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Display DevicesGraphics without display device could not be interactive; there could be the preview of correctness of the image until it is plotted or printed. Basically, the outputs are of following two forms:-

(1) Soft Copy (2) Hard Copy

(1) Soft Copy: The electronic version of output which usually resides in computer memory or on disk is

known as soft copy. Unlike Hard Copy, Soft Copy is not a permanent form of output. It is transient (temporary) and is usually displayed on the screen. This kind of output is not tangible (can not be touched). Soft Copy output includes audio visual form of output which is generated using a computer. In addition, textual or graphical information displayed on the computer screen is also a soft copy form of output.

Soft Copy Output is further divided into two parts:-

(1) CRT (Cathode Ray Tube)(2) Flat Panel Display Devices.

Display Devices

Soft Copy Hard Copy

Soft Copy

CRT

Refresh CRT Non-Refresh CRT

Raster Scan

Random Scan

DVST

Flat Panel Display Devices

Emissive Non-Emissive

Plasma Panel

Thin-Film Electro

Luminescent Display

LED

LCD

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Cathode Ray Tube (CRT):

CRT’s or video monitors are the most common input/output devices on computer today. Following fig (i) illustrate the basic operation of a CRT. A beam of electrons (cathode rays), emitted by an electron gun, passes through focusing and deflection systems that direct the beam towards specified position on the phosphor-coated screen. The phosphor then emits a small spot of light at each position contacted by the electron beam. Because the light emitted by 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 again and again. This type of display is called a refresh CRT.

Components of CRT:-

(i) Electron Gun (EG)(ii) Focusing System(iii) Deflection System (Electrostatic deflection plate/Magnetic deflection coil)(iv) Phosphor Coated Screen

Fig(i) Basic Design of a Magnetic-deflection CRT(i) Electron Gun: - EG contains two basic components: a 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 the electrons to be “boiled off” the hot cathode surface. In the vacuum tube inside the CRT envelope, the free, negatively charged electrons are then accelerated towards phosphor coating by a high positive voltage. The accelerating voltage can be generated with a positive charged metal coating on the inside of the CRT envelope near the phosphor screen, or an accelerating anode can be used, as shown in fig(ii). Sometimes the EG is built to contain the accelerated anode and focusing system within the same unit. Fig(ii) Operation of an EG with an accelerating Anode

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Intensity of the electron beam is controlled by setting voltage levels on the control grid, which is a metal cylinder that fits over the cathode. A high negative voltage applied to the control grid will shut off the beam by repelling electrons and stopping them from passing through the small hole at the end of the control grid structure. A smaller negative voltage on the control grid simply decreases the number of electrons passing through. Since the amount of light emitted by the phosphor coating depends on the number of electrons striking the screen, we control the brightness of a display by varying the voltage on the control grid. We specify the intensity level for individual screen positions with graphics software commands.

The purpose of the electron gun in the CRT is to produce an electric beam with the following properties:-

(a) It must be accurately focused so that it produces a sharp spot of light where it strikes the phosphor.

(b) It must have high velocity, since the brightness of the image depends on the velocity of the electron beam.

(c) Means must be provided to control the flow of electrons so that the intensity of the trace of the beam can be controlled.

(ii) Focusing System: - The focusing system in a CRT is needed to force the electron beam to converge into a small spot as it strikes the phosphor. Otherwise, the electron beam would spread out as it approaches the screen. Focusing is accomplished with either electric or magnetic fields. Electrostatic focusing, the electron beam passes through a positively charged metal cylinder that forms an electrostatic lens. The action of the electrostatic lens focuses the electron beam at the center of the screen, in exactly the same way that an optical lens focuses a beam of light at a particular foal distance. Similar lens focusing effects can be accomplished with a magnetic field set up a coil mounted around the outside of the

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CRT envelope. Magnetic lens focusing produces the smallest spot size ion the screen and is used in special purpose devices.

(iii) Deflection System: - It is used to control the direction of the electron beam. As with focusing, deflection of the electron beam can be controlled either by electric or magnetic fields. CRT’s are now commonly constructed with magnetic deflection coils mounted on the outside of the CRT envelope as shown if fig (i) above. Two pairs of coils are used, with the coils in each pair mounted on opposite sides of the neck of the CRT envelope. One pair is mounted on the top and bottom of the neck and the other pair is mounted on opposite sides on the neck. The magnetic field produced by each pair of coils results in a traverse deflection force that is perpendicular both to the direction of the magnetic field and to the direction of travel of the electron beam. Horizontal deflection is accomplished with one pair of coils and vertical deflection by the other pair. The proper deflection amounts are attained by adjusting the current through the coils. When electrostatic deflection is used, two pairs of parallel plates are mounted inside the CRT envelope. One pair of plates is mounted horizontally to control the vertical deflection and the other pair is mounted vertically to control horizontal deflection as shown in fig(iii).

Fig(iii) Electrostatic deflection of the electron beam in a CRT

(iv) Phosphorus-coated Screen: - At the very rare end of CRT is the phosphorus-coated Screen, which has a unique property that allows the entire system to work. Phosphorus glow when they are attacked by a high-energy electron beam. They continue to glow for a distinct period of time after being exposed to electron beam. The glow given off after the electron beam is removed is known as phosphorescence and the duration of phosphorescence is

known as the phosphorus persistence.Lower persistence phosphorus require higher refresh rate to

maintain a picture on the screen without flicker. Higher persistence phosphorus require lower refresh rate to maintain a picture on the screen without flicker. A phosphor with low persistence is useful for animation. A phosphor with high persistence is useful for highly complex display.

Properties of Display Devices (Video Display Devices): -

(i) Persistence: - Persistence is the duration of the phosphorescence. Where phosphorescence is the glow given off by the phosphor after the electron beam is removed. Different kinds of phosphor are available for use in a CRT. Besides color, a major difference between phosphorus is their persistence; how long they continue to emit light

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after the electron beam is removed. Lower persistence phosphorus require higher refresh rate to maintain a picture on the screen without flicker. Higher persistence phosphorus require lower refresh rate to maintain a picture on the screen without flicker. A phosphor with low persistence is useful for animation. A phosphor with high persistence is useful for highly complex display, static pictures.

(ii) Resolution: - Resolution indicates the maximum number of pixels that can be displayed without overlap on the CRT. It is defined as number of pixels per unit length (e.g. inch) in the horizontal as well as the vertical direction is refereed to resolution. Thus a 3x2 inch image at a resolution of 300 pixels per inch would have a total of 540,000 pixels (number of pixels in the horizontal direction is 3x300=900 and in the vertical direction is 2x300=600 so 900x600=540,000). Fig(iv) shows the intensity distribution of a pixel on the screen.

The intensity is greatest at the centre of the spot, and decrease to the edges of the pixel. Two illuminated phosphor spots are distinguishable when their separation is greater than the diameter at which a spot intensity has fallen to 60% of maximum. The overlap position is shown in fig(v). The resolution of a CRT is depending on the following points: -(i) Type of Phosphor(ii) Intensity to be displayed(iii) Focusing System(iv) Deflection System

(iii) Aspect Ratio: - It is the ratio of vertical pixels to horizontal pixels to produce equal length lines in both the directions on the screen. An aspect ration of 4/5 means that a vertical line plotted with 4 pixels has the same length as a horizontal line plotted with 5 points. Most standard CRT have a display area with an aspect ratio 4:3.

Refresh CRT Techniques

There are two techniques used for producing images on the CRT screen using Refresh CRT: -

(a) Raster Scan Display(b) Random Scan (Vector Scan) Display(a) Raster Scan Display : - The most common type of graphics monitor employing a refresh CRT

is the raster-scan display, based on television technology. In a raster-scan system, the electron beam is swept across the screen, one row at a time from top to bottom. As the electron beam moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots. Picture definition is stored in a memory area called the refresh buffer or frame buffer. This memory area holds the set of intensity values for all the screen points. Stored

Fig(iv)

Fig(v)

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intensity values are then retrieved from the refresh buffer and painted on the screen one row (scan line) at a time as shown in following fig (vi).

Fig(vi) A raster-scan system displays an object as a set of discrete points across each scan line

When the beam is moved from left to right, it is ON. The beam is OFF when it is moved from right to left as shown by dotted line in following fig(vii). The return to the left of the screen after refreshing each scan line, is called the horizontal retrace of thee electron beam. When the electron beam reaches the bottom (or completing one frame), it is made OFF and rapidly retraced back to the top left of the screen to start again refreshing procedure. This is known as vertical retrace.

Horizontal Retrace

Vertical Retrace

1234

5

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Fig (vii)

Each screen point is referred to as pixel or pel (shortened form of picture element). The capability of raster-scan system to store intensity information fro each screen point makes it well suited for realistic display of scenes containing subtle shading and color patterns. Home television sets and printers are examples of other systems using rates-scan methods.

Intensity range for pixel positions depends on the capability of the raster system. In a simple black-and-white system, each screen point is either on or off, so only one bit per pixel is needed to control the intensity of screen positions. For a Bi-level system, a bit value of 1 indicates that the electron beam is to be turned on at that position, and a value of 0 indicates that the electron beam is turned to be off. Additional bits are needed when color and intensity variations can be displayed. Up to 24 bits per pixel are included in high-quality systems, which can require several megabytes of storage for the frame buffer, depending on the resolution of the system. A system with 24 bits per pixel and a screen resolution of 1024 by 1024 requires 3 megabytes of storage for the frame buffer. On a black-and-white system with one bit per pixel, the frame buffer is commonly called a bitmap. For systems with multiple

bits per pixel, the frame buffer is often referred to as a pixmap.

Refreshing on raster-scan display is carried out at the rate of 60 to 80 frames per second, although some systems are designed for higher refresh rates. Sometimes, refresh rates are described in units of cycles per second, or Hertz (Hz), where a cycle corresponds to one frame. So, refresh rate of 60 frames can be simply described as 60 Hz.

On some raster-scan systems (and in TV sets), each frame is displayed in two passes using an interlaced refresh procedure. In the first pass, the beam sweeps across every other scan line from top to bottom. Then after the vertical retrace, the beam sweeps out the remaining scan lines as shown in following fig (viii). Interlacing of the scan lines in this form allows us to see the entire screen displayed in one-half the time it would have taken o sweep across all the lines at once from top to bottom. This is an effective technique for avoiding flicker.

Pass 1 (Scanning Odd Rows) Pass 2(Scanning Even Rows)

10

1

34

5

10

2

1

34

5

10

2

Horizontal Retrace

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Advantages of Raster Scan Display: -

1. Can display Realistic images 2. Million different colors can be generated3. Shadow scenes are possible

Disadvantages of Raster Scan Display: -

1. Low Resolution2. Electron beam directed to entire screen not only to those parts of the screen where picture is to

be drawn so time consuming when the drawn image size is very much less than the entire screen.

3. Expensive

(b) Random Scan Displays: - When operated as a random-scan display unit, a CRT has the electron beam directed only to those parts of the screen where a picture is to be drawn. Random scan monitors draw a picture one line at a time and for this reason are also referred to as vector display (or stroke-writing or calligraphic displays). The component lines of a picture can be drawn and refreshed by a random-scan system in any specified order as shown in fig (ix).

Refresh rate on a random-scan depends on the number of lines to be displayed. Picture definition is now stored as a set of line-drawing commands in an area of memory referred to as the refresh display file. Sometimes the refresh display file is called the display list, display program, or simply the refresh buffer.

To display a specified picture, the system cycles through the set of commands in the display file, drawing each component line in turn. After all the line drawing commands have been processed, the system cycles back to the first line command in the list.

Fig (viii) Interlacing Vertical Retrace

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Random-scan displays are designed to draw all the component lines of a picture 30 to 60 times each second (means refresh rate is 30 Hz to 60 Hz). High-quality vector systems are capable of handling

Fig(ix). A Random-Scan system draw the component lines of an object in any order specified

approximately 1, 00,000 “short” lines at this refresh rate. When a small set of lines is to be displayed, each refresh cycle is delayed to avoid refresh rates greater than 60 frames per second. Otherwise, faster refreshing of the set of lines could burn out the phosphorus.

Random-scan systems are designed for line-drawing applications and can not display realistic shaded scenes. Since picture definition is stored as a set of line-drawing instructions and not as a set of intensity values for all screen points, vector displays generally have higher resolution than raster systems. Also, vector displays produce smooth line drawings because the beam directly follows the line path. A raster system, in contrast, produces jagged lines that are plotted as discrete point sets.

Advantages of Random Scan Display: -

1. A CRT has the electron beam directed only to the parts of the screen where a picture is to be drawn.

2. Produce smooth line drawings3. High Resolution

Disadvantages of Random Scan Display: -

1. Random-Scan monitors can not display realistic shaded scenes.

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Difference between Vector Scan Display and Raster Scan Display

Random Scan/Vector Scan Display Raster Scan Display

COLOR CRT MONITORS

A CRT monitor displays color pictures by using a combination of phosphorus that emits different-colored light. It generates a range of colors by combining the emitted light from the different phosphorus. There are two basic techniques used for producing color displays:

(1) Beam-Penetration Technique(2) Shadow-Mask Technique

(1) Beam-Penetration Technique: - The Beam-penetration method for displaying color pictures has been used with random-scan monitors. Two layers of phosphorus, usually red and green, are coated onto the inside of the CRT screen, and the displayed color depends on how far the electron beam penetrates into the phosphorus layers.

a. A beam of slow electrons excite only outer red layer and produce red color.b. A beam of very fast electrons penetrates through the red layer and excites the

inner green layer and produce green color.c. At intermediate beam speeds, combination of red and green light is emitted to

show two additional colors, orange and yellow.

The speed of electrons, and hence the screen color at any point, is controlled by the beam-acceleration voltage.

(1) In Vector scan display the beam is moved only to those parts of the screen where a picture is to be drawn.

(2) Vector display flickers when the number of primitives in the buffer becomes too large.

(3) Cost is more.(4) Vector display only lines and

characters.(5) Higher resolution.(6) Produce smooth Line drawings.(7) Less Color

(1) In raster scan display the electron beam is moved over the screen one scan line at a time, from top to bottom and then back to top.

(2) In raster display, the refresh process is independent of the complexity of the image.

(3) Cost is low.(4) Raster display has ability to display areas

filled with solid colors or patterns.(5) Less Resolution(6) Produce Jagged Lines.(7) More Color

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Advantages of Beam-Penetration Technique: -

1. It is an inexpensive technique to produce color in random-scan monitors.

Disadvantages of Beam-Penetration Technique: -

1. It can display only four colors.2. The quality of picture produced by this technique is not as good as compared to other

techniques.3. The hardware and software must be designed to introduce adequate delays between changes in

color, so that there is time for voltages to settle.

(2) Shadow-Mask Technique: - Shadow-mask methods are commonly used in raster-scan systems (including color TV) because they produced a much wider range of colors than the beam-penetration method. A shadow-mask CRT has three phosphorus color dots at each pixel position. One phosphorus dot emits a red light, another emits a green light, and the third emits a blue light. This type of CRT has three electron guns, one for each color dot, and a shadow-mask grid just behind the phosphorus-coated screen. Following fig (x) illustrates the delta-delta shadow-mask method, commonly used in color CRT systems.

Fig (x). operation of a delta-delta, Shadow-Mask CRT

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The three electron beams are deflected and focused as a group onto the shadow-mask method, commonly used in color CRT systems. The three electron beams are deflected and focused as a group onto the shadow-mask, which contains a series of holes aligned with the phosphor-dot patterns. When the three beams pass through a hole in the shadow mask, they activate a dot triangle, which appears as a small color spot on the screen. The phosphor dots in the triangles are arranged so that each electron beam can activate only its corresponding color dot when it passes through the shadow mask. Another configuration for the three electron guns is an in-line arrangement in which the three electron guns, and the corresponding red-green-blue color dots on the screen, are aligned along one scan line instead in a triangular pattern. This in-line arrangement of electron guns is easier to keep in alignment and is commonly used in high-resolution color CRT’s.

Triangular Pattern Line Pattern

We obtain color variations in a shadow-mask CRT by varying the intensity levels of the three electron beams. By turning off the red and green guns, we get only the color coming from the blue phosphor. Other combinations of beam intensities produce a small light spot for each pixel position, since our eyes tend to merge the three colors into one composite. The color we see depends on the amount of excitation of the red, green, and blue phosphorus. A white area is the result of activating all three dots with equal intensity. Yellow is produced with green and red dots only, magenta is produced with the blue and red dots, and cyan shows up when blue and green are activated equally.

In some low cost systems, the electron beam can only be set to on or off, limiting displays to eight colors only. More sophisticated systems can set intermediate intensity levels for the electron beams, allowing several million different colors to be generated.

Color CRT’s in graphics systems are designed as RGB monitors. These monitors use shadow-mask methods and take the intensity level for each electron gun (red, green, and blue) directly from the computer system without any intermediate processing. High-quality raster-graphics systems have 24 bits per pixel in the frame buffer, allowing 256 voltage settings for each electron gun and nearly 17 million color choices for each pixel. An RGB color system with 24 bits of storage per pixel e\is generally referred to as a full-color system or a true-color system.

Advantages of Shadow-Mask Technique: -

1. Can display realistic images.2. Million different colors can be generated.3. Shadow scenes are possible.

R G B

R

G B

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Disadvantages of Shadow-Mask Technique: -

1. Relatively expensive compared with the monochrome CRT.2. Relatively poor resolution.3. Light output of the shadow0mask CRT (Mask tends to block a large proportion of the available

beam energy, reducing the total brightness.)4. Convergence problem. It is extremely difficult to adjust three guns and deflection system so that

electron beams are deflected together, all converging on same hole in the shadow mask.

Non-Refresh CRT In Refresh-CRT we do refreshing of the screen to maintain a screen image. An alternate method for maintaining a screen image is to store the picture information inside the CRT instead of refreshing the screen. Example: - Direct View Storage Tube (DVST)

Direct View Storage Tube (DVST): - A direct-view storage tube (DVST) uses the storage grid which stores the picture information as a charge distribution just behind the phosphor-coated screen.

Above Fig (xi) shows the general arrangement of the DVST. It consists of two electron guns: a primary gun and a flood gun. A primary gun stores the picture pattern and flood gun maintains the picture display. A primary gun produces high speed electrons which strike on the storage grid to draw the picture pattern. As electron beam strikes on the storage grid with high speed, it knocks out electrons from the storage grid keeping the net positive charge. The knocked out electrons are attracted towards the collector. The net positive charge on the storage grid is nothing but the picture pattern. The continuous slow speed electrons from the flood gun pass through the control grid and are attracted to the positive charged area of the storage grid. The low speed electrons then penetrate the storage grid and strike the phosphor coating without affecting the

Screen

Collector

Storage grid

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positive charge pattern on the storage grid. During this process the collector just behind the storage grid smoothes outs the flow of flood electrons.

Advantages of DVST: -

1. Refreshing of CRT is not required.2. Because no refreshing is required, very complex pictures can be displayed at very high

resolution without flicker.3. It has flat screen.

Disadvantages of DVST: -

1. They do not display colors. 2. Selective or part erasing of screen is not possible.3. Erasing requires removal of charge on the storage grid. This erasing and redrawing process takes

several seconds.4. It has poor contrast as a result of the comparatively low accelerating potential applied to the

flood electrons.5. The performance of DVST is some what inferior (lesser) to the refresh CRT.

Difference between DVST and Refresh CRT

DVST Refresh CRT

Flat Panel Display Devices

The term Flat-Panel Display refers to a class of video devices that have reduced volume, weight, and power requirements compared to a CRT. The important feature of Flat-Panel Display is that they are thinner than CRT’s. There are two types of Flat-Panel Displays: - emissive displays and non-emissive displays.

1. DVST has storage tube to retain picture until it is erased.

2. Refreshing is not required.3. Performance of DVST inferior to refresh

CRT.4. DVST has no refresh buffer thus to

eliminate a picture section, the entire screen must be erased and to modified.

5. High persistence.6. Creates bright picture.

1. Refresh CRT has no storage tube.

2. Refreshing is required.

3. Performance of Refresh CRT superior to DVST.

4. Since Refresh buffer is there so erasing a screen portion does not require entire screen to be erased and modified.

5. Comparatively low persistence.6. Creates fine picture.

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(1) Emissive Displays: - they convert electrical energy into light energy. Plasma panels, thin-film electro luminescent displays, and light emitting diode are examples of emissive displays.

A. Plasma Panel Display (PPD): - plasma panels, also called gas-discharge displays, are constructed by filling region between two glass plates with a mixture of gases that usually includes neon. A series of vertical conducting ribbon is placed on one glass panel, and a set of horizontal ribbons is built into the other glass panel as shown in Fig (xii). By applying voltages between the horizontal and vertical conductors the gas at the intersection of two conductors is made to behave as if it were divided into tiny cells, each one independent of its neighbors. These independent cells are made to glow by placing a firing voltage of about 120 volts across it by means of electrodes. The glow can be sustained by maintaining a high frequency alternating voltage of about 90 volts across the cell. Due to this refreshing is not required. Fig (xii). Basic design of a plasma-panel display device.

Advantages of PPD: -

1. Refreshing is not required.2. Produces a very steady image, totally free of flicker.3. Less bulky than a CRT.4. Allows selective writing and selective erasing, at speed of about 20 µsec per cell.5. It has the flat screen.6. Large screen size is possible.

Disadvantages of PPD: -

1. Relatively poor resolution of about 60dpi (dots per inch).2. It requires complex addressing and wiring.3. Costlier than the CRT’s.4. It uses a lot of power.5. It does not show sharp contrast.

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6. They are monochromatic devices (i.e. single color devices) but systems have been developed that are now capable of displaying color and grayscale.

7. Technology is expensive.

B. Thin-Film Electroluminescent Displays: - Thin-film electroluminescent displays are similar in construction to a plasma panel. The difference is that the region between the glass plates is filled with a phosphor, such as zinc doped with manganese, instead of a gas as shown in fig (xiii). When a sufficient high voltage is applied to a pair of crossing electrodes, the phosphor become conductor in the area of intersection of the two electrodes. Electrical energy is then absorbed by the manganese atoms, which then release the energy as a spot of light similar to the glowing plasma effect in a plasma panel, electroluminescent displays require more power than plasma panels, and good color and gray scale display are hard to achieve. Fig (xiii). Basic design of a thin-film electroluminescent display device

Disadvantages of Thin-film Electroluminescent: -1. Require more power than plasma panel.2. Good color and gray scale display are hard to achieve

C. Light-Emitting Diode (LED): - A matrix of diodes is arranged to form the pixel positions in the display, and picture definition is stored in refresh buffer. Information is read from the refresh buffer and converted to voltage levels that are applied to the diodes to produce the light patterns in the display.

Fig (xv). Laptop Computer

Fig (xvi). The light-twisting, shutter effect used in the design of most LCD devices.

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(2) Non-Emissive Displays: - They convert optical effects to convert sunlight or light from other source into graphics patterns. LCD is an example of non-emissive displays.

(A) Liquid Crystal Display (LCD): - LCD’s are commonly used in small systems, such as calculators and portable, laptop computers fig (xiv). These non-emissive devices produce a picture by passing polarized light from the surroundings or from an internal light source through a liquid-crystal that can be aligned to either block or transmit the light.

The term liquid crystal refers to the fact that these compounds have a crstalline arrangement of molecules, yet they flow like a liquid. These devices commonly use nematic (threadlike) liquid-crystal compounds that tend to keep the long axes of the rod-shaped molecules aligned. An LCD can then be constructed with a nematic liquid crystal

as shown in fig (xv). Two glass plates, each containing a light polarizer at right angles to the other plate, sandwich the liquid-crystal material. Rows of horizontal transparent conductors are built into one glass plate, and columns of vertical conductors are put into the other plate. The intersection of two conductors defines a pixel position. Normally, the molecules are aligned as shown in the “on state” of fig (xvi). Polarized light passing through the material is twisted so that it will pass through the opposite polarizer. The light is then reflected back to the viewer.

To turn off the pixel, we apply a voltage to the two intersecting conductors to align the molecules so that the light is not twisted. This type of flat-panel display is referred to as a passive-matrix LCD.

Picture definitions are stored in a refresh buffer, and the screen is refreshed at the arte of 60 frames per second, as in the emissive devices. Back lighting is also commonly applied using solid-state electronic devices, so that the system is not completely dependent on out light source. Colors

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can be displayed by using different materials or dyes and placing a triad of color pixels at each screen location.

Another method for constructing LCD’s is to place a transistor at each pixel location, using thin-film transistor technology. The transistors are used to control the voltage at pixel locations and to prevent charge from gradually leaking out of the liquid-crystal cells. These devices are called active-matrix displays.

Advantages of LCD: -

1. Low weight.2. Low power consumption.3. LCD’s are flat.4. Small size.5. Less eyestrain.

Disadvantages of LCD: -

1. LCD’s can be viewed only from a limited angle. To observe this, hold a calculator that uses an LCD at arm’s length in front of you. Next, turn the calculator slowly back and forth along its vertical axis. You should observe the numbers on the display “fading” as the calculator display makes a greater angle with your line of sight.

2. LCD’s are temperature dependent (0-70oC).3. LCD’s do not emit light; as a result, the image has very little contrast.4. The resolution is not as good as that of a CRT.

(2)Hard Copy: - Output which is produced on a paper is known as Hard Copy output. They are permanent in nature. Among the wide variety of the hard copy output devices, printers and plotters are the most commonly used. A printer is used to produce printouts of the documents stored on a computer's disk drive. A plotter is a pen-based output device, which is used for producing high quality output by moving ink pens across the paper. Hard copy display devices are shown if fig (xvii).

(I) Printers: -Printers are further classified into two categories: impact printers and non-impact printers.

(1) 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. Impact printers are the oldest printers and are still in use. Impact printers can print only one character at a time while some impact printers can print an entire line. The three most commonly used impact printers are dot matrix printers, daisy wheel printers and drum printers. The printer that prints the characters by striking the ribbon and onto the paper, are called impact printers. These printers are of two types, (i) Character and (ii) Line printers.

Characteristics of impact printers: 1. In impact printers, there is physical contact with the paper to produce an image. 2. They have relatively low consumable costs. The primary recurring costs for these printers are

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the ink ribbons and paper. 3. Due to being robust and low cost, they are useful for bulk printing. 4. They can withstand dusty environment, vibrations and extreme temperature. 5. Impact printers are ideal for printing multiple copies (that is, carbon copies) because they can

easily print through many layers of paper. 6. Due to its striking activity, impact printers are very noisy. 7. Since they are mechanical in nature, they tend to be slow. 8. Impact printers do not support transparencies.

Fig (xvii) Hard Copy Display Devices

1.1 Dot Matrix Printer (DMP): -A dot matrix printer (also known as the wire matrix printer) uses the oldest printing technology and it prints one character at a time. Each character printed is in form of pattern of dot and head consists of a matrix of pins of size (5x7, 7x9, 9x7 or 9x9) which comes out to form a character that is why it is called Dot-Matrix Printer. Usually, dot matrix printers can print any shape of character, which a user can specify. This allows the printer to print many special characters, different sizes of print and enables it to print graphics, such as charts and graphs. The speed of dot matrix printers is measured in characters per second (cps).

Working: -Printer head moves on a carriage. Paper is sandwiched between ribbon and head. Say A’ is to be printed, then Pins corresponding to ‘A’ moves out, strike the ribbon and ‘A’ is printed. This is the way one character is printed, but we need to print the whole line. For this a buffer (temporary storage) of size equal to the size of paper is used. Say for a paper of width 80

Hard Copy

Plotters

Drum Flatbed

Printers

Impact Non-Impact

Dot Matrix Printer (DMP)

Daisy Wheel Printer

Character Line Ink-Jet Laser

Drum Printer

Chain Printer

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columns a buffer of 80 characters is used. All character corresponding to a line are stored from 1st to 80th position in buffer and printing starts from left to right from 1 st to 80th position on paper. After printing one line, paper is moved up by carriage and again printing starts from 1 st to 80th. This way of printing always in one direction left to right is called Unidirectional. New printers print bidirectional which is achieved as: first line printed starting from 1 st position of buffer storage, as soon as 1st character is printed, 80th position of next line is stored in it and son on. After printing one line, on return, head starts printing from right to left using buffer positions 80th to 1st again. Thus, it prints in both directions. All modern Dot Matrix Printers print this way.

DMP are available in two sizes 80 column and 1342 column. Many vendors supplying them are: EPSON, WIPRO, TVSE and GODREJ etc. The speed can vary from about 200 to over 360 cps (character per second). Advantages of DMP: -

a. Inexpensiveb. Widely usedc. Other language characters can be printed

Disadvantages of DMP: -a. Slow speedb. Poor qualityc. Noisy

1.2 Daisy Wheel Printers: - Head is lying on a wheel and Pins are corresponding to characters are like petals of Daisy that is why it is called Daisy Wheel Printer. A motor spins the wheel at a very fast speed. The daisy wheel is a disk made of plastic or metal on which characters stand out in relief along the outer edge.

Working: -To print a character, the printer rotates the disk until the desired letter is facing the paper. Then a hammer strikes the disk, forcing the character to hit an ink ribbon, leaving an impression of the character on the paper. You can change the daisy wheel to print different fonts.

These printers have very usually very slow because of the time required to rotate the print wheel for each character desired. Speed of daisy wheel printers is in the order of 10 to 50 cps. Quality of printing is very good. These printers are generally used for word processing in offices which require a few letters to be printed with very nice quality representation. Daisy wheel printers produce high-resolution output and are more reliable than DMP’s. These printers are also called smart printers because of its bidirectional printing and built-in microprocessor control features.Advantages of Daisy Wheel Printers: -

a. Are more reliable than DMP’s.b. Better qualityc. The fonts of characters can easily be changed

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Disadvantages of Daisy Wheel Printers: -

a. Slower than DMP’s.b. More expensive than DMP’s.c. Noisy

2. Line Printers: -Line printers are printers which print one line at a time. These are impact printers for producing very large volume paper output. They are very fast and speed varies from a range of 300 to 3000 lines per minute. Two most commonly used printers in this category are: Drum Printer and Chain Printer.

2.1 Drum Printer: -This line printer consists of a drum. The surface of the drum is divided into tracks. Total tracks are equal to size of paper i.e., for a paper width of 132 characters, the drum will have 132 tracks. A characters set is embossed on track. Character set is not of character that printer will support. Different characters sets are available in the market, 48 character set, 64 and 96 characters set. Drum is made to rotate at a very fast speed. Paper is wrapped in drum and each track has a hammer attached to it. Ribbon is sandwiched between paper and hammer. Say if letter 'j' is to be printed. As soon as 'j' comes in front of hammer attached to j th track, it is fired. Thus, 'j' is embossed on paper.

In this way one character is printed, but we need a whole line to be printed at once. For that a buffer is used, which has storage capacity equal to size of paper. All character in a line is stored in buffer and as soon as all come in front of hammers, all hammers is fired at once. It is very important that all hammers should be fired at once and thus a line is printed. If synchronization is not achieved in firing of hammers, then wavy output will occur.

One rotation of drum prints one line. Drum printers are fast in speed and speed is between 300 to 2000 lines per minute. All printing features are available in this line printer.

Working: -The basics of a line printer like drum printer are similar to those of a serial printer, except that multiple hammers strike multiple type elements against the paper almost simultaneously, so that an entire line is printed in one operation. A typical arrangement of a drum printer involves a large rotating drum mounted horizontally and positioned in front of a very wide, inked ribbon, which in turn is positioned in front of the paper itself. The drum contains characters molded onto its surface in columns around its circumference; each column contains a complete set of characters (letters, digits, etc.) running around the circumference of the drum. The drum spins continuously at high speed when the printer is operating. In order to print a line, hammers positioned behind the paper ram the paper against the ribbon and against the drum beyond it at exactly the right instant; such that the appropriate character is printed in each column, as it spins past on the drum. Once every column has been printed, the paper is advanced upward so that the next line can be printed.

Advantages of Drum Printer: - • Very high speed.

Disadvantages of Drum Printer: -

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Very expensive. Characters, fonts cannot be changed.

As the strike should be very precise, the mistiming causes wavy printing.

2.2 Chain Printers: -They are also line printers, because of use of a chain of character sets, they are called Chain Printers. They can be thought of as a type wound on two spools with a character set repeated, three or four times. A standard character set may have 48, 64 and 96 characters.In addition to the chain, the printer has a set of hammer mounted in front of the chain in a manner that an inked ribbon and paper can be placed between the hammer and the chain. The total number of hammers is equal to the total number of print positions. Therefore, if there are 132 print positions, the printer will have 132 hammers. The chain rotates at a high speed, and a character at a print position is printed by activating the appropriate hammer, when the character embossed on the chain passes below it. Because character set is repeated three to four times, speed of chain printer increases, it is not necessary to wait for a whole cycle as in drum for a particular character to come in front of hammer. Speed varies from 400 to 3000 lines per minute).

Advantages of Chain Printer: - 1. Character fonts can easily be changed. 2. The problem of wavy printing does not arise as the chain rotates horizontally where a

drum rotates vertically. 3. Different scripts (languages) can be used with the same printer.

Disadvantages of Chain Printer: - 1. The timing of hammer strike is very critical. 2. Do not have the ability to print any shape of characters, different sizes of prints and

graphics.3. Noisy.

(2) Non-Impact Printers: -The printers that print the characters without striking against the ribbon and onto the paper are called Non-Impact Printers. These printers print a complete page at a time, therefore, also called Page Printers. Unlike impact printers, a non-impact printer forms characters and images without making direct physical contact between printing mechanism and paper. In this printer, the print head does not make contact with the paper, and no inked ribbon is required. Ink can be sprayed against the paper and then pressure is used to fuse a fine black powder into the shape of a character. They use techniques other than physically striking the page to transfer ink onto the page. The major technologies competing in the non-impact market are ink-jet and laser. Page printers are of two types(i) Laser Printers(ii) Ink Jet Printers

Characteristics of Non-impact Printers: - 1. Non-impact printers are faster than impact printers.

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2. They are quieter than impact printers because there is no striking mechanism involved and only few moving parts are used.

3. They possess the ability to change typefaces automatically. 4. These printers produce high-quality graphics. 5. These printers usually support the transparencies. 6. These printers cannot print multipart forms because no impact is being made on the

paper.(1) Ink-Jet Printers: -It is the most commonly used printer. Being a non-impact it does not touch the paper while creating an image. It uses a series of nozzles to spray onto the paper. Originally it was made black and white only. However, the print head has now been expanded and the nozzle accommodates CMYK. The combination of these four colors will be the resultant color. These printers are costlier than the dot matrix printers, but the quality is much better. 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-users. These printers print documents at a medium pace, but slow down if printing a document in multicolor. These printers can print about 6-15 pages per minute. Moreover, they can also program to print unusual symbols such as Japanese or Chinese characters.

Working: -An ink-jet printer has a print cartridge with a series of tiny electrically heated chambers. These Cartridges are attached to print heads with a series of small nozzles that spray ink onto the surface of the paper. As print head moves back and forth across the page, software gives instructions regarding the type and the quantity of colors. It also tells the position where the dots of ink should be 'sprayed’. There are two main ways to drop the ink droplets, namely, the bubble-jet and piezoelectric technology.' Bubble-jet printers use heat to fire ink onto the paper. Piezoelectric technology uses a piezo crystal at the back of the ink reservoir. Advantages of Ink-Jet Printer:

a. High quality printingb. More reliable

Disadvantages of Ink-Jet Printer:a. Expensiveb. Slow as compare to laser printers

(2) Laser Printers: - These are non- impact printers. They use laser lights to produce the dots needed to form the character be printed on a page and hence are known as laser printers. A laser printer provides the highest quality text and images for personal computers today, operates on the same principle as that of a photocopy machine. They are also known as page printers because they process and store the entire page before they actually print it.

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The output is generated in following steps: Step 1 The bits of data sent by processing unit act as

triggers to turn the laser beam on and off. Step 2 The output device has a drum which is cleared

and is given a positive electric charge. To print a page the modulated laser beam passing from the laser, scans back and forth the surface of the drum. The positive electric charge on the drum is stored on just those parts of the charges on the exposed drum surface.

Step 3 The laser exposed parts of the drum attract an ink powder known as toner.

Step 4 The attracted ink powder is transferred to paper.

Step 5 The ink particles are permanently fixed to the paper by using either heat or pressure technique.

Step 6 The drum rotates back to the cleaner where a rubber blade cleanse off the excess ink and prepares the drum to print the next page.

Characteristics of Laser printer:

1 It is a very fast printer.

2 It can print text and graphics with a very high quality resolution from 300 to 1200 dpi.

3 It can print in different fonts, that is, type styles and sizes.

4 It is more expensive to buy and maintain than the other printers.

Working: -The core component of laser printing system is the photoreceptor drum. A rotating mirror inside the printer causes the beam of a laser to sweep across the photoconductive drum. Initially, the beam of laser charges the photoconductive drum positively. When the charged photoconductor is exposed to an optical image through a beam of light to discharge, a latent or invisible image is formed. At the point where the laser strikes the surface of drum, it creates a dot of positive charge. These points are represented by a black dot, which will be printed on the paper. After this, the printer coats the drum with a container, which contains a black powder called toner. This toner is negatively charged, and so it clings to the positive areas of the drum surface. When the powder pattern gets fixed, the drum is rotated and the paper is fed into the drum surface via a pressure roller. This pressure roller transfers the black toner. Since, the paper is moving at the same speed as the drum, the paper picks up the image pattern precisely. Finally, the printer passes the paper through the fuser, a pair of heated rollers. As the paper passes through these rollers, the loose toner powder gets melted and fuses with the fibers in the paper. Advantages

Very high speed. Very high quality output. Give good graphics quality. • Support many fonts and different character size.

Disadvantages Expensive. Cannot be used to produce multiple copies of a document in a single printing.

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(II) 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. Plotters are used to print high quality graphics and drawings. Plotters are generally used for printing drawing graphical images such as charts, drawings, maps, etc. of engineering and scientific applications. Some important types of plotters are: Drum Plotters and Flatbed Plotters.

(1) Drum Plotters: - In drum plotters, the paper on which the design is to be made is placed over a drum. These plotters consist of one or more pen(s) that are mounted on a carriage and this carriage is horizontally placed across the drum. The drum can rotate in either clockwise or anticlockwise direction under the control of plotting instructions sent by the computer. Drum plotters are used to produce continuous output, such as plotting earthquake activity, or for long graphic output, such as tall building structures. The drum plotters are generally the least expensive.

(2) Flatbed Plotters: -

Flatbed 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 line. In flatbed 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 flatbed surface, these are used in designing of ships, aircraft, build-ings, etc. The size of these automated drafting tables can range up to roughly 5ft. (I.5m) by 20ft. (6.lm) with plotting accuracies approaching ±0.001 in. (±0.025 mm).

The major disadvantage of this plotter is that it is a slow output device and can take hours to com-plete a complex drawing. The flatbed plotter is also more expensive.

Comparison between Plotters and Printers: - Plotters are similar to printers, but they draw lines using a pen. As a result, they can produce continuous lines, whereas printers can only simulate lines by printing a closely spaced series of dots. Multicolor plotters use different-colored pens to draw different colors. Color plots can be made by using four pens (cyan, magenta, yellow, and black) and need no human intervention to change them. 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 Manufacturer (CAM) applications such as printing out plans for houses or car parts. These are also used with programs like AUTO CAD (computer assisted drafting) to give graphic outputs.