3. Output Devices
Output devices are hardware components, which are used to display or print the processed information. The processed data, presented to the user via the output devices could be text, graphics, audio or video. Common output devices include monitor, printer, plotter, speaker etc.
i. Printer
Printers are the most popular output devices used today for producing hard-copy output. The print quality of the printer is determined by the resolution of the printer. Resolution is measured in dots per inch (dpi). Printers with high resolution i.e. more dpi provide better image quality. Different types of printers are described below.
a. Dot Matrix Printer
Dot-matrix printers are character printers that print one character at a time. They are the kind of impact printers that form characters and all kinds of images as patterns of dots. Since dot matrix printers produce printed output as patterns of dots, they can print. any shape of character that a program can describe. They, therefore, can print many special characters, different sizes of print, and also have the ability to print graphics such as charts and graphs. Dot-matrix printers are impact printers because they print by hammering the pins on the inked ribbon to leave ink impressions on a paper. Hence, they can be used to produce multiple copies by using carbon paper or Its equivalent. Due to impact printing, dot-matrix printers are noisy as compared to non-Impact printers. The speed of dot matrix printer lies between 200 and 600 characters per second (cps) an their resolution ranges from 72 to 360 dpi. Dot matrix printers normally come in two sizes – 80 column printer and 132 column printer.
b. Drum Printers
They are impact printers because they print by hammering on a paper and inked ribbon against the characters embossed on the drum. Due to impact printing, drum printers are noisy in operation and often use a cover to reduce the noise level. Printing speeds of drum printers are in the range of 300 to 2000 lines per minute.
c. Inkjet Printers
Inkjet printers are character printers that form characters and all kind of images on a paper. They are non impact printers. The print head of an inkjet printer contains up to 64 tiny nozzles that can be heated up selectively in a few microseconds by an integrated circuit resistor. When the resistor heats up, the ink near it vaporizes and is ejected through the nozzle making a dot on the paper placed in front of the print head. A high resolution inkjet printer has as many as 64 nozzles within a height of 7mm providing print resolution of around 360 dots per inch.
d. Laser Printers
Laser printer is a non-impact printer that creates latent image by means of a laser beam and then makes it visible by a toner, transfers and then fixes it on paper. The laser printer can print 5-24 pages of text per minute. The resolution of laser printers is 600 dpi . Some high model laser printers have resolution of 1200 dpi. The speed of laser printer is very high with a very low level of noise.
e. Chain/Band Printers
Chain/band printers are line printers that print one line at a time. It consists of a metallic chain/band on which all characters of the character set supported by the printer are embossed. A standard character set may have 48, 64 or 96 characters. The printer has a set of hammers mounted in front of the chain/band in a manner that an inked ribbon and paper can be placed between the hammers and chain/band. The total number of hammers is equal to the total number of print positions. Therefore, a printer supporting 132 print positions will have 132 hammers.
f. Thermal Printer
In this printer, heat elements produce dot-matrix images on heat sensitive paper. The output of thermal printer fades quality when exposed to light.
g. Electrostatic printer
Electrostatic printers are devices in which the parts of paper which are to be printed are electrostatically charged and attract a fine dust which is then focused to the paper by the application of heat.
ii. Plotter
Dot matrix, inkjet, and laser printers are capable of producing graphics output. However, many engineering design applications like architectural plan of a building, design of mechanical components of an aircraft or a car, etc., often require high-quality, perfectly-proportioned graphic output on large sheets. The various types of printers discussed above are not suitable for meeting this output requirement of such applications. A special type of output device, called plotters, is used for this purpose. Plotters are an ideal output device for architects, engineers, city planners and others who need to routinely generate high precision, hard copy, graphic output of widely varying sizes. They are mainly used for drawings in AUTOCAD (computer assisted drafting), Computer Aided Design (CAD) and Computer Aided Manufacturing (CAM) applications. Plotters are of two types:
a. Drum plotter
b. Flatbed plotter
a. Drum Plotter
In a drum plotter, the paper on which the design is to be made is placed over a drum that can rotate in both clockwise and anti-clockwise. directions to produce vertical motion, The mechanism also consists of one or more penholders mounted perpendicular to the drum s surface. The pen(s) clamped in the holder(s) can move left to right or right to left to produce horizontal motion. A graph-plotting program controls the movements of the drum and pens). That is, under computer control, the drum and pen(s) move simultaneously to draw designs and graphs on the sheet placed on the drum. The plotter can also annotate the designs and graphs so drawn by using the pen to draw characters of various sizes. Since each pen is program selectable, pens having ink of different colors can be mounted in different holders to produce multi-colored designs.
b. Flatbed Plotter
A flatbed plotter plots a design or graph on a sheet of paper spread and fixed over a rectangular flatbed table. In his type of plotter, normally the paper does not move and the pen holding mechanism provides all types of motions necessary to draw complex designs and graphs. That is, under computer control, the pen(s) move in the required manner to draw designs and graphs on the sheet placed on the flatbed table. The plotter can also annotate the designs and graphs so drawn by using the pen to draw characters of various sizes. Provision is also there to mount more than one pen in the pen(s) holding mechanism. Since each pen is program selectable, pens having ink of different colors can be mounted in different holders to produce multi-colored designs. The plot size is restricted by the area of the bed. Some may be as small as A4 size (8″x 11″ page) while some very large beds used in the design of cars, ships, aircrafts, buildings, highways, etc. can be up to 20 ft. by 50 ft. Some plotters can also etch on plastic or metal plates. in this case, the plastic or metal sheet is spread on the bed and the drawing pen has a sharp- edged needle.
iii. Screen Image Projector
Screen image projector is an output device used to project information from a computer on a large screen (such as a cloth screen or wall) so that a group of people can view it simultaneously. It is very useful for making presentations to a group of people with direct use of a computer. Before such an output device was available, the contents of a presentation were prepared using a computer. The presentation material was then printed on a printer and the printout was next reproduced on overhead projector transparency sheets by using a copier machine. Finally, the presentation was made by using an overhead projector. Special marker pens had to be used for marking certain portions of the contents on the transparency sheets during the presentation, .
iv. Monitor
Commonly known as monitor, the visual display unit (VDU) is a very important output device. It is made up of tiny dots called pixels, which are arranged in a rectangular form. The resolution of the monitor, which is nothing but the sharpness of an image, depends upon these pixels. Inside the monitor, there is wither a cathode ray tube (CRT) or liquid crystal display (LCD) . Therefore, monitors are either CRT or LCD type.
Based on the quality of resolution, monitors are classified into the following six categories:
a. Colour graphic adapter (CGA)
It displays both text and graphics with graphics of resolution 320X250 (no. of pixels). Its text resolution s of fair quality.
b. Monochrome display adapter (MAD)
It can display only text with good quality resolution.
c. Enhanced graphic adapter (EGA)
It can display both text and enhanced graphics with graphics resolution of 640X350. Its text resolution is also of good quality.
d. Hercules graphics adapter (HGA)
It can display both text and mono graphics with graphics resolution of 320X200. Its text resolution is of fair quality.
e. Video graphics adapter (VGA)
It can display both text and video graphics with graphics resolution of 640X480. Its text resolution is much better than all above discussed types.
f. Super VGA (SVGA)
This is the most popularly used monitor, which can display both text and video graphics with graphics while a monochrome monitor displays it in single colour
The clarity of image on the computer screen depends on three factor:
i. Resolution of screen
The number of pixels in horizontal and vertical direction. More the number of pixels, the sharper is the image. The common resolution screen is 800X600 and 1024X768
ii. Dot pitch
The diagonal distance between two coloured pixels on a display screen.
iii. Refresh Rate
The number of times per second the pixels are recharged so that their glow remains bright.
Visual Display Terminal (VDA) A monitor and keyboard together are known as Visual Display Terminal.
v. Computer Output Microfilm (COM)
COM is a technique to produce output on a microfilm media (microfilm reel or microfiche card). A microfiche card is a 4by 6 inch film sheet, which can store several hundred pages. COM is used for storing output in banking and insurance applications, medical X rays etc.
vi. Voice Response System
Just as a speech recognition system allows a user to talk to a computer, a voice response system enables a computer to talk to a A voice response system has an audio-response device that produces audio output. Obviously, the output is temporary, soft-copy output. Voice response systems are of two types – voice reproduction system and speech synthesizer. They are described below.
Voice Reproduction System
A voice reproduction system produces audio output by selecting an appropriate audio output from a set of pre- recorded audio responses. The set of pre-recorded audio responses may include words, phrases, or sentences spoken by human beings, music or alarms generated by musical instruments, or any other type of sound., The actual analog recordings of the pre-recorded sounds are converted into digital data first and then stored on a computer’s disk, or in its memory chip permanently. When audio output is to be produced. the computer selects the appropriate sound from the set of pre-recorded sounds. The selected sound is converted and then routed to a speaker to produce the audio output. back into analog form and then routed to a speaker to produce the audio output.
Voice reproduction systems are very useful in a wide range of applications. Their uses include:
a. Audio help for guiding how to operate a system. For example, banking industry uses voice reproduction systems in automatic teller machines to provide systematic guidance to customers on how to transact with the bank by using an ATM.
b. Automatic answering machines. For example, telephone enquiries for new telephone numbers in place of old numbers, or vacancy status of a flight or train is often taken care of by an automatic answering machine.
c. Video games are made exciting and interesting by playing an event-based sound from a set of pre- recorded sounds.
d. Talking alarm clocks. For example, every hour the clock speaks out what is the time by selecting the appropriate voice message corresponding to that hour from the set of pre-recorded voice messages: In addition, the clock may speak “its time to wake up” at the time set for alarm.
e. Talking toys and home appliances also use a voice reproduction system.
f. Often personal computers with audio facility are used for automated multimedia presentations during exhibitions.
Speech Synthesizer
A speech synthesizer converts text information into spoken sentences. To produce speech, these devices combine basic sound units called phonemes. From a given text information, sequence of words are combined into phonemes, amplified, and output through a speaker attached to the system. Speech synthesizers are still in their infancy because currently they can produce only limited unique sounds with only limited vocal inflections and phrasing. However, they are very useful in a wide range of applications. Their uses include:
a. For reading out text information to blind persons. For example, a recently published book may be scanned using a scanner, converted into text using OCR software, and read out to blind persons using a speech synthesizer. This allows blind persons to know the latest information published in a book as soon as it is printed.
b. For allowing those persons who cannot speak, to communicate effectively. For example, a person with this type of disability simply types the information and the speech synthesizer converts it into spoken words.
iii. For translation systems that convert an entered text into spoken words in a selected language. For example, a foreigner coming to India may enter a text he/she wants to communicate to an Indian, and the speech synthesizer converts it into spoken words of the selected Indian language.
