I/O Devices

CT101 – Computing Systems

Contents• Input devices

– Keyboard– Mouse– Optical Readers– Card Readers

• Output devices– Printer

• Inkjet printers• Laser Printers

– Monitor• CRT

Input Devices

• The following section discusses the functioning of a number of input devices

• Input devices allow the user to input information (data) into the computer for analysis or storage, as well as give commands to the computer

• Examples of input devices are keyboards, scanners, mice, bar-wands, and touch screens

Keyboard• The keyboard is the most widely used means of entering

information into a computer. • Pressing a key on the keyboard generates a code that

represents the character associated with the key• The two main codes associated with computers are ASCII

(American Standard Code for Information Interchange) and EBCDIC (Extended Binary Coded Decimal Interchange Code) – ASCII is a seven bit code, so characters generated by the keyboard

are made available as a seven bit code (a total of 128 different combinations)

– EBCDIC used by IBM and is not used anymore

Keyboard• A typical keyboard has four basic types of keys:

– Typing keys – Numeric keypad – Function keys – Control keys

• ASCII codes:– 00-1F Control Codes – 20-3F Punctuation and digits – 40-5F Uppercase – 60-7F Lowercase

• Control codes are used to control devices like printers and modems, and to position the cursor. For instance, FF stands for form feed, and HT stands for horizontal tab. Control codes are generated on a standard keyboard by holding down the Ctrl key, and whilst it is depressed, pressing another key– For example, pressing the key combination CTRL-A generates the code

sequence SOH, and pressing CTRL-R generates the code sequence DC2

Keyboard• Interface

– RS232, PS2 and USB– Usually power, ground, clock and data signals

• Types– QWERTY (used in PC)– DVORAK– Microsoft ergonomic keyboard

Keyboard Technologies

• Keyboards use a variety of switch technologies. • It is interesting to note that we generally like to have some

audible and tactile response to our typing on a keyboard. – We want to hear the keys "click" as we type, and we want the keys

to feel firm and spring back quickly as we press them.

• Different technologies:– Contact switch keyboard

– Ferrite core switch keyboard

– Magnetic reed switch keyboard

– Mercury contact switch keyboard

Contact switch keyboard

• Contact switch is the most common form of keyboard switch

• Pressing the plunger causes the contacts to touch (or separate), thus providing a circuit closure (or circuit open)

• Circuitry is often used to eliminate the “bouncing” effect, else false readings are obtained

Ferrite core switch

• A small ferrite core, mounted on the base of the switch, has two wires (a drive and sense wire) attached through its center

• A current pulse is applied to the drive wire, which is sensed on the sense wire

• The ferrite core is held in a state of saturation by a small magnet attached to the plunger. This prevents the current pulse from being transferred to the sense wire

• When the plunger is pressed, the magnet moves away from the ferrite core, bringing the core out of saturation and allowing the sense wire to pick up the current pulse on the drive wire

• This type of switch is non-contacting, zero bounce and has a long life

Other contact switch keyboards

• Magnetic reed switch– A reed switch comprises a pair of contacts

enclosed in sealed glass envelope– The contacts are forced together or apart by

applying an external magnetic field. The magnetic field can be generated by a small permanent magnet attached to the plunger

• Mercury contact switch– Two contacts are enclosed in a small

flexible tube, which is filled with a small amount of mercury

– When the plunger is pressed, the flexible tube is compressed and causes the mercury to short with the two contacts, completing the circuit.

Keyboard encoders

• The encoder has the task of detecting when a key has been depressed, identifying the key, and generating the code for the key

• The encoder enables each row in turn, and checks for a change in state in the column lines. If there is a change in state, this means a key has been depressed. The row and column values are used to look up a table which returns the key value

• The keyboard encoder controls the keyboard switches and generates a unique code for each key• It is an integrated circuit or small processor, and handles the key switches in a matrix of rows and columns

Keyboard encoders

• The key matrix is the grid of circuits underneath the keys. In all keyboards each circuit is broken at the point below a specific key.

• Pressing the key bridges the gap in the circuit, allowing a tiny amount of current to flow through. The processor monitors the key matrix for signs of continuity at any point on the grid.

• When it finds a circuit that is closed, it compares the location of that circuit on the key matrix to the character map in its ROM. The character map is basically a comparison chart for the processor that tells it what the key at x,y coordinates in the key matrix represents.

• If more than one key is pressed at the same time, the processor checks to see if that combination of keys has a designation in the character map. For example, pressing the a key by itself would result in a small letter "a" being sent to the computer. If you press and hold down the Shift key while pressing the a key, the processor compares that combination with the character map and produces a capital letter "A.“

• The character map in the keyboard can be superseded by a different character map provided by the computer. This is done quite often in languages whose characters do not have English equivalents. Also, there are utilities for changing the character map from the traditional QWERTY to DVORAK or another custom version

Keyboard summary

• The standard PC has a QWERTY keyboard which uses ASCII codes.

• Control codes are used to control devices like printers, modems and terminals and are generated using the CTRL key

• A keyboard encoder is a device which handles the key presses and converts them to an ASCII code

Mouse• The main goal of any mouse is to translate the motion of your hand

into signals that the computer can use. • Traditional mice do the translation using five components:

– A ball inside the mouse touches the desktop and rolls when the mouse moves– Two rollers inside the mouse touch the ball. One of the rollers is oriented so

that it detects motion in the X direction, and the other is oriented 90 degrees to the first roller so it detects motion in the Y direction. When the ball rotates, one or both of these rollers rotate as well

Mouse• Components:

– The rollers each connect to a shaft, and the shaft spins a disk with holes in it. When a roller rolls, its shaft and disk spin

– On either side of the disk there is an infrared LED and an infrared sensor. The holes in the disk break the beam of light coming from the LED so that the infrared sensor sees pulses of light. The rate of the pulsing is directly related to the speed of the mouse and the distance it travels

– An on-board processor chip reads the pulses from the infrared sensors and turns them into binary data that the computer can understand. The chip sends the binary data to the computer through the mouse's cord

Mouse data interface• Whenever the mouse moves or the user clicks a button, the mouse

sends 3 bytes of data to the computer. • The first byte's 8 bits contain:

– Left button state (0 = off, 1 = on) – Right button state (0 = off, 1 = on) – 0 – 1 – X direction (positive or negative) – Y direction – X overflow (the mouse moved more than 255 pulses in 1/40th of a second) – Y overflow

• The next 2 bytes contain the X and Y movement values, respectively. These 2 bytes contain the number of pulses that have been detected in the X and Y direction since the last packet was sent. The data is sent from the mouse to the computer serially on the data line, with the clock line pulsing to tell the computer where each bit starts and stops.

• Eleven bits are sent for each byte (1 start bit, 8 data bits, 1 parity bit and 1 stop bit). The PS/2 mouse sends on the order of 1,200 bits per second. That allows it to report mouse position to the computer at a maximum rate of about 40 reports per second. If you are moving the mouse very rapidly, the mouse may travel an inch or more in one-fortieth of a second. This is why there is a byte allocated for X and Y motion in the data protocol.

Pins:1. Unused 2. +5 volts (to

power the chip and LEDs)

3. Unused 4. Clock 5. Ground 6. Data

Optical Readers

• Convert printed or hand written information to computer data

• Information is read in one two ways – OCR (Optical Character Recognition)

• Printed or typewritten characters

• Handwritten characters

– OMR (Optical Mark Readers)marks which are placed in predefined areas of the document

• Pre-printed paper is normally used for hand-printed marks or characters

• Readers are designed to read marks or characters positioned in a matrix pattern

• Colored printing is used in pre-printing, which is insensitive to the reader

OCR devices• OCR is the scanning of text documents into graphic images,

then using software to decode the graphic picture elements back into text.

• When the scanner scans the document, it is read as a series of black and white pixel (dots) elements. This process often tends to degrade the edges of the text characters, and is more pronounced when the characters on the original are too small. Edge degradations makes it harder for the OCR software to convert the pixel elements back into text later on.

• The OCR software reads the bitmap of pixels created in step 1 and averages out the white spaces on the page, effectively identifying paragraphs and eliminating graphics. The white spaces between each line of text is used as a baseline reference for recognizing the characters on that line.

OCR devices• First, the OCR software tries to match each character on a line in the

bitmap against character templates that it knows about. • The remaining unidentified characters have a technique known as

feature extraction applied to them. The OCR software calculates the characters height, lines, curves and other features. It can then make close guesses as to the characters value.

• For the remaining characters the OCR software cannot recognize, the software can either apply contextual analysis, which basically means looking at the syntax and construction of the words and making a guess (for example, changing thi5 to this), or give up and substitute the unknown character with a distinctive symbol such as ~ or @.

• The finished information is normally able to be saved in a number of different formats, text or Rich Text Format (RTF). OCR software which support RTF can also recognize bold, italics, retain tabs and whitespace, as well as recognize a limited number of different fonts.

Bar Codes• Represents numeric data as a series of bars • Bars have varying thickness and separations • Numeric data is often written underneath the bar-code • Easily read by light-pen or scanner device

The light pen has a sensitive tip which contains a light source and light detector. When the pen is stroked along the bar-code symbol, the light from the pen bounces off the dark bars to produce a corresponding set of binary pulses. This sequence is decoded to give the numeric data that the bar-code represents

• Examples of Bar-Code Applications– Consumer goods (super-markets) – Stock inventory – Library systems for cataloging books – Drug dispensing at your local pharmacy

Card Readers

• Magnetic Card Readers– Magnetic card swipe readers read the information

contained on a magnetic strip on a plastic card

• Smart Card Readers– Smart cards are intelligent cards which replace the

magnetic stripe with a controller and memory

• Optical Card Readers– Optical Smart Cards are the same size and shape as

standard plastic credit cards, but hold up to six megabytes of updateable digital information in a secure, inexpensive, and compact personal package

Magnetic Card Readers

• Characteristics– the data is stored on the magnetic stripe – the number of characters stored is typically about 1K – the card often includes a name and signature, or other details (like a photo) for added security

• Connection– The magnetic card reader is inserted between the keyboard and the base unit of the computer. The

magnetic card unit converts the information on the magnetic card and presents it to the computer as a series of ASCII characters.

– The computer thinks that the characters came from the keyboard. This simplifies writing software to read magnetic cards.

• Applications– Electronic Funds Transfer, Point of Sale (EFT-POS) – Drivers License – Telecom Phone Cards – Credit Cards, VISA, MASTER-CARD, AMERICAN EXPRESS

Smart Card Readers

• Characteristics– include built-in electronics and memory storage (often 16KB or more) – logs details and transaction information – numerous applications, including shopping, banking, SKY-TV, medical – potential security risks

• A scheme (1996) involved a medical center which placed customer information on smart cards. – This included medical and dentist information from the center. Even if cards are

protected by the use of PIN (Personal Identification Numbers) information, which is normally limited to 4 digits, the potential for disaster is comparatively high if someone has the time and resources to fraudulently obtain a card and access the information on it for criminal reasons

Optical Cards• Optical Smart Cards are the same size and shape as standard plastic credit

cards, but hold up to six-megabytes of updateable digital information in a secure, inexpensive, and compact personal package.

• Data that can be stored on Optical Smart Cards include – cardholder name, address, and other personal information – digitized cardholder photographs – signatures – medical images or x-rays – updateable account balances and transaction audit trails – security information

• Advantages of optical smart cards over chip and magnetic-stripe cards include: – Large storage capacity of six-megabytes – Off-Line card verification with NO dependence on telephone or other links to a

central database – Faster card updates (30 times faster than chip-cards) – Permanent and VERY secure fraud-proof operation using the latest crypto

technology – No possible data loss from exposure of cards to static electricity, water, magnetic or

electrical fields, or to x-rays (for example -- during airport security checks) – Less expensive long-term system operating costs – Global card standards with multiple sources

• The Canadian Government has adopted the Canon Optical Card as a positive identification card-"CANPASS" for international travelers. The CANPASS Card contains traveler's photo and fingerprint templates and reduces the time required to clear Customs by about 80%.

Printers• There are several major printer technologies available. These

technologies can be broken down into two main categories with several types in each:

• Impact - These printers have a mechanism that touches the paper in order to create an image: – Dot matrix printers use a series of small pins to strike a ribbon coated with ink,

causing the ink to transfer to the paper at the point of impact– Character printers are basically computerized typewriters. They have a ball or

series of bars with actual characters (letters and numbers) embossed on the surface. The appropriate character is struck against the ink ribbon, transferring the character's image to the paper. Character printers are fast and sharp for basic text, but very limited for other use.

• Non-impact - These printers do not touch the paper when creating an image. Inkjet printers are part of this group, which includes: – Inkjet printers, use a series of nozzles to spray drops of ink directly on the

paper– Laser printers, use dry ink (toner), static electricity, and heat to place and bond

the ink onto the paper

Inkjet Printer Components

• Print head assembly – Print head - The core of an inkjet printer, the print head contains a series of nozzles that

are used to spray drops of ink• Ink cartridges

– Depending on the manufacturer and model of the printer, ink cartridges come in various combinations

• separate black and color cartridges• color and black in a single cartridge or even a cartridge for each ink color. The cartridges of

some inkjet printers include the print head itself • Print head stepper motor - A stepper motor moves the print head assembly (print

head and ink cartridges) back and forth across the paper – Some printers have another stepper motor to park the print head assembly when the

printer is not in use. Parking means that the print head assembly is restricted from accidentally moving, like a parking break on a car

• Belt - A belt is used to attach the print head assembly to the stepper motor. • Stabilizer bar - The print head assembly uses a stabilizer bar to ensure that

movement is precise and controlled

Inkjet Printer Components• Paper feed assembly

– Paper tray/feeder• Most inkjet printers have a tray that you load the paper into.• Some printers dispense with the standard tray for a feeder instead. The feeder

typically snaps open at an angle on the back of the printer, allowing you to place paper in it. Feeders generally do not hold as much paper as a traditional paper tray.

– Rollers - A set of rollers pull the paper in from the tray or feeder and advance the paper when the print head assembly is ready for another pass

– Paper feed stepper motor - This stepper motor powers the rollers to move the paper in the exact increment needed to ensure a continuous image is printed

• Control circuitry - A small but sophisticated amount of circuitry is built into the printer to control all the mechanical aspects of operation, as well as decode the information sent to the printer from the computer

• Interface –parallel port, USB (Universal Serial Bus) or sometimes network technologies (wired or wireless)

Inkjet Printer Components

• Left – print head, belt and paper feed mechanisms• Right – control circuitry

Inkjet Printers Technology• Thermal bubble - Used by manufacturers such as Canon,

this method is commonly referred to as bubble jet– tiny resistors create heat, and this heat vaporizes ink to create a

bubble. As the bubble expands, some of the ink is pushed out of a nozzle onto the paper. When the bubble "pops" (collapses), a vacuum is created. This pulls more ink into the print head from the cartridge

– A typical bubble jet print head has 300 or 600 tiny nozzles, and all of them can fire a droplet simultaneously

• Piezoelectric – Epson patent, this technology uses piezo crystals– A crystal is located at the back of the ink reservoir of each nozzle.

The crystal receives a tiny electric charge that causes it to vibrate. When the crystal vibrates inward, expanding, it forces a tiny amount of ink out of the nozzle. When it vibrates out, it pulls some more ink from the reservoir to replace the ink sprayed out

Inkjet Printer Technology

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Laser Printer• The primary principle at work in a laser printer is static

electricity, the same energy that makes clothes in the dryer stick together or a lightning bolt travel from a thundercloud to the ground.

• Static electricity is simply an electrical charge built up on an insulated object, such as a balloon or your body. Since oppositely charged atoms are attracted to each other, objects with opposite static electricity fields cling together.

• A laser printer uses this phenomenon as a sort of "temporary glue." The core component of this system is the photoreceptor, typically a revolving drum or cylinder. This drum assembly is made out of highly photoconductive material that is discharged by light photons.

Laser Printer

Initially, the drum is given a total positive charge by the charge corona wire, a wire with an electrical current running through it. (Some printers use a charged roller instead of a corona wire, but the principle is the same.) As the drum revolves, the printer shines a tiny laser beam across the surface to discharge certain points. In this way, the laser "draws" the letters and images to be printed as a pattern of electrical charges -- an electrostatic image. The system can also work with the charges reversed -- that is, a positive electrostatic image on a negative background

After the pattern is set, the printer coats the drum with positively charged toner -- a fine, black powder. Since it has a positive charge, the toner clings to the negative discharged areas of the drum, but not to the positively charged "background." This is something like writing on a soda can with glue and then rolling it over some flour: The flour only sticks to the glue-coated part of the can, so you end up with a message written in powder.

With the powder pattern affixed, the drum rolls over a sheet of paper, which is moving along a belt below. Before the paper rolls under the drum, it is given a negative charge by the transfer corona wire (charged roller). This charge is stronger than the negative charge of the electrostatic image, so the paper can pull the toner powder away. Since it is moving at the same speed as the drum, the paper picks up the image pattern exactly. To keep the paper from sticking to the drum, it is discharged by the detac corona wire immediately after picking up the toner.

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 melts, fusing with the fibers in the paper. The fuser rolls the paper to the output tray, and you have your finished page. The fuser also heats up the paper itself, of course, which is why pages are always hot when they come out of a laser printer or photocopier; After depositing toner on the paper, the drum surface passes the discharge lamp. This bright light exposes the entire photoreceptor surface, erasing the electrical image. The drum surface then passes the charge corona wire, which reapplies the positive charge

Laser Printer Controller• Before a laser printer can do anything else, it needs to receive the page data and

figure out how it's going to put everything on the paper. This is the job of the printer controller.

• The printer controller is the laser printer's main onboard computer. It talks to the host computer through a communications port, such as a parallel port or USB port. At the start of the printing job, the laser printer establishes with the host computer how they will exchange data. The controller may have to start and stop the host computer periodically to process the information it has received

• In an office, a laser printer will probably be connected to several separate host computers, so multiple users can print documents from their machine. The controller handles each one separately, but may be carrying on many "conversations" concurrently. This ability to handle several jobs at once is one of the reasons why laser printers are so popular

• For the printer controller and the host computer to communicate, they need to speak the same page description language. In earlier printers, the computer sent a special sort of text file and a simple code giving the printer some basic formatting information. Since these early printers had only a few fonts, this was a very straightforward process. Today there are hundreds of different fonts to choose from, as well as complex graphics. To handle all of this diverse information, the printer needs to speak a more advanced language

Laser Printer Controller• The primary printer languages these days are Hewlett Packard's Printer Command

Language (PCL) and Adobe's Postscript. Both of these languages describe the page in vector form

– that is, as mathematical values of geometric shapes, rather than as a series of dots (a bitmap image). • The printer itself takes the vector images and converts them into a bitmap page. With this

system, the printer can receive elaborate, complex pages, featuring any sort of font or image. Also, since the printer creates the bitmap image itself, it can use its maximum printer resolution.

• Some printers use a graphical device interface (GDI) format instead of a standard PCL. In this system, the host computer creates the dot array itself, so the controller doesn't have to process anything

– it just sends the dot instructions on to the laser. • But in most laser printers, the controller must organize all of the data it receives from the host

computer. This includes all of the commands that tell the printer what to do – what paper to use, how to format the page, how to handle the font, etc. For the controller to work

with this data, it has to get it in the right order. • Once the data is structured, the controller begins putting the page together. It sets the text

margins, arranges the words and places any graphics. When the page is arranged, the raster image processor (RIP) takes the page data, either as a whole or piece by piece, and breaks it down into an array of tiny dots. The printer needs the page in this form so the laser can write it out on the photoreceptor drum.

• In most laser printers, the controller saves all print-job data in its own memory. This lets the controller put different printing jobs into a queue so it can work through them one at a time. It also saves time when printing multiple copies of a document

Laser Printer Assembly• The laser part of a laser printer is made out of:

– A laser – A movable mirror – A lens

• The laser receives the page data (the tiny dots that make up the text and images) one horizontal line at a time. As the beam moves across the drum, the laser emits a pulse of light for every dot to be printed, and no pulse for every dot of empty space. The laser doesn't actually move the beam itself. It bounces the beam off a movable mirror instead. As the mirror moves, it shines the beam through a series of lenses. This system compensates for the image distortion caused by the varying distance between the mirror and points along the drum.

• The laser assembly moves in only one plane, horizontally. After each horizontal scan, the printer moves the photoreceptor drum up a notch so the laser assembly can draw the next line. A small print-engine computer synchronizes all of this perfectly, even at high speeds.

Advantages of Laser Printers• The main advantages of laser printers are speed, precision and

economy. A laser can move very quickly, so it can "write" with much greater speed than an ink jet. And because the laser beam has an unvarying diameter, it can draw more precisely, without spilling any excess ink. Laser printers tend to be more expensive than inkjet printers, but it doesn't cost as much to keep them running – toner powder is cheap and lasts a long time, while you can use up expensive ink

cartridges very quickly. This is why offices typically use a laser printer. – When they were first introduced, laser printers were too expensive to use as a

personal printer. Since that time, however, laser printers have gotten much more affordable. Now you can pick up a basic model for just a little bit more than a nice inkjet printer.

• As technology advances, laser-printer prices should continue to drop, while performance improves. We'll also see a number of innovative design variations, and possibly brand-new applications of electrostatic printing. Many inventors believe we've only scratched the surface of what we can do with simple static electricity!

Other Printers• Solid ink printers contain sticks of wax-like ink that are melted and applied to the

paper. The ink then hardens in place. • Dye sublimation printers have a long roll of transparent film that resembles sheets

of red-, blue-, yellow- and gray-colored cellophane stuck together end to end. Embedded in this film are solid dyes corresponding to the four basic colors used in printing: cyan, magenta, yellow and black (CMYK). The print head uses a heating element that varies in temperature, depending on the amount of a particular color that needs to be applied. The dyes vaporize and permeate the glossy surface of the paper before they return to solid form. The printer does a complete pass over the paper for each of the basic colors, gradually building the image.

• Thermal wax printers are something of a hybrid of dye-sublimation and solid ink technologies. They use a ribbon with alternating CMYK color bands. The ribbon passes in front of a print head that has a series of tiny heated pins. The pins cause the wax to melt and adhere to the paper, where it hardens in place.

• Thermal auto chrome printers have the color in the paper instead of in the printer. There are three layers (cyan, magenta and yellow) in the paper, and each layer is activated by the application of a specific amount of heat. The print head has a heating element that can vary in temperature. The print head passes over the paper three times, providing the appropriate temperature for each color layer as needed

The Monitor• Often referred to as a monitor when packaged in a separate case, the display is the

most-used output device on a computer. The display provides instant feedback by showing you text and graphic images as you work or play.

• Display technologies:– Desktops were usually using Cathode Ray Tube (CRT) display. – Portable computing devices such as laptops incorporate Liquid Crystal Dispay (LCD), light-

emitting diode (LED), gas plasma or other image projection technology. • Because of their slimmer design and smaller energy consumption, monitors

using LCD technologies are beginning to replace the venerable CRT on many desktops.

• Main characteristics of a monitor are:– Display technology - Currently, the choices are mainly between CRT and LCD technologies

(more LCD than CRT). – Viewable area (usually measured diagonally) – Aspect ratio and orientation (landscape or portrait) – Maximum resolution – Dot pitch – Refresh rate – Color depth – Amount of power consumption

Display Technology• Displays have come a long way since the blinking green monitors in text-based computer

systems of the 1970s. • In 1981, IBM introduced the Color Graphics Adapter (CGA), which was capable of

rendering four colors, and had a maximum resolution of 320 pixels horizontally by 200 pixels vertically.

• IBM introduced the Enhanced Graphics Adapter (EGA) display in 1984. EGA allowed up to 16 different colors and increased the resolution to 640x350 pixels, improving the appearance of the display and making it easier to read text.

• In 1987, IBM introduced the Video Graphics Array (VGA) display system. Most computers today support the VGA standard and many VGA monitors are still in use (640x480)

• IBM introduced the Extended Graphics Array (XGA) display in 1990, offering 800x600 pixel resolution in true color (16.8 million colors) and 1,024x768 resolution in 65,536 colors.

• Most displays sold today support the Ultra Extended Graphics Array (UXGA) standard. UXGA can support a palette of up to 16.8 million colors and resolutions of up to 1600x1200 pixels, depending on the video memory of the graphics card in your computer.

• A typical UXGA adapter takes the digital data sent by application programs, stores it in video random access memory (VRAM) or some equivalent, and uses a digital-to-analog converter (DAC) to convert it to analog data for the display scanning mechanism. Once it is in analog form, the information is sent to the monitor through a VGA cable.

Display Characteristics• Viewable area

– Aspect ratio: typical 4:3 or 16:9– Screen size: 15, 17, 19, 21, 22, 24, 26, etc…

• Resolution– refers to the number of individual dots, known as pixels, contained on a display.

Resolution is expressed by identifying the number of pixels on the horizontal axis (rows) and the number on the vertical axis (columns), such as 640x480. The monitor's viewable area, refresh rate and dot pitch all directly affect the maximum resolution a monitor can display

• Dot Pitch– is the measure of how much space there is between a display's pixels. When considering

dot pitch, remember that smaller is better. Packing the pixels closer together is fundamental to achieving higher resolutions

• Refresh Rate– In monitors based on CRT, is the number of times that the image on the display is drawn

each second. If your CRT monitor has a refresh rate of 72 Hertz (Hz), then it cycles through all the pixels from top to bottom 72 times a second. Refresh rates are very important because they control flicker, and you want the refresh rate as high as possible. Too few cycles per second and you will notice a flickering, which can lead to headaches and eye strain.

Display Characteristics• Color depth

– The combination of the display modes supported by your graphics adapter and the color capability of your monitor determine how many colors can be displayed.

• For example, a display that can operate in SuperVGA (SVGA) mode can display up to 16,777,216 (usually rounded to 16.8 million) colors because it can process a 24-bit-long description of a pixel. The number of bits used to describe a pixel is known as its bit depth.

– With a 24-bit bit depth, 8 bits are dedicated to each of the three additive primary colors (red, green and blue).

• This bit depth is also called true color because it can produce the 10,000,000 colors discernible to the human eye, while a 16-bit display is only capable of producing 65,536 colors. Displays jumped from 16-bit color to 24-bit color because working in 8-bit increments makes things a whole lot easier for developers and programmers.

• Power consumption – varies greatly with different technologies. CRTs are somewhat power-hungry, at

about 110 watts for a typical display, especially when compared to LCDs, which average between 30 and 40 watts.

CRT• the CRT consists of a vacuum tube

enclosed in glass. At the end of the tube is an electron gun (which generates an electron beam), focusing and deflection assembly. At the other end of the tube is the screen surface which is coated in phosphorous which emits a dot of light when struck by the electronic beam.

• the electron gun emits a high stream of electrons.

• the electron beam is deflected by signals applied to the deflection assembly.

• when the electron beam strikes the phosphorous coating on the inner surface of the screen, light is emitted.

• the electron beam is swept across the screen from left to right in horizontal lines, from top to bottom, in a pattern called a raster.

• the screen image is repeated at the refresh rate, as the image dissipates quickly.

CRT

• color systems use three electron beams, one each for red, blue and green.

• in color systems, three different phosphors are used which match each of the three beams. This collection of three phosphors is called a triad.

• a shadow mask is used to prevent the beams from striking the wrong phosphors

• Interlaced Displays– the screen picture is made up of two full passes of the electron beam from top to bottom – the first pass builds up one screen doing only odd line numbers – the second pass completes the screen doing all the even line numbers – the technique relies upon the persistence of the image on the screen so that the human eye blends

both passes together. Persistence is a measure of how long the image remains on the screen before it fades away. This is because phosphor only emits light when struck by electrons, when it stops being struck, it stops emitting light.

– this technique is commonly used for Television • Non-Interlaced Displays

– In non-interlaced displays, the screen picture is made from one single pass of the electron beam from top to bottom.