatively stable form of output. This type of output is also highly portable. Paper is one of the most widely used hard copy output media. The principal examples are printouts, whether text or graphics, from printers
2. Soft Copy: The electronic version of an output, which usually resides in computer memory and/or on disk, is known as soft copy. Unlike hard copy, soft copy is not a permanent form of output. It is transient and is usually displayed on the screen. This kind of output is not tangible, that is, it cannot be touched. Soft copy output includes audio and visual form of output, which is generated using a computer. In addition, textual or graphical information displayed on a computer monitor is also soft copy form of output.

Based on the hard copy and soft copy outputs, the output devices are classified into: hard copy and soft copy output devices. Printers and plotters are the most commonly used hard copy output devices. The commonly used soft copy output device is computer monitor.

• Since the dawn of the computer age, producing printed output on paper has been one of the computer’s principal functions. A printer prints information and data from the computer onto a paper. Generally,the printer prints 80 or l32 columns of characters in each line, and prints either on single sheets, or on a continuous roll of paper, depending upon the printer itself. The quality of a printer is determined by the clarity of a print it can produce, that is, its resolution.
• Resolution is used to describe the sharpness and clarity of an image. The higher the resolution, the better the image. For printers, the resolution is measured in dpi (dots per inch). The more dots per inch, the better will be the quality of image. The dots are so small and close together that they project the image as a solid one. If a printer has a resolution of 600 dpi, it means that the printer is capable of printing 360,000 dots per square inch.
• Printers are divided into two basic categories: impact printers and non-impact printers. As their names specify, impact printers work by physically striking a head or needle against an ink ribbon to make a mark on the paper. This includes dot matrix printers, daisy wheel printers, and drum printers. In contrast, ink-jet and laser printers are non-impact printers. They use techniques other than physically striking the page to transfer ink onto the page.

• Dot matrix printer (also known as the wire matrix printer) uses the oldest printing technology and it prints one character at a time. It prints characters and images as pattern of dots. The speed of dot matrix printers is measured in characters per second (cps).
• Most dot matrix printers offer different speeds depending on the quality of print desired. The speed can vary from about 200 to over 500 cps. The print quality is determined by the number of pins (the mechanisms that print the dots), which can vary from 9 to 24. The more pins per inch, the higher the print resolution.
• The best dot matrix printers (24 pins) can produce near letter-quality type image. Most dot matrix printers have a resolution ranging from 72-360 dpi.Dot matrix printers are inexpensive and have low operating costs. These printers are able to use different types of fonts, different line densities, and different types of paper. Many dot matrix printers are bi-directional, that is, they can print the characters from either direction, left or right.

• The major drawback of dot matrix printer is that the pattern of dots that make up each character are visible on the print produced by it, making it look unprofessional. If you need a printer that can produce professional letter quality documents, you need a daisy wheel printer.Daisy wheel printer is named so because the print head of this printer resembles a daisy flower, with the printing arms that appear like the petals of the flower.These printers are commonly referred to as letter quality printers as the print quality is as good as that of a high-quality typewriter.
• Daisy wheel printers produce high-resolution output and are more reliable than dot matrix printers. They can have speed up to 90 cps. These printers are also called smart printers because of its bidirectional printing and built-in microprocessor control features. However, daisy wheel printers give only alphanumeric output. They cannot print graphics and cannot change fonts unless the print wheel is physically replaced. These printers are usually very slow because of the time required to rotate the print wheel for each character desired. Daisy wheel printers are slower and more expensive than dot matrix printers. However, if the appearance of the correspondence is important and you do not need graphics,a daisy wheel printer is a better choice.

Drum Printer

• The most common type of printer found in homes today is the ink-jet printer. An ink-jet printer is a printer that places extremely small droplets of ink onto paper to create an image. Being a non-impact printer, it does not touch the paper while creating an image.
• Instead, it uses a series of nozzles to spray drops of ink directly onto the paper. Inkjets were originally manufactured to print in monochrome (black and white) only. However, the print head has now been expanded and the nozzles increased to accommodate cyan (C), magenta (M), yellow (Y), and black (K). This combination of colours is called CMYK.
• It allows for printing images with nearly the same quality as a photo development lab using certain types of coated paper.
• Ink-jet printers are costlier than dot matrix printers but the quality is much better. These printers can print any shape of character, which a user can specify as they produce printed output as pattern of tiny dots. This allows the printer to print many special characters, different sizes of print, and enables it to print graphics such as charts and graphs. Ink-jet printers typically print with a resolution of 600 dpi or more. Due to the high resolution, these printers produce high quality graphics and text printouts.
• They are also affordable, which appeals to small businesses and home offices.These printers print documents at a medium pace but slow down if printing a document with multi-colours. These printers can print about 6 pages a minute and can be programmed to print symbols such as Japanese or Chinese characters.

Laser Printer

• A laser printer provides the highest quality text and images for personal computers today.It is a very fast printer,which operates on the same principle as that of a photocopy machine.Most laser printers can print text and graphics with a very high quality resolution.They are also known as page printers because they process and store the entire page before they actually print it.
• They produce sharp, crisp images of both text and graphics, providing resolutions from 300 to 2400 dpi.Today, the resolution of most printers is 600 dpi.They are quiet and fast, able to print 4-32 text only pages per minute for individual microcomputers and up to 200 pages per minute for mainframes. Laser printers can print in excess of 2000 lines per minute. Furthermore, they can print in different fonts, that is, type styles and sizes.
• Laser printers are often faster than ink-jet printers but are more expensive to buy and maintain than the other printers. The cost of these printers depends on a combination of costs of paper, toner replacement, and drum replacement. These printers are useful for volume printing because of their speed.

 Plotters

• A plotter is a pen-based output device that is attached to a computer for making vector graphics, that is, images created by a series of many straight lines. It is used to draw high-resolution charts, graphs,blueprints, maps, circuit diagrams, and other line-based diagrams. It is similar to printer, but it draws lines using a pen.
• As a result, it can produce continuous lines, whereas printer can only simulate lines by printing a closely spaced series of dots. Multicolour plotter uses different-coloured pens to draw different colours. Colour plots can be made by using four pens (cyan, magenta, yellow, and black) and need no human intervention to change them.
• Being vector-based, a plotter tends to draw much crisper lines and graphics. The lines drawn by these devices are continuous and very accurate. However, plotter is considered a very slow output device because it requires excessive mechanical movement to plot. Furthermore, it is unable to produce solid fills and shading.
• Plotters are relatively expensive as compared to printers but can produce more printouts than standard printers. They are mainly used for Computer Aided Design (CAD) and Computer Aided Manufacturing (CAM) applications such as printing out plans for houses or car parts. These are also used with programs like AUTOCAD (computer assisted drafting) to give graphic outputs. There are two different types of plotters, drum plotter (where the paper moves) and flat-bed plotter (where the
  paper is stationary).

1.Drum Plotter

• In drum plotters, the paper on which the design is to be printed is placed over a drum. These plotters consist of one or more pen(s) that are mounted on a carriage which is horizontally placed across the drum. The drum can rotate in either clockwise or anti-clockwise direction under the control of plotting instructions sent by computer. In case, a horizontal line is to be drawn, the horizontal movement of a pen is combined with the vertical movement of a page via the drum. The curves can also be drawn by creating a sequence of very short straight lines. In these plotters, each pen can have ink of different colour to produce multicolour designs. Drum plotters are used to produce continuous output such as plotting earthquake activity or for long graphic output such as tall building structures.

2.Flat-bed Plotter

• Flat-bed plotters consist of a stationary horizontal plotting surface on which paper is fixed. The pen is mounted on a carriage, which can move horizontally, vertically, leftwards or rightwards to draw lines. In flat-bed plotters, the paper does not move, the pen-holding mechanism provides all the motion. These plotters are instructed by the computer on the movement of pens in the x-y coordinates on the page. These plotters are capable of working on any standard, that is, from A4 size paper to some very big beds. Depending on the size of the flat-bed surface, these are used in designing of ships, aircrafts, buildings, and so on. The major disadvantage of this plotter is that it is a slow output device and can take hours to complete a complex drawing.

Computer Monitor

• The monitor is the most frequently used output device for producing soft-copy output. A computer monitor is a TV like display attached to the computer on which the output can be displayed and viewed.The computer monitor can either be a monochrome display or a colour display. A monochrome screen uses only one colour (usually white, green, amber or black) to display text on contrasting background.
• Colour screens commonly display 256 colours at one time from a selection of over 256,000 choices. Monitors are available in various sizes like 14, 15, 17, 19, and 21 inches. The size of the display is described based on two parameters: aspect ratio and screen size. Aspect ratio is the ratio of the width of the display screen to the height, that is, the ratio of vertical points to the horizontal points necessary to produce equal-length lines in both directions on the screen.Generally, computer displays have an aspect ratio of 4:3. Like televisions, screen sizes are normally measured diagonally (in inches), the distance from one corner to the opposite corner.
• Sometimes, while watching television, you may notice that the picture looks a bit blurred. The reason behind this is that the displayed image is not solid but is created by the configurations of dots. These dots are known as picture elements, pels, or simply pixels. The golden rule of a sharp image is that the more the pixels, the sharper the picture. The screen clarity depends on three basic qualities:

1. Resolution: It refers to the number of pixels in the horizontal and vertical directions on the screen. In medium resolution graphics, pixels are large, whereas in high-resolution graphics, pixels are small. The average CRT display is currently 800 × 600 or 1024 × 768. The more dots, or pixels,available to create the image, the sharper it will be. Therefore, a resolution of 1024 × 768 will produce sharper images (for example, smaller icons and more information) than one of 640 × 480.
2. Dot Pitch: It is the measurement of the diagonal distance between two like-coloured (red, green or blue) pixels on a display screen. It is measured in millimetres and common dot pitches are .51 mm,.31 mm, .28 mm, .27mm, .26 mm, and .25 mm. Smaller the dot pitch, sharper will be the image when displayed on the monitor. Generally, a dot pitch of less than .31 mm provides clear images.Multimedia and desktop-publishing users typically use .25 mm dot-pitch monitors.
3. Refresh Rate: It is the number of times per second the pixels are recharged so that their glow remains bright. Normally, screen pixels are made from phosphor. An electron beam strikes the phosphor and causes it to emit light, resulting in the display of the image. However, it needs to be refreshed periodically because the phosphors hold their glow for just a fraction of a second. The refresh rate for a monitor is measured in Hertz (Hz) and varies from 60-75Hz. A refresh rate of 60HZ means image is redrawn 60 times a second. The higher the refresh rate, the more solid the image looks on the screen, that is, it does not flicker.

Cathode Ray Tube (CRT) Monitor

• Nowadays, most computer monitors are based on Cathode Ray Tube (CRT) technology. The basic operation of these tubes is similar to that in television sets. Figure 1 illustrates the basic components of a CRT.

• A beam of electrons (cathode rays) emitted by an electron gun passes through focusing and deflection systems that direct the beam toward specified positions on the phosphor-coated screen. The phosphor then emits a small spot of light at each position contacted by the beam. When the electron beam strikes the phosphors, the light is emitted for a short period of time, this condition is known as persistence.
• Technically, persistence is defined as the time it takes the emitted light from the screen to decay to 1/10 of its original intensity. Graphics monitors are usually constructed with persistence in the range from 10 to 60 microseconds. Since the light emitted by the phosphor fades very rapidly, some method is needed for maintaining the screen picture. One way to keep the phosphor glowing is to redraw the picture repeatedly by quickly directing the electron beam back over the same points. This type of display is called a refresh CRT.
• The primary components of an electron gun in a CRT are the heated metal cathode and a control grid. Heat is supplied to the cathode by directing a current through a coil of wire,called the filament,inside the cylindrical cathode structure.This causes electrons to be ‘boiled off’ the hot cathode surface.In the vacuum inside the CRT envelope, the free, negatively charged electrons are then accelerated toward the phosphor coating by a highly positive voltage.
• The accelerating voltage can be generated with a positively charged metal coating on the inside of the CRT envelope near the phosphor screen, or an accelerating anode can be used, as in Figure 1 Note that sometimes the electron gun is built to contain the accelerating anode and focusing system within the same unit.
• Before reaching the phosphor-coated screen, the electrons have to be passed through the monitor’s focusing system. The focusing system is initially set up to focus the electron flow into a very thin beam and then in a specific direction. Focusing can be accomplished either by electric or by magnetic fields.
• When the electrons in the beams collide with the phosphor coating, their kinetic energy is absorbed by the phosphor. Some of this energy is converted into heat while rest of the energy causes the electrons in the phosphors to move up to the higher energy levels. After this, when these electrons begin to return to the ground state, they emit light at certain frequencies. These frequencies are proportionate to the energy difference between the higher state and the ground state. As a result, the image, which we see on the screen, is the combination of all the electron light emissions.

Liquid Crystal Display (LCD) Monitor