Types of output devices of the computer system

1. Hard Copy: We refer to the physical form of output as a hard copy. It typically means recorded information copied from a computer onto paper or another durable surface, such as microfilm. Hard copy output provides a permanent and relatively stable format. This type of output is also highly portable. Among various hard copy media, paper remains the most widely used. Printouts—whether text or graphics—from printers serve as the main examples.
2. Soft Copy: We call the electronic version of output a soft copy. It usually resides in computer memory or on a disk. Unlike hard copy, soft copy does not provide a permanent form of output. It remains transient and typically appears on the screen. Since users cannot touch it, soft copy output is intangible. Soft copy includes audio and visual outputs generated by a computer, as well as textual or graphical information displayed on a monitor.

We classify output devices based on whether they produce hard copy or soft copy output. Printers and plotters serve as the most common hard copy output devices. The computer monitor acts as the most commonly used soft copy output device.

• Since the dawn of the computer age, computers have primarily produced printed output on paper. A printer transfers information and data from the computer onto paper. Generally, printers print 80 or 132 columns of characters per line, using either single sheets or continuous rolls of paper, depending on the printer model. The printer’s quality depends on the clarity of its prints—that is, its resolution.
• Resolution describes the sharpness and clarity of an image. Higher resolution produces a better image. Printers measure resolution in dpi (dots per inch). The more dots per inch a printer places, the higher the image quality. Because the dots are tiny and closely packed, they form a solid-looking image. For example, a printer with a resolution of 600 dpi can print 360,000 dots per square inch.
• We divide printers into two basic categories: impact printers and non-impact printers. Impact printers create marks on paper by physically striking a head or needle against an ink ribbon. Examples include dot matrix, daisy wheel, and drum printers. In contrast, non-impact printers—such as ink-jet and laser printers—use methods other than physical striking to transfer ink onto the page.

• The dot matrix printer, also called the wire matrix printer, uses the oldest printing technology and prints one character at a time. It creates characters and images by forming patterns of dots. We measure the speed of dot matrix printers in characters per second (cps).
• Most dot matrix printers provide different speeds depending on the desired print quality. The printers can print anywhere from about 200 to over 500 characters per second (cps). Print quality depends on the number of pins—the mechanisms that create the dots—which range from 9 to 24. The more pins a printer uses per inch, the higher its print resolution.
• The best dot matrix printers use 24 pins and can produce near letter-quality images. Most dot matrix printers offer resolutions ranging from 72 to 360 dpi. They cost less to buy and operate compared to other printers. These printers can print with different fonts, line densities, and on various types of paper. Many dot matrix printers print bi-directionally, meaning they can print characters both from left to right and right to left.

• The main drawback of dot matrix printers lies in the visible dot patterns that form each character, which gives the prints an unprofessional appearance. For professional, letter-quality documents, users prefer daisy wheel printers. Named after their print head that resembles a daisy flower with petal-like arms, these printers produce typewriter-quality output.
• Daisy wheel printers produce high-resolution output and offer greater reliability than dot matrix printers. They can print at speeds up to 90 characters per second (cps). People also call them smart printers because they print bidirectionally and include built-in microprocessor controls. However, daisy wheel printers print only alphanumeric characters. They cannot print graphics or change fonts unless users physically replace the print wheel.
• These printers run slowly because the print wheel must rotate to the correct position for each character. Daisy wheel printers operate slower and cost more than dot matrix printers. However, if you prioritize the appearance of your documents and don’t need graphics, a daisy wheel printer offers a better option.

Drum Printer

• Most homes use ink-jet printers today. An ink-jet printer sprays extremely small droplets of ink onto paper to create an image. Since it does not physically touch the paper while printing, it belongs to the non-impact printer category.
• Instead, the printer uses a series of nozzles that spray ink drops directly onto the paper. Manufacturers originally designed inkjets to print only in monochrome (black and white). However, they have expanded the print head and increased the number of nozzles to handle cyan (C), magenta (M), yellow (Y), and black (K). We call this combination of colors CMYK.
• This technology lets printers produce images that nearly match the quality of photo development labs when using certain types of coated paper.
• Ink-jet printers cost more than dot matrix printers but deliver much better quality. They print any character shape users specify by forming patterns of tiny dots. This ability lets them print special characters, various font sizes, and graphics like charts and graphs. Ink-jet printers typically print at resolutions of 600 dpi or higher. Because of their high resolution, they produce high-quality graphics and text printouts.
• Small businesses and home offices find these printers affordable. They print documents at a medium pace but slow down when printing multi-colour documents. These printers produce about 6 pages per minute and can print symbols, including Japanese and Chinese characters, when programmed.

Laser Printer

• Laser printers deliver the highest quality text and images for personal computers today. They operate very quickly, using the same principle as photocopy machines. Most laser printers produce text and graphics with very high-resolution quality. People also call them page printers because they process and store the entire page before printing it.
• Laser printers produce sharp, crisp images of both text and graphics, offering resolutions from 300 to 2400 dpi. Today, most printers run at 600 dpi. They operate quietly and quickly, printing 4 to 32 text-only pages per minute for individual microcomputers and up to 200 pages per minute for mainframes. These printers can produce over 2,000 lines per minute. Additionally, laser printers print in various fonts, including different styles and sizes.
• Laser printers often print faster than ink-jet printers but cost more to buy and maintain than other types. The overall cost depends on paper, toner, and drum replacements. Because of their speed, users rely on laser printers for high-volume printing.

 Plotters

• A plotter connects to a computer and uses pens to create vector graphics—images made from many straight lines. People use plotters to draw high-resolution charts, graphs, blueprints, maps, circuit diagrams, and other line-based diagrams. Unlike printers that print dots, plotters draw lines directly with a pen.
• Because it uses a pen, a plotter produces continuous lines, while printers simulate lines by printing closely spaced dots. Multicolor plotters use different-colored pens to draw various colors. They create color plots with four pens—cyan, magenta, yellow, and black—and automatically switch between them without human intervention.
• Because plotters use vector-based technology, they draw much crisper lines and graphics. These devices produce continuous and highly accurate lines. However, plotters operate slowly due to the extensive mechanical movement required to plot images. Additionally, they cannot create solid fills or shading.
• Plotters cost more than printers but produce larger printouts than standard printers. People mainly use them for Computer Aided Design (CAD) and Computer Aided Manufacturing (CAM) applications, such as printing house plans or car parts. They also work with programs like AUTOCAD to generate graphic outputs. Plotters come in two types: drum plotters, where the paper moves, and flat-bed plotters, where the paper stays stationary.

1.Drum Plotter

• In drum plotters, users place the paper over a rotating drum. One or more pens mount on a carriage that moves horizontally across the drum. The drum rotates clockwise or counterclockwise based on plotting instructions from the computer. To draw a horizontal line, the plotter combines the pen’s horizontal movement with the drum’s vertical rotation. The plotter creates curves by drawing many short straight lines in sequence. Each pen can hold a different ink color to produce multicolor designs. People use drum plotters to produce continuous outputs, such as plotting earthquake activity or creating long graphic designs like tall building structures.

2.Flat-bed Plotter

• Flat-bed plotters have a stationary horizontal surface where users fix the paper. A pen mounts on a carriage that moves horizontally, vertically, left, or right to draw lines. Since the paper stays still, the plotter moves the pen mechanism to create the image. The computer sends instructions to control the pen’s movement along the x-y coordinates on the page.
• These plotters can work on any standard paper size, from A4 to very large beds. Designers use them for ships, aircraft, buildings, and other large projects depending on the flat-bed surface size. The major drawback of these plotters is their slow speed—they can take hours to complete complex drawings.

Computer Monitor

• The monitor serves as the most frequently used output device for producing soft-copy output. A computer connects to a TV-like display where users view the output. The computer monitor can display either monochrome or color images. A monochrome screen uses a single color—usually white, green, amber, or black—to show text against a contrasting background.
• Colour screens display 256 colours at a time from a selection of over 256,000 choices. Manufacturers offer monitors in various sizes, including 14, 15, 17, 19, and 21 inches. We describe the display size using two parameters: aspect ratio and screen size. The aspect ratio compares the width of the screen to its height—that is, the ratio of vertical points to horizontal points needed to produce equal-length lines in both directions. Most computer displays have a 4:3 aspect ratio. Like televisions, people measure screen sizes diagonally in inches, from one corner to the opposite corner.
• Sometimes, you may notice a blurred picture while watching television. This happens because the display creates the image using patterns of dots, not solid images. These dots, called picture elements, pels, or simply pixels, form the image. The golden rule for a sharp image says: the more pixels, the sharper the picture. Three basic qualities determine the screen’s clarity:

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

• In the above section, we discussed the most popular CRT monitors that are used as the display devices. With the widespread use of smaller computers like PDAs and laptops, a new type of display Liquid Crystal Display (LCD) has made a big impact on computer market. LCD screens have been used since long on notebook computers but are also becoming popular as a desktop monitor.
• The term liquid crystal sounds like a contradiction. We generally conceive a crystal as a solid material like quartz and a liquid as water like fluid. However, some substances can exist in an odd state that is semi-liquid and semi-solid. When they are in this state, their molecules tend to maintain their orientation like the molecules in a solid, but also move around to different positions like the molecules in a liquid.Thus, liquid crystals are neither a solid nor a liquid. Manufacturers use this amazing ability of liquid crystals to display images.
• A LCD screen is a collection of multiple layers. A fluorescent light source, known as the backlight,makes up the rearmost layer. Light passes through the first of two polarising filters. The polarised light then passes through a layer that contains thousands of liquid crystal blobs aligned in tiny containers called cells. These cells are aligned in rows across the screen; one or more cells make up one pixel. Electric leads around the edge of the LCD create an electric field that twists the crystal molecule, which lines the light up with the second polarising filter and allows it to pass through.

Figure 2

• The process illustrated in Figure 2 is followed for a simple monochrome LCD. However, colour LCD is more complex. In a coloured LCD panel, each pixel is made up of three liquid crystal cells. In front of each of these cells, there is a red, green, or blue filter. Light passing through the filtered cells creates the colours on the LCD. Nowadays, nearly every colour LCD uses a thin-film transistor (TFT),also known as an active matrix, to activate each cell. TFT-based LCD creates sharp, bright images as compared to previous LCD technologies. The oldest of the matrix technologies, passive-matrix, offers sharp text but leaves “ghost images” on the screen when the display changes rapidly, making it less than optimal for moving video.
• A LCD addresses each pixel individually. As a result, they can create sharper text than CRTs. However, LCD has only one ‘natural’ resolution, limited by the number of pixels physically built into the display. If you want to move up to, say, 1024 by 768 LCD on an 800 by 600 LCD, you have to emulate it with software, which will work only at certain resolutions.

Conclusion