1.2.2.7: Output Devices

Output Devices

The final stage of information processing involves the use of output devices to transform computer-readable data back into an information format that can be processed by humans. As with input devices, when deciding on an output device you need to consider what sort of information is to be displayed, and who is intended to receive it.

One distinction that can be drawn between output devices is that of hardcopy versus softcopy devices. Hardcopy devices (printers) produce a tangible and permanent output whereas softcopy devices (display screens) present a temporary, fleeting image.

 

Display screens

The desk-based computer screen is the most popular output device. The standard monitor works on the same principle as the normal TV tube: a “ray” gun fires electrically charged particles onto a specially coated tube (hence the name Cathode-Ray Tube or CRT). Where the particles hit the coating, the “coating” is being “excited” and emits light. A strong magnetic field guides the particle stream to form the text or graphics on your familiar monitor.

CRTs vary substantially in size and resolution. Screen size is usually measured in inches diagonally across from corner to corner and varies from as little as 12 or 14 inches for standard PCs, to as much as 40+ inches for large demonstration and video-conferencing screens. The screen resolution depends on a number of technical factors.

A technology that has received much impetus from the fast-growing laptop and notebook market is the liquid crystal display (LCD). LCDs have matured quickly, increasing in resolution, contrast, and colour quality. Their main advantages are lower energy requirements and their thin, flat size. Although alternative technologies are already being explored in research laboratories, they currently dominate the “flat display” market.

Organic light-emitting diodes (OLED) can generate brighter and faster images than LED technology, and require thinner screens, but they have less stable colour characteristics, making them more suitable for cellular telephone displays than for computers.

Another screen-related technology is the video projection unit. Originally developed for the projection of video films, the current trend towards more portable LCD-based lightweight projectors is fuelled by the needs of computer-driven public presentations. Today’s units fit easily into a small suitcase and project a computer presentation in very much the same way a slide projector shows a slide presentation. They are rapidly replacing the flat transparent LCD panels that needed to be placed on top of an overhead projection unit. Though the LCD panels are more compact, weigh less and are much cheaper, their image is generally of much poorer quality and less bright.

 

Printers and plotters

Printers are the most popular output device for producing permanent, paper-based computer output. Although they are all hardcopy devices, a distinction can be made between impact and non-impact printers. With impact printers, a hammer or needle physically hits an inked ribbon to leave an ink impression of the desired shape on the paper. The advantage of the impact printer is that it can produce more than one simultaneous copy by using carbon or chemically-coated paper. Non-impact printers, on the other hand, have far fewer mechanically moving parts and are therefore much quieter and tend to be more reliable.

The following are the main types of printers currently in use.

• Dot-matrix printers used to be the familiar low-cost printers connected to many personal computers. The print head consists of a vertical row of needles each of which is individually controlled by a magnet. As the print head moves horizontally across the paper, the individual needles strike the paper (and ribbon in between) as directed by the control mechanism to produce text characters or graphics. A close inspection of a dot-matrix printout will reveal the constituent dots that make up the text. Although it is one of the cheapest printer options, its print quality is generally much lower that that of laser and ink-jet printers. However, today’s models are quick and give a much better quality by increasing the number of needles.
• Laser printers are quickly growing in market share. They work on the same principle as the photocopier. A laser beam, toggled on and off very quickly, illuminates selected areas on a photo-sensitive drum, where the light is converted into electrical charge. As the drum rotates into a “bed” of carbon particles (“toner”) with the opposite charge, these particles will adhere to the drum. The blank paper is then pressed against the drum so that the particles “rub off onto the paper sheet. The sheet then passes through a high-temperature area so that the carbon particles are permanently fused onto the paper. Current high-end laser printers can cope with extremely large printing volumes, as is required e.g. by banks to print their millions of monthly account statements. The laser technology continues to develop in tandem with photocopier technology. Laser printers can now handle colour printing, double-sided printing or combine with mail equipment to perforate, fold, address and seal automatically into envelopes. At the lower end of the scale are the low-cost “personal” laser printers, which give a very good printing quality at a relatively modest cost.
• Thermal printers use heat to print. The older thermal printers used heat-sensitive paper, similar to the special fax paper. A slight heat or pressure will leave a darker area. This produced very cheap but low-quality output. Currently, thermal-printing technology is used mainly for high-quality color printing. These new thermal printers use colored wax sticks and melt the wax onto the paper. Although they are slower than competing color laser and inkjet technologies, they give a much more vibrant, color-saturated image.
• Inkjet printers are probably the most popular low-cost printing technology. Liquid ink is squirted onto the paper in the form of tiny droplets. These printers are about the same price as dot-matrix printers, albeit more expensive in terms of consumables. Their quality is close to that of the laser printers. Their great advantage is that the printers can easily be adapted to use coloured ink, thus making popular color printers.
• Plotters are mainly used for engineering and architectural drawings. A plotter consists of one (or several – in the case of color plotters) pen(s) affixed to an arm. As the arm moves across the sheet of paper, the pen draws lines onto the paper. It is ideal for line drawings such as plans, especially in cases where the paper size exceeds that which can be accommodated by the other types of printers.
• Chain and line printers are still popular in mainframe environments for the quick production of large volumes of internal printing. The line printer consists of a horizontal, rotating “drum” with 132 cylinders, each containing a full character set. As the 132-column wide paper moves up past the drum, a line at a time, each one of the 132 hammers on the other side of the paper strikes at the exact moment that the corresponding cylinder “shows” the correct character. The hammer hits the drum (and ink ribbon) and leaves an imprint of the character on the paper. The chain printer works on the same principle, but uses a horizontally rotating chain with engraved characters, instead of a drum. As anyone with some working experience in a large organization knows, the print quality of these “computer printouts” is not very high.

 

Figure 4-4 compares the various output devices in terms of a number of characteristics.

Figure 4: Comparison of output devices

Device	Technology	Quality	Speed	Duplicates?	Graphics?	Fonts?	Colour?
CRT	softcopy	high	very fast	n/a	yes	yes	yes
LCD	softcopy	fair	very fast	n/a	yes	yes	yes
Plotter	hardcopy	fair	slow	no	yes	yes	yes
Chain/line printer	hardcopy	low	very fast	yes	no	no	no
Laser printer	hardcopy	high	fast/fair	no	yes	yes	yes
Dot-Matrix printer	hardcopy	fair	fast/fair	yes	yes	yes	some
Inkjet printer	hardcopy	good	fair	no	yes	yes	yes

 

Audio-output devices

A type of output that is becoming increasingly popular different types of audio output. is audio output. There are many different types of audio output.

• Sound output is required by most multimedia applications and sophisticated games. The sound card in many of today’s personal computers synthesizes sound by drawing from a library of stored sounds, essentially using the same process as found in music keyboards. More advanced multimedia workstations are equipped for full stereo multi-channel surround sound and easily surpass many a modern hi-fi system in cabling and speaker complexity.
• MIDI in/output. Modern day music production would be impossible without a vast array of electronic instruments and keyboards. These are typically controlled by a personal computer by means of Musical Instrument Digital Interface (MIDI), a common standard for linking, controlling and processing electronic music.
• Speech synthesis is the production of speech-like output using an artificial voice. Although the lack of intonation still makes the voice sound artificial, the technology is reasonably mature and can be found anywhere from talking clocks and luxury cars to automated responses for telephonic directory enquiries.

 

Other Output Devices

Many other, extremely specialized input and output devices have been developed. Process control, for example, is a very specialized field but extremely important for automated factories (car manufacturing, canneries), continuous process environments (nuclear plants, refineries) or hazardous places (microbiological research laboratories, space exploration). For these applications, the computer relies on a multitude of sensors for its inputs: temperatures, speed, pressure, flow rates, weight, position, … These sensor inputs are then processed by the computers, which in turn control directly robot arms and other mechanical devices such as cutters, welding equipment, valves, switches, mixers etc.