Computer Fundamentals
Unit-II
Computer
Hardware
Input Devices: - An input device
can be defined as an electromechanical device that allows the user to feed
information (data) into the computer for analysis, storage and to give commands
to the computer. Data and instructions are entered into the main memory of
computer through an input device. Input device captures information and
translates it into a form that can be processed and used by other parts of the
computer. After processing the input data, the computer provides the results
with the help of output devices. Input devices play a major role in the
processing of any data via the computer system because the output of the computer
is always based on the given input. A computer can accept input in two ways,
either manually or directly. In case of manual data entry, the user enters the
data into computer by hand such as by using keyboard and mouse. User can also
enter data directly by transferring information automatically from a source
document (like barcode) into the computer.
Keyboard: - Keyboard
devices are the most commonly used input devices today. Using a keyboard, the
user can type text and execute commands. Keyboard is designed to resemble a
regular typewriter with a few additional keys. Data is entered into the
computer by simply pressing various keys. The layout of a keyboard come in
various styles such as QWERTY, DVORAK, AZERTY but the most common layout is the
QWERTY. It is named so because the first six keys on the top row of letters are
Q, W, E, R, T and Y. The number of keys on a typical keyboard varies from 82
keys to 108 keys. Portable computers such as laptops quite often have custom
keyboards that have slightly different key arrangements than a standard
keyboard.
Keyboard is the easiest input device, as it does not
require any special skill. Usually, it is supplied with a computer so no
additional cost is incurred. The maintenance and operation cost of keyboard is
also less.
Layout of the keyboard: - The layout of
the keyboard can be divided into the following five sections:
- Typing Keys (Alphanumeric Keys):- The
alphanumeric keys contain the keys for the letters of English alphabet,
the numbers 0 to 9, and several special characters like ?, /, $, &
etc.
- Numeric Keypad: - The numeric keypad is a set
of keys that looks like an adding machine with its ten digits and
mathematical operators (+, -, * and /). It is usually located on the right
side of the keyboard.
- Functions Keys: - The function keys (F1, F2, F3
etc) are arranged in a row along the top of the keyboard and could be
assigned specific commands by the current applications or the operating
system. For example, most of the Microsoft programs use F1 to display
helps.
- Control Keys: - These keys provide cursor and
screen control. It includes four directional arrow keys that are arranged
in an inverted T formation between the typing keys and the numeric keypad.
These keys allow the user to move the cursor on the display area one space
at a time. In either an up, down, left or right direction. Control keys
also include Home, End, Insert, Delete, Page up, Page Down, Control (Ctrl),
Alternate (Alt) and Escape (Esc). The Windows keyboard also consists of
two Windows or Start keys, and an application key.
- Special Purpose Keys: - A keyboard also
contains some special purpose keys such as Enter, Shift, Caps Lock, Num
Lock, Spacebar, Tab and Print Screen.
Working of a Keyboard: - A keyboard is
a series of switches connected to a small keyboard microprocessor that monitors
the state of each switch and initiates a specific response to a change in that
state. When the user presses a key, it causes a change in the amount of current
flowing through the circuit associated specifically with that key. The keyboard
microprocessor detects this change in current flow. By doing this, the
processor can tell when a key has been pressed and when it is being released.
Depending upon which key’s circuit carries a signal to the microprocessor; the
processor generates the associative code, known as scan code, of the key and
sends it to the operating system. A copy of this code is also stored in the
keyboard’s memory. When the operating system reads the scan code, it informs
the same to the keyboard and the scan code stored in keyboard’s memory is then
erased. Initially, the processor filters all the tiny current fluctuations out
of the signal and treats it as a single key press. If the user continues to
hold down a key, the processor determines that he wish to send that character
repeatedly to the computer. In this process, the delay between each instance of
a character can normally be set in operating system, typically ranging from 2
to 30 characters per second (cps).
Keyboard Buffer: - A keyboard buffer
is a small memory inside either the terminal or the keyboard in which each keystroke
is first saved before being transferred to the computer’s main memory for
processing. The keyboard buffer usually has capacity to save from a few tens to
a few hundreds of keystrokes. Storing of keystrokes in keyboard buffer instead
of sending them directly to the main memory makes the user interaction with the
system much more efficient and smoother. This is because when the keystroke is
made, the processor may be busy doing something else and may not be in a
position to immediately transfer the keyboard input to the main memory.
Pointing Devices: - Most computers
come with an alphanumeric keyboard but in some applications, keyboard is not
convenient. For example, if the user wants to select an item from a list, the
user can identify that items position by selecting it through the keyboard.
However, this action could be performed quickly by pointing devices. A pointing
device is used to communicate with the computer by pointing to locations on the
monitor screen. Some of the commonly used pointing devices are mouse,
trackball, joystick, light pen and touch screen.
Mouse: - Mouse is a
small hand-held pointing device. Usually, a mouse contains two or three
buttons, which can be used to input commands or information. The mouse may be
classified as a mechanical mouse or an optical mouse. A mechanical mouse uses a
rubber ball at the bottom surface, which rotates as the mouse is moved along a
flat surface, to move the cursor. Mechanical mouse is the most common and least
expensive pointing device. Microsoft, IBM and Logitech are some well-known
makers of the mechanical mouse.
An optical mouse uses a light beam instead of a rotating
ball to detect movement across a specially patterned mouse pad. As the user rolls
the mouse on a flat surface, the cursor on the screen also moves in the
direction of mouse’s movement. An optical mouse has the following benefits over
the mechanical mouse:
- No moving part means less wear and a lower
chance of failure.
- Dirt can not get inside the mouse and hence no
interference with the tracking sensors.
- They do not require a special surface such as a
mouse pad.
Depending on the
application, the cursor of the mouse can be text cursor or graphic cursor. The
text cursor (I) is used for text while the graphic cursor is used for pointing
and drawing. The graphic cursor is displayed on the screen by the variety of
symbols such as arrow or a pointing finger.
Like keyboard, usually it is also supplied with a
computer; therefore, no additional cost is incurred. However, it needs a flat
space close to the computer. The mouse cannot easily be used with laptop,
notebook or palmtop computers.
Some of the common mouse
actions are: Pointing, Click, Right Click, Double Click and Drag and Drop.
Working of a Mouse: - Principally,
the mouse works by measuring how much it moves in a given direction. A
mechanical mouse has a rubber ball in the bottom. When the user moves the
mouse, the balls roll along the surface of the mouse pad, and the mouse keeps
track of how far the balls rolls. This allows it to tell how far it has moved.
Inside the bottom of the mouse are three rollers. One of them, the one mounted
at a 450 angle to the other two, is spring loaded. This
roller is usually the smallest of the three. It is there simply to hold the
ball against the other two rollers. The other two rollers are usually larger
and of different color. These rollers are mounted at a 900 angle to
the one other, one roller measures how fast the ball is turning horizontally and
the other measures how fast it is turning vertically. When the ball rolls, it
turns these two rollers. The rollers are connected to axles and the axles are
connected to a small sensor that measures how fast the axle is turning. Both sets
of information are passed to the electronics inside the mouse. This little
processor, usually consisting of little more than a single chip, uses the
information to determine how fast the mouse itself is moving, and in what
direction. This information is passed to the computer via mouse cord, where the
operating system then moves the pointer accordingly.
The optical mouse uses an infrared light and special
mouse pads with fine grid lines to measure the rotation of the axle. The axle
in optical mouse is connected to a little photo-interrupter wheel with a number
of tiny holes in it. in front of this wheel is a light and on the other side of
the wheel is a light meter. As the wheel turns, the light flashes through the
holes in the wheel. By measuring how often these flashes occur, the light
sensor can measure how fast the wheel is turning and sends the corresponding
coordinates to the computer. The computer moves the cursor on the screen based
on the coordinates received from the mouse. This happens hundreds of times each
second, making the cursor appear to move very smoothly.
Trackball: - A trackball
is a pointing device that is similar to a mechanical mouse. The ball that is
placed in the base of a mechanical mouse is placed on the top along with the
buttons in case of a trackball. To move the graphics cursor around the screen,
the ball is rolled with the fingers. Because the whole device is not moved for
moving the graphics cursor, a trackball requires less space than a mouse for
operation. Since it need not be moved for moving the graphics cursor, it is often
attached to or built into the keyboard. Trackball built into the keyboard are
commonly used in laptop and notebook computers because a mouse is not practical
for laptop users in a small space. A trackball comes in various shapes and
forms with the same functionality. The
three commonly used shapes are a ball, a button, and a square. In case of a
ball, the ball is rolled with the help of fingers to move the graphics cursor.
In case of a button, the button is pushed with a finger in the desired
direction of the graphics cursor movement.
In case of a square plastic, the fingers is placed on top of it and
moved in the desired direction of the graphics cursor movement.
Working of a trackball: - A trackball
works in the same way as a mouse, with the ball turning rollers, the rollers
turning axles(axis) , which are in turn connected to either mechanical or
optical sensors that measure their rotation. A trackball consists of a number
of components. As one moves the trackball, it starts a chain of events inside
the box that results in the pointer moving on the computer screen. In a normal
trackball, on one side of each encoding wheel is a pair of LED (light emitting
diode) that emits infrared light. On the opposite side of each pair of LEDs,
there is a light sensor. Every time light from the LEDs shines through a hole
in the encoding wheel, a pulse of electricity is sent from the light sensor to
the microprocessor. When the trackball rolls side-to-side, the horizontal (x-axis)
shaft rotates, spinning the attached encoder wheel. Similarly, when the
trackball is rolled up and down, the vertical (y-axis) shaft rotates, spinning
the attached encoder wheel. Due to this spinning (rotating), the light blinks
which can be detected by the light sensor. The microprocessor counts how many
times the light sensors detect light each second and send this information to
the computer along the cord.
Joystick: - A joystick is
a pointing device that works on the same principle as a trackball. Instead of
using the fingers in case of a trackball, the user of a joystick moves the
spherical ball with the help of the stick with his/her hand. The stick can be
moved forward or backward, left or right, to move and position the graphics
cursor at the desired position. The joystick offers three types of control:
digital, glide and direct. Digital control allows movement in a limited number
of directions such as up, down, left and right. Glide and direct control allow
movements in all directions (360 degrees). Direct control joysticks have the added
ability to respond to the distance and speed with which the user moves the
stick.
The basic design of a joystick consists of a stick that
is attached to a plastic base with a flexible rubber sheath. This plastic base
houses a circuit board that sits beneath the stick. Joysticks are mainly used
for computer games, for other applications, which includes flight simulators,
training simulators, CAD/CAM systems, and for controlling industrial robots.
Working of a Joystick: - Various
joystick technologies are available and they differ mainly in how much
information they pass on. All the joysticks are designed to inform the computer
about the positioning of the handle at any given time. This is done by
providing the x-y coordinates of the handle. The x-axis represents the
side-to-side position and the y-axis represents the forward block position.
This basic design consists of a stick that is attached to a plastic base with a
flexible rubber sheath. The base houses a circuit board that sits directly
underneath the stick, which carry electricity from one contact point to
another. When the joystick is in the neutral position, all but one of the
individual circuits is broken. Each broken section is covered with a simple plastic
button containing a tiny metal disc. When the stick is moved in any direction,
it pushes down on one of these buttons, pressing the conductive metal disc
against the circuit board. This closes the circuit, that is, it completes the
connection between the two wire sections. When the circuit is closed, electricity
can flow down a wire from the computer and to another wire leading back to the
computer. When the computer picks up a charge on a particular wire, it knows
that the joystick is in the right position to complete that particular circuit.
The joystick buttons work exactly the same way. When a button is pressed down,
it completes a circuit and the computer recognizes a command.
Digitizing Tablet: - A digitizer
is an input device used for converting (digitizing) pictures, maps, and drawings
into digital form for storage in computers. For example, the x and y
coordinates of points in a drawing may be stored in digital form. This enables
re-creation of the drawing from the stored information whenever required as
well as easy of changes in the drawing as and when required.
A digitizer consists of a digitizing tablet (also known
as graphics tablet) associated with a stylus. The digitizing tablet is a flat
surface that contains hundreds of fine copper wires forming a grid. Each copper
wire receives electric pulses. The digitizing tablet can be spread over a
working table and is connected to a computer. The stylus is like a pen or a
lens-like cursor with a cross-hair and button. The stylus is connected to the
tablet and can be pressed down at a point on the tablet to input the (x, y)
coordinates of the point. When the stylus is moved on the tablet, the cursor on
the computer’s screen moves simultaneously to a corresponding position on the
screen to provide visual feedback to the operator. This enables the operator to
draw sketches directly or to input sketched drawings very easily. Inputting
drawings or developing sketches using a digitizer is further simplified by the
fact that poorly sketched lines, arcs, and other graphical objects are automatically
input as mathematically precise objects, like straight lines and smooth curves.
Digitizer are commonly used in the area of Computer Aided Design (CAD) by
architects and engineers to design cars, buildings, medical devices, robots,
mechanical parts, etc. They are also used in the area of Geographical
Information System (GIS) for digitizing of maps available in paper form.
Scanners: - There are a
number of situations when some information (picture or text) is available on
paper and is needed on the computer disk for further manipulation. A scanner is
an input device that translates paper documents into an electronic format that
can be stored in a computer. The input documents may be typed text, pictures,
graphics or even handwritten material. This input device has been found to be
very useful in preserving paper documents in electronic form. The copy of a
document stored in a computer in this manner will never deteriorate in quality
or become yellow with age and can be displayed or printed whenever desired. If
the computer in which the scanned document is stored has the right kind of
software, the stored images can be altered and manipulated in interesting ways.
Scanners are also called optical scanners because they
use optical technology for converting an image into electronic form. They can
store images in both gray-scale and color mode. The two most common types of
scanners are hand-held scanner and flat-bed scanner.
Hand-Held
Scanner: - A hand-held scanner consists of light emitting diodes, which are
placed over the material to be scanned. This scanner performs the scanning of
the document very slowly from the top of the bottom, with its light on. In this
process, all the documents are converted and then stored as an image. While
working, the scanner is dragged very steadily (gradually) and carefully over
the document and it should move at a constant speed without stopping or jerking
in order to obtain best results. Due to
this reason, hand held scanners are widely used where high accuracy is not of
much importance. The size of hand held scanners is small. They come in various resolutions,
up to about 800 dpi (dots per inch) and are available in either grey scale or
colour. Furthermore, they are also used when the volume of the documents to be
scanned is low. The typical application of this scanner includes the storing
and reproducing of the images in publications. These devices read the data on
the price tags, shipping labels, inventory part number, and so on.
Working of a hand-held
scanner: - When the hand-held scanner’s scan button is pressed, a light
emitting diode illuminates the document underneath it. An inverted angled
mirror directly over the scanner’s window reflects the image onto the scanner’s
lens, which is located at the back of the scanner. The lens focuses a single
line of the image onto a charged coupled device (CCD), which contains a row of
light detectors. As the light shines through these detectors, each of them
records the amount of light as a voltage that corresponds to white, black, and
grey or to a colour. These voltages are sent to a specialized analog chip,
which corrects any colour detections error. After that, a single line image is
passed to analog to digital converter (ADC), which converts the analog signals
into binary form that can be sent to the computer. In this way, the converter
clears itself of the data so that it can receive the next line of the image.
Flat-Bed
Scanner: - Flat-bed scanners look similar to a photocopier machine. It
consists of a box containing a glass plate on its top and a lid that covers the
glass plate. This glass plate is primarily used for placing the document to be
scanned. The light beam is placed below the glass plate and when it is
activated, it moves from left to right horizontally. After scanning one line,
the beam of light moves in order to scan the next line and thus, the procedure
is repeated until all the lines are scanned. For scanning, an A4 size document
takes about 20 seconds. These scanners are capable of scanning black and white
as well as colour images. The flat-bed scanners are larger in size and more
expensive than the hand-held scanners. However, they usually produce better
quality images because they employ better scanning technology.
Working of a flat-bed
scanner: - To scan a document, first it is placed on the glass plate and the
cover is closed. A lamp is used to illuminate the document. The scan head
(mirrors, lens, filter and CCD array constitutes a scan head) is moved slowly
across the document by a belt that is attached to a stepper motor. The head is
attached to a stabilizer bar to ensure that there is no wobble (vibrate) or
deviation in the pass. In scanning
terms, a pass means that the scan head has completed a single complete scan of
the document. The image of the document is reflected by an angled mirror to
another mirror. Each mirror is slightly curved to focus the image it reflects
onto a smaller surface. The last mirror reflects the image onto a lens. The
lens focuses the image through a filter on the CCD array. It is a collection of
tiny light-sensitive diodes (also called photosites), which converts light into
electrical charge. The brighter the light that hits a single photosite, the
greater the electrical charge that will accumulate at that site. Some scanners
use a three pass scanning method. Each pass uses a different colour filter (red,
green or blue) between the lens and CCD array. After the three passes are
completed, the scanner software assembles the three filtered images into a
single full-colour image. Nowadays, most scanners use the single pass method.
The lens splits the image into three smaller versions of the original image.
Each smaller version passes through a colour filter (either red, green, blue)
onto a discrete section of the CCD array. The scanner combines the data from
the three parts of the CCD array into a single full-colour image, which is then
sent to the computer.
Digital
Camera:-Digital camera stores images digitally rather than recording them on
a film. Once a picture has been taken, it can be downloaded to a computer
system and then manipulated with an image editing software and printed. The big
advantage of digital cameras is that making photos is both inexpensive and fast
because there is no film processing.
Working of a digital camera: - All digital
cameras record images in an electronic form, that is, the image is represented
in computer’s languages, the language of bits and bytes. Essentially, a digital
image is just a long string of 1’s and 0’s that represent all the tiny colored
dots or pixels that collectively make up the image. Just like a conventional camera, it has a series of
lenses that focus light to create an image of a scene. However, instead of
focusing this light onto a piece of film, it focuses it onto a semiconductor
device that records light electronically. A computer then breaks this
electronic information down into digital data. The key difference between a
digital camera and a film-based camera is that the digital camera does not have
a film; instead, it has a sensor that converts light into electrical charges.
The image sensor employed by most digital cameras is a charge-coupled device (CCD).
Some low-end cameras use complementary metal oxide semiconductor (CMOS) technology.
The CCD is a collection of tiny light sensitive diodes, which convert photons
(light) into electrons (electrical charges).
MICR: - MICR is an acronym for Magnetic-Ink Character Recognition. It
refers to the special magnetic encoding, printed on the bottom of a negotiable
check. This information is machine readable via bank reader, which read the
visual patterns and magnetic waveforms of the MICR encoding. The characters are
printed using special ink, which contains iron particles that can be magnetized.
Magnetic ink character readers are used generally in banks to process the
cheques. In case of bank cheques, the numbers written at the bottom are
recorded in MICR (using special magnetic ink), representing unique cheque
numbers, bank and branch code etc. A MICR reads these characters by examining
their shapes in a matrix form and the information is then passed on to the
computer.
The banking industry prefers
MICR because as compared to the OCR, it gives extra security against forgeries
such as colour copies of payroll cheques or hand-altered characters on a
cheque. If a document has been forged, say a counterfeit(fake) check produced
using a colour photocopying machine, the magnetic ink line will either not
respond to magnetic fields, or will produce an incorrect code when scanned
using a device designed to recover the information in the magnetic characters.
The reading speed of the MICR is also higher. This method is very efficient and
time saving for data processing.
OCR: - Optical Character Recognition is a process of scanning printed pages as
images on a flatbed scanner and then using OCR software to recognize the
letters as ASCII text. The OCR software has tools for both acquiring the image
from a scanner and recognizing the text. In the OCR system, a book or a
magazine article is fed directly into an electronic computer file and then this
file is edited by using a word processor. Advanced OCR systems can read text in
a large variety of fonts, but they still have difficulty with handwritten text.
OCR works best with originals or very clear copies and
mono-spaced fonts like courier. For a good OCR, one should use 12 point or
greater font size. The text should be laid out in single column and it should
be printed/written in black on a white background. OCR has been used to enter
data automatically into a computer for dissemination (fNrjkuk]
fc[ksjuk ) and processing. The earliest of system was dedicated to high volume
variable data entry. The first major use of OCR was in processing petroleum
credit card sales drafts. Over time, other application evolved including cash
register tape readers, page scanners etc. using an OCR system, one can
consolidate data entry and reduce data entry errors. The result of OCR is very
human readable and can be used with many printing techniques. However, it is a
very expensive input device and if the document is not typed properly, it will
become difficult for the OCR to recognize the characters. Furthermore, except
for tab stops and paragraph marks, most document formatting is lost during text
scanning. The output from a finished text scan will be a single column editable
text file. This text file will always require spell checking and proof reading
as well as reformatting to desired final layout.
Working
of an OCR: - All the OCR systems include an optical scanner for reading text and
sophisticated software for converting the text into machine-readable form.
During the OCR processing the text is analyzed for light and dark areas in
order to identify each alphabetic letter or numeric digit. When a character is
recognized, it is converted into an ASCII code. There are two basic methods
used for OCR: matrix matching and feature extraction. The matrix matching
technique compares what the OCR scanner sees as a character with a library of
character matrices or templates. When an image matches one of these prescribed
matrices of dots within a given level of similarity, the computer labels that
images as the corresponding ASCII character. Feature extraction OCR does not
require strict matching to prescribed templates. This method varies depending
on how much ‘computer intelligence’ is applied by the manufacturer. The
computer looks for general features such as open areas, closed shapes, diagonal
lines, line intersections, etc. This method is much more versatile than matrix
matching. Matrix matching works best
when the OCR encounters a limited repertoire of type styles; with little or no
variation within each style where the characters are less predictable, feature
extraction is superior. At the end of the OCR processing, the final information
can 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 white space, as well as recognize a limited number of different fonts.
However, using OCR, the computer can not interpret the special characters and
images. In addition, the storage capacity required for storing the document as
an image is much more than required for storing the document as a text.
OMR:- Optical Mark
Recognition(OMR) is the process of detecting the presence of intended marked
responses. A mark registers significantly less light than the surrounding
paper. Optical mark reading is done by a special device known as optical mark
reader. In order to be detected by the OMR reader, a mark has to be positioned
correctly on the paper and should be significantly darker than the surrounding
paper. The OMR technology enables a high speed reading of large quantities of
data and transferring this data to computer without using a keyboard. The OMR
reader scans the form, detects the presence of marks, and passes this
information to the computer for processing by application software. Generally this technology is used to read
answer sheets (objective type tests). In this method, special printed
forms/documents are printed with boxes, which can be marked with dark pencil or
ink. These forms are then passed under a light source and the presence of dark
ink is transformed into electric pulses, which are transmitted to the computer.
OMR is also used for standardized testing as well as
course enrolment and attendance in education. Human resource department across
industries use OMR for applications such as benefit enrolment, employee testing,
payroll deductions, and user training. Healthcare providers use the technology
for registration and surveys, and medical labs for patient evaluations and
tracking supply orders and lab services. OMR is also used for time and attendance,
labour tracking, inventory management, voting applications, exit surveys,
polling and all manner of questionnaires and evaluation studies. Since it is
easy to use and is cost effective for opinion tracking, the technology has
become a tool for non-location and direct-mail marketing. OMR has a better
recognition rate then OCR because fewer mistakes are made by machines to read
marks than in reading handwritten characters. Large volumes of data can be
collected quickly and easily without the need for specially trained staff. Usually,
an OMR reader can maintain a throughput of 1500 to 10000 forms per hour.
However, the designing of documents for OMR is complicated and the OMR reader
needs to be reprogrammed for each new document design. OMR readers are
relatively slow because the person putting marks on the documents must follow the
instructions precisely. Any folding or dirt on a form may prevent the form from
being read correctly. In addition, it requires accurate alignment of printing
on forms and need a paper of good quality.
Working if a OMR:-
Essentially a method of mark reading, it can be performed in two different
ways:
- The first method is based on the conductivity
of graphic in order to determine the presence of pencil mark. The marks
must be made only in pencil because the number of magnetic particles in the
lead pencils is large.
- The second method is based on the reflection of
light. In this, a thin beam of light is directed on the surface of the
paper. When lesser amount of light is transmitted through the dot, the filled
box can be recognized. OMR can evaluate only those documents, which are
printed with the marked positions in the specified areas.
OMR is traditionally
performed using reflective light method where a beam of light is reflected on a
sheet with marks, to capture the reflection (presence of marks) or absence of
reflection (absence of marks). The OMR data entry system converts the
information about the presence or absence of marks into a computer data file. A
simple pen or pencil mark is made on the form to indicate each selected response
such as answers to survey questions. The completed forms are scanned by an
optical mark reader, which detects the presence of a mark by measuring the
reflected light. The OMR reader then interprets the pattern of marks into a
data record and sends this to your computer for storage, analysis and
reporting.
BCR:- Bar code is a
machine-readable code in the form of a pattern of parallel vertical lines of
varying widths. They are commonly used for labeling goods that are available in
super markets, numberings books in libraries etc. These codes/stripes are
sensed and read by a photoelectric device( bar code reader) that reads the code
by means of reflective light. The information recorded in bar code reader is
then fed into the computer, which recognizes the information from the thickness
and spacing of bars. BCR are either hand-held or fixed-mount. Hand-held
scanners are used to read bar codes on stationary items. With fixed-mount
scanners, items having a bar code are passed by the scanner- by hand as in
retail scanning applications or by conveyor belt in many industrial
applications. BCR provide enormous benefits for just about every business. With
a BCR, capturing data is faster and more accurate. A bar code scanner can record
data five to seven times faster than a skilled typist can record. A BCR entry
has an error rate of about 1 in 3 million. Bar coding also reduces cost in
terms of labour and reduced revenue losses resulting from data collection
errors. Bar code readers are widely used in supermarkets, department stores, libraries
etc. You must have seen bar code on the back cover of certain books and
greetings cards. Retail and grocery stores use a bar code reader to determine
the item being sold and to retrieve the item prices from a computer system.
Working of a BCR:- Bar code
scanners are electro-optical systems that include a means of illuminating the
symbol and measuring reflected light. The light waveform data is converted from
analog to digital, in order to be processed by a decoder (which is either built
into the scanner or a separate plug-in device), and then transmitted to the
computer-based application software. The process begins when a device directs a
light beam through a bar code. The device contains a small sensory reading
element. This sensor detects the light being reflected back from the bar code,
and converts light energy into electrical energy. The result is an electrical
signal that can be converted into an alphanumeric data. The pen in the bar code
unit reads the information stored in the bar code and converts it into a series
of ASCII characters by which operating system gets the information stored in
the bar code.
Voice Recognition:-Speech recognition
is one of the most interactive systems to communicate with the computer. The
user can simply instruct the computer with the help of a microphone ( along
with speech recognition software) what task to perform. It is the technology by
which sounds, words, or phrases spoken by humans are converted into digital
signals, and these signals are transformed into computer generated text or
commands. Most speech recognition systems are speaker-dependent so they must be
separately trained for each individual user. The speech recognition system
learns the voice of the user, who speaks isolated words repeatedly. Then, these
voiced words are recognizable in the future.
Speech recognition is gaining popularity in the corporate
world among non-typists, people with disabilities, and business travelers who
tape-record information for later transcription. Most frequently used for
dictation, screen navigation and web browsing, computer-based
speech-recognition systems have relatively high accuracy rates. It allows the
user to communicate with the computer directly without using a keyboard or a
mouse. It is an ideal system for people who are visually impaired and those
whose hands are disabled. Speech recognition systems can be used to create text
documents such as letters or E-mail, to browse the Internet, and to navigate
among application by voice commands. However, as compared to other input
devices, the reliability of speech recognizer is less. Sometimes it is unable
to differentiate between two similar sounding words such as see and sea. It is
also not suitable for noisy places.
For the last two decades, computer scientists have been
working on speech recognition systems. The major difficulty in developing these
systems is that the people communicate with each other using languages with
different accents and intonations. For this reason, most successful speech
recognition systems require a period of training to be accustomed to an
individual’s accent and intonation. Although this technology is still in its
development stage, it may someday eliminate the need for keying the data into
the computer’s memory completely.
Light Pen:- A light pen is
a hand held electro-optical pointing device which when touched to or aimed
closely at a connected computer monitor, will allow the computer to determine
where on that screen the pen is aimed. It facilitates drawing images and
selects objects on the display screen by directly pointing to the objects with
the pen. It is a pen like device, which is connected to the machine by a cable.
Although named light pen, it actually does not emit light but its light
sensitive-diode would sense the light coming from the screen. The light coming
from the screen causes the photocell to respond by generating a pulse. This
electric response is transmitted to the processor that identifies the position to
which the light pen is pointing. With the movement of light pen over the
screen, the lines or images are drawn. Light
pens give the user the full range of mouse capabilities, without the use of a
pad or any horizontal surface. Using light pens, users can interact more easily
with applications, in such modes as drag and drop, or highlighting. It is used
directly on the monitor screen and it does not require any special hand/eye
coordinating skills. Pushing the light pen tip against the screen activities a
switch, which allow the user to make menu selections, draw, paint, and perform
other input functions. Light pens are perfect for applications where desk space
is limited, in harsh workplace environments, and any situation where fast
accurate input is desired. It is very useful to identify a specific location on
the screen. However, it does not provide any information when held over a blank
part of the screen. Light pen is economically priced compared to a touch
screen, and requires little or no maintenance.
Working of a light pen:- The
light pen, in conjunction with its interface and software, makes a time
measurement that is translated into x-y coordinates representative of a
position on the monitor. The light pen contains a lens and a photo detector located
in its tip. When the electron beam that sweeps the monitor strikes the phosphor
within the light pen’s field of view, the light emitted by the phosphor is
focused through the lens and onto the photo detector. Due to this, the signal
current is increased and is transmitted to the computer. The position of the
beam is tracked by the horizontal and vertical counters, which relay this
information to a register. This cycle is repeated for every frame produced by
the electron beam. By noting when a scan
goes by and measuring the interval between scan lines or entire screen refreshes,
an accurate position of the photo detector on the screen is determined. The
light pen software generates x-y vectors corresponding to a point on the
screen, which may be used to make a selection by activating a switch on the
light pen.
Touch Screen:- A touch screen
is a special kind of display screen device, which is placed on the computer
monitor in order to allow the direct selection or activation of the computer
when somebody touches the screen. Touch screen is normally used when
information has to be accessed with minimum effort. However, the touch screen
is not suitable for input of large amounts of data. Typically, they are used in
information-providing systems like the hospitals, airlines and railway
reservation counters amusement parks etc.
Working of a touch screen:-
A basic touch screen has three main components: a touch sensor, a controller,
and a software driver. The touch sensor/panel is a clear glass panel with a
touch responsive surface. It is placed over a display screen so that the
responsive area of the panel with a touch responsive surface. It is placed over
a display screen so that the responsive area of the panel covers the viewable
area of the video screen. There are several different touch sensor technologies
in the market today, each using a different method to detect touch input. The
two most famous touch screen techniques are infrared beams and ultrasonic
acoustic waves. The infrared beams interlace the surface of the screen, and
when a light beam is broken, that particular location is recorded. On the
contrary, the ultrasonic acoustic waves pass over the surface of the screen and
when the wave signals are interrupted by some contact with the screen, the location
is recorded. The panel generally has an electrical current going through it and
touching the screen causes a voltage change, which is used to determine the
location of the touch to the screen. The controller, which is a small card,
connects the touch sensor and the computer. It takes information from the touch
sensor and the computer. It takes information from the touch sensor and
translates it into information that computer can understand. The driver is a
software update for the computer system that allows the touch screen and
computer to work together. It tells the operating system how to interpret the
touch event information that is sent from the controller.
Output Devices:-
Monitor – Monitors are
the most popular output devices used today for producing soft-copy output. A
monitor is usually associated with a keyboard and together they form a video
display terminal (VDT). A VDT is the most popular input/output device used with
today’s computers. The name terminal comes from the fact that a terminal is at
the terminal or end, point of a communication path. The term monitor usually
refers to the entire box, whereas display screen can mean just the screen. In
addition, the term monitor often implies graphics capabilities.
There are many ways to classify monitors. The Most basic
is in terms of color capabilities, which separates monitors into three classes:
Monochrome – Monochrome
monitors actually display two colors, one for the background and one for the
foreground. The colors can be black and white, green and black or amber and
black.
Gray-scale – A gray-scale
monitor is a special type of monochrome monitor capable of displaying different
shades of gray. The use of many shades of gray to represent an image is called
gray scaling. Continuous-tone images, such as black and white photographs, use
an almost unlimited number of shades of gray. Conventional computer hardware
and software can only represent a limited number of shades of gray (typically
16 or 256). Gray scaling is the process of converting a continuous –tone image
to an image that a computer can manipulate.
Color – Color monitors
can display anywhere from 16 to over 1 million different colors. Color monitors
are sometimes called RGB monitors because they accept separate signals – red,
green and blue.
After this classification, the most important aspect of a
monitor is its screen size. Like televisions, screen sizes are measured in
diagonal inches, the distance from one corner to the opposite corner
diagonally. A typical size for small VGA monitors is 14 inches. Monitors that
are 16 or more inches diagonally are often called full-page monitors. The
screen size is sometimes misleading because there is always an area around the
edge of the screen that can’t be used. Therefore, monitor manufacturer must now
also state the viewable area – that is the area of screen that is actually
used.
Another way of classifying monitors is in terms of the
type of signal they accept: analog or digital. Nearly all-modern monitors
accept analog signals, which is required by the VGA, SVGA and other high-resolution
color standards.
Clarity of picture on a
screen – Screen clarity depends on three qualities: resolution, dot pitch and
refresh rate.
Resolution – It refers to
the sharpness and clarity of an image. The term is most often used to describe
monitors, printers and bit-mapped graphic images. For graphics monitors, the
screen resolution signifies the number of dots (pixels) on the entire screen.
For example, a 640-by-480-pixel screen is capable of displaying 640 distinct
dots on each of 480 lines, or about 300,000 pixels. This translates into
different dpi measurements depending on the size of the screen. For example, a
15-inch VGA monitor (640*480) displays about 50 dots per inch.
Dot Pitch – Dot pitch is
the amount of space between pixels, the closer the dots, and the crisper the
image. This is a measurement that indicates the diagonal distance between
like-colored phosphor dots on a display screen. Measured in millimeters, the
dot pitch is one of the principal characteristics that determine the quality of
display monitors. The dot pitch of color monitors for personal computers ranges
from about 0.15 mm to 0.30 mm.
Refresh Rate – Refresh rate
is the number of times per second that the pixels are recharged so that their
glow remains bright. The refresh rate for a monitor is measured in hertz (Hz)
and is also called the vertical frequency; vertical scans rate, frame rate or
vertical refresh rate. The old standard for monitor refresh rates was 60 Hz,
but a new standard developed by VESA sets the refresh rate at 75 Hz for monitors
displaying resolutions of 640*480 or greater. This means that the monitor
redraws the display 75 times per second.
Interlacing – A display
technique that enables a monitor to provide more resolution inexpensively. With
interlacing monitors, the electron guns draw only half the horizontal lines
with each pass (for example, all odd lines on one pass and all even lines on
the next pass). Because an interlacing monitor refreshes only half the lines at
one time, it can display twice as many lines per refresh cycle. Another way of looking at it is that
interlacing provides the same resolution as non-interlacing but less
expensively. A shortcoming of interlacing is that the reaction time is slower,
so programs that depend on quick refresh rate (for example, animation and
video) may experience flickering or streaking.
Video Display Adapters – To display
graphics, a display screen must have a video display adapter. A video display
adapter also called a graphics adapter card is a circuit board that determines
the resolution, number of colors, and how fast images appear on the display
screen. Video display adapters come with their own memory chips, which
determine how fast the card processes images and how many colors it can
display. The video display adapter is often built into the motherboard,
although it may also be an expansion card that plugs into an expansion slot.
VGA – VGA stands for
Video Graphics Adapter, a graphics display systems for PCs developed by IBM.
VGA has become one of the de facto standards for PCs. In text mode, VGA systems
provide resolutions of 720 by 400 pixels. In graphics mode, the resolution is
either 640 by 480 (with 16 colors) or 320 by 200 (with 256 colors).
Since its introduction in 1987, several other
standards have been developed that offer greater resolution and more colors
(SVGA, XGA) but VGA remains the lowest common denominator. All PCs made today
support VGA and possibly some other more advanced standard.
SVGA – Short for super
VGA, a set of graphics standards designed to offer greater resolution than VGA.
There are several varieties of SVGA, each providing a different resolution:
800 by 600 pixels
1024 by 768 pixels
1280 by 1024 pixels
1600 by 1200 pixels
All SVGA standards support a
palette of 16 million colors, but the number of colors that can be displayed
simultaneously is limited by the amount of video memory installed in a system.
One SVGA system might display only 256 simultaneous colors while another display
the entire palette of 16 million colors.
XGA: Short for
extended graphics array, a high-resolution graphics standard introduced by IBM
in 1990. XGA was designed to replace the older 8514/A video standard. It
provides the same resolutions (640 by 480 or 1024 by 768 pixels), but supports
more simultaneous colors. In addition,
XGA allows monitors to be non-interlaced.
For any of these displays to
work video display adapters and monitors must be compatible. Your computers
software and the video display adapter must also be compatible.
Printers:- Printers
can be divided into two distinct categories on the basis of producing
impression over the paper:
1 Impact Printers
2 Non-impact Printers
Impact
Printers:- In impact printer, a character is printed on
the paper through physical contact between the print head and paper. Either the
needle or a character is stuck on the paper through the ribbon. This creates a
lot of noise when these printers work. Impact printers may also be categorized
into two types on the basis of produced (impression) pattern.
1 Solid Font:- In a solid font printer, a complete character
strikes a carbon ribbon or other inked surface against paper to produce an
image of the character.
2 Dot Matrix:- Dot matrix printer has a set of printing needles
or pins. Selected print needles strike the inked ribbon against paper to
produce an image of the character.
Impact
Printers can further be categorized into two
categories:
Character Printer :Character
printer prints character by character. It may work on both technologies: Dot
Matrix as well as Solid Font.
Line Printer :Line
printer prints one complete line at a time. It works on both the technologies :
Dot Matrix and Solid Font. Dot matrix type line printers are relatively slower
than solid font impact line printers. Speed may be 300 lines per minute or
more.
Dot
Matrix Printer:- DMP is the oldest printing technology and it
prints one character at a time. Usually, DMP can print any shape of character,
which a user can specify. In a dot matrix printer, the character is formed with
closely packed dots. The printing head contains a vertical array of pins.
Formation of character is done by the movement of head across the paper.
Selected print needles strike the inked ribbon against paper to produce an image
of the character. Dot matrix printer supports printing of graphics. The speed
of dot matrix printers is measured in characters per second(cps). It is faster
than daisy wheel printer and the printing speed lies between 30 to 600 cps. The
print quality is determined by the number of pins. It comes in two print head
specifications, 9 pin and 24 pin. The more pins per inch, the higher the print
resolution. Most DMP printers have a resolution ranging from 72-360 dpi. DMPs
are inexpensive and have low operating costs. The major limitation of DMP is
that it prints only in black and white. They can handle applications such as
accounting, personnel, and payroll very well.
Examples are EPSON EX, 1000, EPSON LQ 1050, CITIZEN MSP 55, GODREJ, etc.
Working
of A DMP:- The
technology behind dot matrix printing is quite simple. The paper is pressed
against a drum and is intermittently(from time to time) pulled forward as printing progresses. The
printer consists of an electro-magnetically driven print head, which is made up
of numerous print wires(pins). The characters are formed by moving the
electro-magnetically driven print head across the paper, which strikes the
printer ribbon situated between the paper and print head pin. As the head
stamps onto the peper through the inked ribbon, a character is produced that is
made up of these dots. These dots seem to be very small for the normal vision
and appear like solid human-readable characters.
Daisy
Wheel Printer :- It is a solid font type character printer.
Daisy wheel printer is named as such because the print head resembles a daisy
flower, with the printing arms appearing like the petals of the flower. Speed
lies between 30 cps to 90 cps. Print quality is better than dot matrix. It is a
bi-directional printer, i.e. the head of the printer prints while moving in
forward direction as well as in backward direction. It also supports graphics
such as curves which can also be produced.
Daisy wheel printer is a letter quality
printer because it produces solid characters unlike broken characters formed by
a dot matrix printer. The font (i.e. style of character) is of fixed type for a
Daisy Wheel printer.
Working
of a Daisy Wheel Printer:- These
printer have print heads composed of metallic or plastic wheels. A raised
character is placed on the tip of each of the daisy wheels petals. Each petal
has an apperance of a letter, number of punctuation mark on it. to rpint the
print wheel is rotated around until the desired character is under the print
hammer. The petal is then struck from behind by the print hammer, which strikes
the character, pushing it against the ink ribbon, and onto the paper, creating
the character.
Line
Printers:- Line printers are high-speed printers capable of printing an entire line
at one time. A fast line printer can print as many as 3000 lines per minute.
The disadvantages of line printers are that they can print only one font, they
can not print graphics, the print quality is low, and they are very noisy.
Non-impact Printers:-
In Non-impact
printers, the head does not come directly in contact with the paper. There is
no impact or hitting of needles so non-impact printers don't make any noise
while printing. They come in many of varieties:
1 Thermal printer
2 Laser printer
3 Ink Jet printer
4 Electrostatic printer
5 Electro graphic printer
Thermal
Printer:- In a thermal printer the characters are
formed by pressing an array of electrically heated needles against heat
sensitive paper. Such papers have a special heat sensitive coating which
becomes dark when a spot is heated. Character is printed with a matrix of dots
which are heated by the needles.
It is not possible to produce multiple copies simultaneously with this
type of printer. A special type of paper is used with this printer which is
costly. This has reduced the popularity of thermal printers.
Laser
Printer:- A laser printer provides the highest quality
text and images for personal computers today. Is is a very fast printer, which
operates on the same principle as that of a photocopy machine. Most laser
printers can print text and graphics with a very high quality resolution. Laser
printer works on the concept of using laser beams to create an image on a
photosensitive surface. Initially the desired output image is written on a
copier drum with a laser beam that operates under the control of the computer.
The laser exposed drum areas attract a toner that attaches itself to the
laser-generated charges on the drum. The toner is permanently fused on paper
with heat and/or pressure by rolling the drum over the blank paper. Laser
printers are quiet and produce very high quality of output. They are capable of
printing 4-30 pages per minute. Laser printers are often faster than ink-jet
printers, but are more expensive to buy and maintain than the other printers.
The cost of these printers depends on a combination of costs of paper, toner
replacement, and drum replacement. These printers are useful for volume
printing because of their speed.
Working
of a laser printer:- 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
onto the paper. 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. The paper is then brought out of the printer.
Ink Jet Printers:- The most common
type of printer found in homes today is the ink-jet printer. Ink Jet printer’s
uses dot matrix approach to print text and graphics. Nozzles in the print head
produce tiny ink droplets. These droplets are charged which are deflected and
then directed to the desired spots on the paper to form the impression of a
character. It has a speed of 40-300 cps (character per second) with software
controls on size and style of characters. These printers support color printing
and are very quiet and noiseless in operation. The print quality of such
printers is very near letter-quality. 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. They are also affordable, which
appeals to small businesses and home offices. These printers can print about 6
pages a minute.
Working of an Ink-Jet
Printer:
- 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 printer head
moves back and forth across the page, software gives instruction 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 piezo-electric technology.
Bubble-jet printers use heat to fire ink onto the paper.
There are three main stages with this method. The squirt is initiated by
heating the ink to create a bubble until the pressure forces it to burst and
hit the paper. The bubble then collapse as the element cools, and the resulting
vacuum draws ink from the reservoir to replace the ink that was ejected.
Piezo-electric technology uses a piezo crystal at the
back of the ink reservoir. It flexes when an electric current flows through it.
Therefore, whenever a dot is required, a current is applied to the piezo
element, the element contracts and in doing so forces a drop of ink out of the
nozzle.
Electrostatic Printers: - An electrostatic
printer moves a continuous sheet of paper over the printing pins which put
small electric charges on the paper. The paper is then passed through a bath of
oppositely charged toner particles. As the opposite charges attract, the paper
picks up the toner on the spots charged by the print pins. The paper is then
passed through the fusing process and the toner is melted onto the paper to
form the character impression. Some electrostatic printers print up to 5000
lines per minute. Such printers use dot-matrix approach for printing. The print
head contains a vertical array (i.e. a vertical column) of pins. Such printers
can also produce graphics.
Comparative
View of Printers:
Printer
Type Advantages Disadvantages
Dot
Matrix Inexpensive, fast,
prints graphics Poor quality
printing
Daisy
Wheel High quality printing Slow, noisy, expensive
Thermal Light weight, battery
powered Slow, poor-quality
printing, requires
special
paper.
Plotter Prints colour and
graphics Expensive
Laser Excellent print
quality, prints graphics Expensive
Plotter:- Plotters
are output devices that are used to produce precise and good quality graphics
and drawings under computer control. They use ink pen or ink jet to draw graphics
or drawings. Either single colour or multicolor pens can be employed. The pens
are driven by a motor. The graphics and drawings produced by plotters are
uniform and precise and of very high quality. Plotters are used for complex
engineering drawings and for drawing of maps that require high degree of
accuracy. Flatbed plotters use horizontal flat surface on which paper can be
fixed. The pen moves in X and Y directions which is controlled by the computer.
They are mainly used for Computer Aided Design (CAD) and Computer Aided Manufacturing
(CAM) application such as printing out plans for houses or car parts. These are
also used with programs like AUTOCAD to give graphic outputs.
There are two different types of
plotters, one where the paper moves (drum), and the other where the paper is
stationary (flatbed plotter).
- 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 instruction sent by the computer. In case, a
horizontal line is to be drawn, the horizontal movement of a pen is
combined with the vertical movement of a page via the drum. Moreover,
plotters can draw curves by creating a sequence of very short straight
lines. In these plotters, each pen can have ink of different colour to produce
multicolor designs. Drum plotters are used to produce continuous output,
such as plotting earthquake activity, or for long graphic output, such as
tall building structures.
- 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 lines.
In flatbed plotters, the paper does not move, the pen holding mechanism
provides all the motion. Theses 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, aircrafts, buildings etc. The major disadvantage
of this plotter is that it is a slow output device and can take hours to
complete a complex drawing.
Sound
Card and Speakers:- An expansion board that enables a computer to
manipulate and output sounds. Sound cards are necessary for nearly all CD-ROMs
and have become common place on modern personal computers. Sound cards enable
the computer to output sound through speakers connected to the board, to record
sound input from a microphone connected to the computer, and manipulate sound
stored on a disk.
Nearly all sound cards
- FM (Frequency Modulation) Synthesis mimics
different musical instruments according to built-in formulas.
- Wavetable Synthesis relies on recordings of actual
instruments to produce sound. Wavetable synthesis produces more accurate
sound, but is also more expensive.
Memory Devices: Storage hardware
provides the capability to store data and program instructions, either
temporarily or permanently for quick retrieval and use during computer
processing. You also know that the term media means the materials on which data
can be recorded (magnetic media are the most popular). But to fully appreciate
the storage capabilities available in computer systems, you must understand a
number of storage fundamentals.
The term primary storage refers to the main memory of a
computer, where both data and instructions are held for immediate access and
use by the computer’s central processing unit during processing. Although the
technology is changing, most primary storage today is considered a volatile
form of storage, meaning that the data and instructions are lost when the
computer is turned off. Secondary storage (or auxiliary storage) is any storage
device designed to retain data and instructions (programs) in a more permanent
form. Secondary storage is nonvolatile, meaning that the data and instructions
remain intact when the computer is turned off.
The easiest way to differentiate between primary and
secondary storage is to consider the reason data is placed in them. Data is
placed in primary storage only when it is needed for immediate processing. Data
in secondary storage remains there until overwritten with new data or deleted
and is accessed when needed. In very general terms, a secondary storage device
(file) and place it on the top of our desk (primary storage or main memory),
where we work on it – perhaps writing a few things in it or throwing away a few
papers. When we are finished with the file, we take it off the desktop (out of
primary storage) and return it to the cabinet (secondary storage).
RAM: Random access memory is a
volatile memory. It means the information stored in it remains as long as the
power is switched ON. As soon as the power is switched OFF, the information
contained in it vanishes. RAM can be defined as a block of sequential memory
locations, each of which has a unique address determining the location and
those location contain a data element.
Physically, this
memory consists of some integrated circuit chips either on the motherboard or
on a small circuit board attached to the motherboard. A computer’s motherboard
is designed so that its memory capacity can be easily enhanced by adding more
memory chips. The additional RAM chips, which plug into special sockets on the
motherboard are also known as single in-line memory modules (SIMMs).
There are two types of RAM.
Dynamic Random
Access Memory (DRAM):- This type of RAM holds the data in dynamic manner with the help of a refresh circuitry.
Each second or even less than that the content of each memory cell is read and
the reading action refreshes the contents of the memory. Due to this refreshing
action, the memory is called dynamic RAM. DRAMs are made from transistor and
capacitor. The capacitor holds the electrical charge if the bit contains 1 and
no charge if the bit is 0. the transistors reads the contents of the capacitor.
The charge is held for a short period and than it fades away, that is, when
refresh circuitry comes in.
Static Random Access
Memory(SRAM):- SRAM along with DRAM is essential for a system to run optimally,
because it is very fast as compared to DRAM. It is effective because most
programs access the same data repeatedly and keeping all this information in
the fast SRAM allows the computer to avoid accessing the slower DRAM. Data is first
written to SRAM assuming that it will be used again soon. SRAM is generally
included in a computer system by the name of cache. A static RAM is faster,
costlier, and consumes more power than dynamic RAM. Due to these reason, large
memories use dynamic RAM, and static RAM
is used mainly for specialized applications. The main memory of most computers
uses dynamic RAM.
ROM: A special type of RAM, called
read only memory(ROM) is a non-volatile memory chip in which data is stored
permanently and can not be altered by the programmer. As the name suggests,
read only memory can only be read, not written. The contents of ROM are not
lost even in case of sudden power failure, making it non-volatile in a
nature. Read only memory is also random
access in nature, which means the CPU can randomly access any location within
ROM.
In fact, storing data permanently
into this kind of memory is called “burning in the data”, because data in such
memory is stored by fuse-links. Once a fuse-link is burnt it is permanent. ROMs
are mainly used to store programs and data which do not change and are
frequently used. For example, the most basic computer operations are carried out by wired electronic circuits. However,
there are several higher level operations that are very frequently used but
will require very complicated electronics circuits for their implementations.
Hence, instead of building electronics for these operations, special programs
are written to perform these operations. These programs are called microprograms
because they deal with low-level machine functions and are essentially
substitutes for additional hardware.
Microprograms are written to aid the control unit in directing all the
operations of the computer systems. ROMs are mainly used by computer
manufacturers for storing these microprograms so that they can not be modified
by the users. A good example of a microprogram that is stored in a ROM chip of a
computer is the set of instructions that is needed to make the computer system
ready for use when its power is switched on. This microprograms called system
boot programs, contains a set of start up instructions to check if the system
hardware like memory, I/O devices etc are functioning properly and looks for an
operating system and loads its core part in the volatile RAM of the system to
produce the initial display screen prompt. Note that this microprogram is used
every time the computer is switched on and needs to be retained when the computer
is switched off.
There are various
types of ROM which are given below:
- Programmable Read Only Memory (PROM)- PROM is a
ROM that can be programmed to record information using a facility known as
PROM programmer. Once the chip has been programmed, the recorded
information can not be changed i. e. PROM becomes same as ROM.
- Erasable Programmable Read Only Memory- EPROM
is another type of ROM that can not erased and the chip can be programmed
to record different information using a special PROM program facility.
Erasure to achieved by exposing the chip to ultraviolet light. When an
EPROM is in use, information can only be read and the information remains
on the chip until it is erased. EPROM are of two kinds-UVPROM(Ultraviolet
PROM) and EAPROM(Electrically Alterable PROM).
- Electrically Erasable PROM (EEPROM)- This type
of ROM can be programmed and erased by electrical signals. It does not
require exposure to ultraviolet light to erase its contents as EPROM
memory does and provides an easy means to load and store temporary or
permanent information in a form of ROM memory. Information loaded in this
memory can be retained for many years without any power supplied, one of
its new applications is as a backup to RAM memory whose contents are lost in a power failure. When power
is returned, the EEPROM memory can be used to replace the lost contents of
the RAM memory and the microcomputer can continue working just as if
nothing had happened. Even newer devices are combining RAM and EEPROM
memory in a single integrated circuit.
Due to the ease with which stored programs can be altered, EEPROM
is also known as flash memory. Flash memory is used in many new I/O and
storage devices. The intelligence of these devices can be upgraded by
simply down-loading new software from a
vendor-supplied disk to
flash memory.
Winchester Drive: There is a third type of relatively new
disk storage unit known as Winchester disk. In this unit disks are permanently
housed in sealed, contamination-free containers. The disks are coated with a
special lubricant which reduces the friction when the read/write heads land on
the disk surface. Winchester disks are fast and highly reliable, yet low priced
compared with conventional had-disk devices. They are normally 5.25, 8 or 14
inches in diameter and storage capacities of 10, 20 and 40 megabytes are
typical. Winchester disks are used in all but the smallest computer systems.
They are extensively used to support minicomputers and their smaller versions
are now competing with floppies to make their place in microcomputer systems. It
is another term for hard disk drive. The term
Mass Storage Devices: Mass storage systems are storage systems, which provide access to
hundreds of billions of bytes of stored data. They combine the advantage of
both tape and disk technologies. The storage medium is essentially a length of
flexible plastic material upon which short stripes of magnetic tape are
mounted. These strips are then placed in cartridges, and the cartridges are
loaded into a storage device that is online to the CPU. The same read/write technique
as used with magnetic tape is used here to read and write data. The access
times of mass storage systems are measured in seconds instead of milliseconds
because a transport mechanism must move to retrieve the cartridge upon which
the desired data is stored. It requires several seconds to locate the cartridge
specified and then several more seconds are needed to transfer the data to a
magnetic disk and then to the CPU. Thus an access time of 10 seconds is common for
these storage systems. But a mass storage device has huge storage capacity and a
very small cost per nit stored. Around 500000 books of this size can be stored
in the IBM 3850. Relatively slow access times limit the use of mass storage
system in many applications. However, mass storage systems are cost-effective
alternative to on-line magnetic tape or disk storage in applications that
require huge storage capacity and in which rapid access to data is not
essential. When used for off-line storage, mass storage systems are often referred
to as archival storage because of the very large volumes of historical or
backup data that they can store.
Magnetic Tape:- Magnetic tape
looks like the tape used in music cassettes. It is a plastic tape with magnetic
coating. Magnetic tapes are used as
external storage device to keep back-up copies of precious software and data.
It is a serial access storage device and provides sequential access only. Tape
is a plastic ribbon that is coated on one side with a magnetic material
(iron-oxide). Information is stored using binary code in the form of magnetized
or non-magnetized spots. An electro-magnetic head arranges these magnetic
particles to store data. These particles are interpreted when we read from the
tape and are then converted back to information. They come in the range of 12.5
mm to 25 mm width and 500 meters to 1200 meters length.
Recording Data on Magnetic Tapes:-
Magnetic tapes have their own coding system. Information is recorded on the
tape in the form of tiny invisible magnetized and non-magnetized spots
(representing 1's and 0's). Tape is divided into vertical columns called frames
and horizontal rows called tracks. Only one character is recorded per frame.
Data is recorded in two coding formats – BCD (Binary Coded Decimal) which is 7
bit format (6 bit BCD and 1 bit for parity checking), EBCDIC (Extended Binary
Coded Decimal Interchange Code) which is 9 bit format (8 bit EBCDIC code and 1
bit for parity checking). A parity or check bit is used to detect errors that
may occur due to loss of a bit from a string of 6 bit BCD or 8 bit EBCDIC format
during input or output operation.
Fixed and Variable Length Records:- The
data is normally stored on a tape in blocks. On some tapes, the block is of
fixed length. It may vary in length for others. In fixed length record block,
the data size cannot exceed a predetermined maximum numbers of characters. In
variable length record block, there is no such limitation on number of
characters and the record may contain any number of characters.
Blocking of Records: - A magnetic
tape alternates sections of data called blocks and regions of blank tapes (about
3/4 inch in length) called Inter Record Gap (irg).
The gap has to be provided to allow for stopping and starting of the tape. Data
transfer takes place only when the tape is moving at constant speed. Also it is
transferred blockwise in magnetic tapes. A block may contain one record or more
than one record. Number of records in one block is known as Blocking Factor of
that tape. The data between two IRG forms one physical record while it may
contain number of logical records within it.
Magnetic Tape Drive:-
Magnetic tape drive are the devices that can either read data from a tape into
the CPU or can write information being produced by the CPU onto the tape. Data
is read from the tape with the help of a read-write assembly. There is one
read-write head for each track.
Data Transfer Rate
(DTR):-
It is an important feature of secondary storage devices. DTR is the product of
tape density and tape speed. Data Transfer Rate (bytes/second) = Packing
Density (bytes/inch) x Tape Speed (inches/second). Typical data transfer rate
is 100000 bytes/second or above.
Tape Density:- The
density of tape means number of frames recorded in 1 inch of tape. Typical tape
densities are 556 BPI, 800 BPI, 1000 BPI, 3250 BPI or 6250 BPI. (Bytes per
Inch).
Tape Speed:- The
speed of tape is measured in inches per second. It normally lies between 50-200
inches/second.
Advantages of Magnetic Tapes
u Magnetic tapes provide
virtually unlimited storage. Number of tapes can be used as per requirement for
storing of data.
u A magnetic tape provides high data density. A typical tape can store
6250 characters per inch. So a tape of 28,800 inches can store 180 million
characters.
u Magnetic tapes are economical to use – their cost is very low.
u Rapid data transfer rate is about one million bytes per second.
u Magnetic tapes and cartridges are very easy to handle and use.
u Tape is a convenient way
of carrying large volumes of information from one place to another.
u It can be erased and
reused many times.
Limitations of Magnetic Tapes
One of the main limitations with magnetic
tapes is that they lack direct access to records and are susceptible to
environmental disturbances. Also the require human intervention for operation.
The data transmission in magnetic tapes is slow as compared to disks. There is
variability or tape drives, i.e. different types of tape drives are available
and there is no standardization, which makes it difficult to recover from parity
errors.
Magnetic
Disks:- Magnetic disks are the most widely used and popular storage
medium for direct access secondary storage. Magnetic disks are used as an
input, output or external storage device. They are a popular medium for Direct
Access Storage Devices (DASD). It is a thin, circular metal plate/platter
coated on both sides with a magnetic material. It usually comes in the form of
a disk pack, also known as hard disk. All the disks in the disk pack rotate at
a very high speed of 700 or 3600 rpm. They come in two varieties according to
the operations of the read-write mechanism – fixed head and moving head. Disk
packs also come in two configurations – fixed disk pack, removable disk pack.
Storage of
Information:-
Information is stored on both the surfaces of each
disk platter except the upper surface of the top platter and lower surface of the
bottom platter. Each disk platter is divided into concentric circles known as
tracks. A set of corresponding tracks in all the surfaces of a disk pack is called
a cylinder. Data is stored in one cylinder first and then the head moves to the
next cylinder, this saves time wasted in moving the head track by track for a
particular disk. Information is recorded as a series of magnetized (signifying
a 1-bit) or non-magnetized (signifying a 0-bit) spots. Each track contains
equal number of characters. Information can be erased from anywhere on the disk
and new data can be recorded on it. Normally there are 200 tracks on a disk
surface numbered 0 to 199. Each track is divided into 8 or 12 equal sectors.
Sectors are used to store information – about 512 bytes can be stored per
sector.
Addressing of Records:-
The heads are attached to access arms which are moved in and out
over the spinning disk. The heads can thus be quickly located over any track to
read or write data. These tracks begin at the outer edge and continue towards
the centre. Each track has a unique number (000-199).
Accessing of Data:-
Data is recorded on the tracks of the spinning disk surface and
read from this surface by one or more read/write heads. There are two basic
types of disk systems – the moving head system and the fixed head system.
Accessing of data is different for both.
Moving Head System
Moving head system consists of one read-write head for each disk
surface mounted on an access arm which can be moved in and out. Each read-write
head moves horizontally across the surface of the disk. Every surface of disk
pack has its own head and all heads move together. One cylinder is accessed
simultaneously by the set of read-write head. Then the head moves for the next
cylinder.
Fixed Head system:-
In a fixed head system the access arm is non-movable and a number
of heads are attached on this arm. These read-write heads are distributed over
the disk surface, one head for each track. As a result no head movement is
required and information is accessed more quickly. Because of the space
required for the additional read-write heads, fixed head disks have less
capacity and cost more per byte of data stored.
Access Time:-
Information is accessed from the disk by referencing the disk
address. Disk address is specified in terms of surface number, track number and
sector number. Access time in any disk system is made up of three components –
Seek Time, Latency Time and Data Transfer Time.
u Seek Time Time
required to position the head over the proper track is called the seek time,
normally measured in milliseconds. For fixed head system, seek time is always
zero.
u Latency Time Time
required to rotate the disk pack to bring the correct sector under the
read-write head is known as latency time. Average latency time is of the order
of 8-10 ms.
u Data Transfer Time Time
required to read or write the actual data on the disk is called data transfer
time.
Storage Capacity:-
The storage capacity of a magnetic disk largely depends upon the
number of disks in the disk pack and the number of tracks per inch and bits per
inch of track. Total number of bytes that can be stored in a disk pack =
(Number of cylinders × Tracks per cylinder × sectors per track × Bytes per
sector). Storage capacity of a disk pack is in the range of 200 to 10000
megabytes (Mbytes).
Advantages of Magnetic
Disks:-
Magnetic disks are DASD devices, i.e. time taken to locate a
particular record is independent of the position of that record. Disk storage
is more durable than that of tape storage. Magnetic disks support on-line
processing because of its direct accessing property. In a nut-shell the
advantages of magnetic disks are: easy accessibility, durability, reusability,
compactness and providing sequential as well as direct access.
Disadvantages:-
The disadvantage of magnetic disk packs is that it is costlier than
tape. Tapes are more economical to use. The disk packs are not easily portable,
i.e. there are difficulties in
removing a diskpack. Also they are less secure because they have direct access
and data can get overwritten by mistake any where on the disk or it can get
corrupted anywhere along the file. In the case of tapes, since all of it is not
exposed for accessing it is more secure and safe.
Types of Disks
All magnetic disks are platters coated with a magnetic material.
They come in different sizes. They are portable or permanently mounted on the
computer system. The complete unit to read and write on these disks is called
Disk Drive Unit. Disks can also be differentiated on the basis of their size,
sectors, number of sides, storage capacity and density.
Hard Disk:- hard disk drive (HDD),
commonly referred to as a hard drive, hard disk, or fixed disk
drive, is a non-volatile storage device
which stores digitally encoded data on rapidly rotating platters with magnetic
surfaces.
HDDs
(introduced in 1956 as data storage for an IBM accounting
computer) were originally developed for use with general purpose computers. In
the 21st century, applications for HDDs have expanded to include digital video recorders, digital audio players, personal digital assistants, digital
cameras and video game consoles.
It is a group of
large metal or plastic disks permanently sealed in a container. Read-write
heads and access mechanisms are also inbuilt within the container. These sealed
containers are not generally removed from their disk drives. In operation, the
disk is rotated at a high speed on the spindle. There is one read-write head
for each surface. The heads can move to and fro to select desired track
position. During operation, heads don't touch the magnetic surface; a thin
cushion of air is maintained between a rotating disk and read-write head. To
store or retrieve data, the system finds the disk address used to contain the
data by moving the read/write head to the appropriate track where it waits
until the desired sector passes by. The capacity of hard disks is very high
compared to other disks. Nowadays hard disks of capacity 20 GB and 80 GB are
available. Although hard disks are fixed in PC, porting of disks from one place
to another is more secure than before because all the disks are safely packed
inside a protective covering.
Floppy Diskette
Hard disks are costly and unfit to transfer data from one place to
another so floppy diskettes of low capacity are used. These diskettes are
economical and very flexible to use and are fixed inside the computer system.
The diskette can be frequently taken out and inserted in the disk drive unit While
in operation, floppy disk drive heads actually touch the surface of the
magnetic disk. This results in quick wear and tear of the disks and the
read-write heads. These diskettes are made up of plastic and have a magnetic
coating. The disk is covered in a protective sheath with opening for editing
and writing.
Nowadays two
sizes of disks are commonly available – 5.25 inches and 3.5 inches. The 3.5
inch diskette comes in a rigid packet for protection so they are not really
‘floppy’. These diskettes store more data due to certain advantages. They have
over 5.25 inch diskettes. The 3.5 inch diskette has actually replaced 5.25 inch
diskettes. Floppy diskettes hold only one disk at a time and so only one
diskette can work with the disk drive unit at a time.
The capacity of
floppy disks, mini disks, is very low compared to hard disks. A 5.25 inch disk
has 360 KB and 1.2 MB capacity while that of 3.5 inch disk can store about 1.44
MB of data. The 3.5 inch disk is also referred to as micro floppy disk. The
cover of these disks has a sliding opening for editing and writing of heads
which is automatically closed when not in use to protect the disk from dust and
moisture.
The index role on the recording media is used by the disk drive
unit to locate the starting of the first sector. In a 5.25 inch floppy disk
write protect opening is occurred by a write protect notch and writing to the
disk is permitted only when the notch is open and in 3.5 inch diskette a write
protect tab is used to make the diskettes read only by covering the notch and
opening the tab respectively. Another hole in a 3.5 inch floppy disk is a high
density detection hole. It distinguishes the 1.44 MB diskette from 720 KB low
density diskette, which is of the same size and shape.
Physical
Structure of Floppy & Hard Disk:- A floppy disk is basically a
circular sheet of plastic, coated with magnetic material. A hard disk is made
of a stack of circular metal platters, also coated with magnetic material.
Before a disk can be used it must be formatted. The surface of the disk is
divided up into a number of concentric tracks, each of which is
subdivided into sectors.
Floppy
disks have 80 tracks on each side and each track is split into 18 sectors. A
3.5" floppy disk with 80 tracks and 18 sectors will have 80 x 18 = 1,440
storage units, each uniquely identified by its track and sector position. Each
storage unit can hold 512 bytes of data, so the disk has a capacity of 1,440 x
512 = 737,280 bytes (720 KBytes) per side, or 1,400 KBytes (1.4 MBytes) per
disk.
A hard
disk is a sealed unit containing a stack of circular platters mounted on a
common spindle. Electromagnetic read/write heads are located above and
below each platter. The platters rotate at a constant speed, eg: 7200 rpm.
While they are spinning the heads can move in towards the centre or out towards
the edge. This allows them to reach any location on the platter.
Elementary knowledge of hard disk - Physical
structure of hard disk
Hard drive consists of platter, control circuit board and
interface parts.
A hard disk is a sealed unit containing a number of platters in
a stack. Hard disks may be mounted in a horizontal or a vertical position. In
this description, the hard drive is mounted horizontally.
Electromagnetic read/write heads are positioned above and below
each platter. As the platters spin, the drive heads move in toward the center
surface and out toward the edge. In this way, the drive heads can reach the
entire surface of each platter.
Making Tracks
On a hard disk, data is stored in thin, concentric bands. A
drive head, while in one position can read or write a circular ring, or band
called a track. There can be more than a thousand tracks on a 3.5-inch hard
disk. Sections within each track are called sectors. A sector is the smallest
physical storage unit on a disk, and is almost always 512 bytes (0.5 kB) in
size.
The figure below shows a hard disk with two platters.
The structure of older
hard drives (i.e. prior to Windows 95) will refer to a cylinder/ head/ sector
notation. A cylinder is formed while all drive heads are in the same position
on the disk. The tracks, stacked on top of each other form a cylinder. This
scheme is slowly being eliminated with modern hard drives. All new disks use a translation
factor to make their actual hardware layout appear continuous, as this is the
way that operating systems from Windows 95 onward like to work.
To the operating system
of a computer, tracks are logical rather than physical in structure, and are established
when the disk is low-level formatted. Tracks are numbered, starting at 0 (the
outermost edge of the disk), and going up to the highest numbered track,
typically 1023, (close to the center). Similarly, there are 1,024 cylinders
(numbered from 0 to 1023) on a hard disk.
The stack of platters
rotate at a constant speed. The drive head, while positioned close to the
center of the disk reads from a surface that is passing by more slowly than the
surface at the outer edges of the disk. To compensate for this physical
difference, tracks near the outside of the disk are less-densely populated with
data than the tracks near the center of the disk. The result of the different
data density is that the same amount of data can be read over the same period
of time, from any drive head position.
The disk space is
filled with data according to a standard plan. One side of one platter contains
space reserved for hardware track-positioning information and is not available
to the operating system. Thus, a disk assembly containing two platters has
three sides available for data. Track-positioning data is written to the disk
during assembly at the factory. The system disk controller reads this data to
place the drive heads in the correct sector position.
Sectors and Clusters
A sector, being the
smallest physical storage unit on the disk, is almost always 512 bytes in size
because 512 is a power of 2 (2 to the power of 9). The number 2 is used because
there are two states in the most basic of computer languages - on and off.
Each disk sector is labeled using the factory track-positioning data. Sector
identification data is written to the area immediately before the contents of
the sector and identifies the starting address of the sector.
Optical Disks:-
CD:-
CD-ROM
stands for Compact Disk-Read Only Memory. It is a small optical disk in which a
laser beam is used to store and read information. It is a read only storage
device, i.e. once CD-ROM is produced the information stored on it cannot be
changed by using common CD-ROM drives. It is mainly used to distribute software
data, multimedia presentations, etc. It looks like a shiny aluminum foil
encased in a plastic container. It is a circular disk with a 4.75 inch diameter.
Another version of CD-ROM, the 3.5 inch, is not as popular as the 4.75 inch
disks.
Data Storage and Retrieval:-
The data is stored on a CD-ROM disk in a completely different way
as compared to the data storage of a magnetic disk. On a CD-ROM, data is stored
in a continuous 'spiral' format, like the process of recording songs on the
audio tracks. On a magnetic disk, as you move towards the center, the sector
size decreases, whereas on a CD-ROM disk each sector is of the same size. As
each sector of the CD-ROM is of the same size, the CD-ROM drive is rotated
using a constant linear velocity. This method increases the disk storage
capacity but maintaining a constant linear velocity requires complicated logic
circuits and the disk access operation also slows down as compared to the hard
disk drive.
The data on the
CD-ROM is stored on a recording surface which is made up of aluminum or gold
layer; this layer is protected by a transparent plastic coating. On this
recording surface, data is recorded as ‘pits’ and ‘lands’ as shown in the
following figure. The lands are flat surface on the disk surface and pits are
small depressions on the disk surface. These pits are created by focusing a
sharp laser beam on the disk surface.
The laser beam
actually burns the surface so a pit is created at that place. These pits and
lands are recognized as 1 and 0. For reading, the laser beam is focused on the
disk surface. The beam gets reflected at land and no reflection is there at
pits. Sequence of reflected light is interpreted as 0s and 1s by the drive unit
and sent to the computer.
Merits
and Demerits of CD-ROM:-
The main advantage
of these disks is that unlike the magnetic storage media, if these disks are
kept properly, the stored information on these disks will last forever. These
CD-ROMs are not susceptible to electrical and magnetic field disturbances
because whatever is stored is permanent and it is not in the form of electrical
charge or magnetic polarity of particles. Another big advantage of the CD-ROM
disk is its high storage capacity. One CD-ROM can store 600 MB of data. The
only demerit associated with it is that it is read only storage device and data
cannot be changed by using a normal CD-ROM drive.
One of the most common applications of the CD-ROM is
entertainment software such as video games, etc. Many types of educational
software encyclopedias requiring lot of space are now a days available on
CD-ROMs. They are also being used extensively for multimedia application.
CD-R:-
CD-R
drive, which is short for Compact Disk-Recordable drive, is a type of disk
drive that can create CD-ROMs and audio CDs. This allows the users to master a
CD-ROM or audio CD for publishing. Until recently, CD-R drives were quite
expensive, but processes have dropped dramatically. A feature of many CD-R
drives, called multisession recording, enables you to keep adding data to a CD-ROM
over time. This is extremely important if you want to use the CD-R drive to
create backup CD-ROMs. To create CD-ROMs and audio CDs, you will need not only a
Cd-R drive, but also a CD-R software package. Often, it is the software
package, not the drive itself that determines how easy or difficult it is to
create CD-ROMs. Cd-R drives can also read CD-ROMs and play audio CDs.
CD-RW:- CD-RW disk is short for CD-Rewritable disk and this is a new type of CD
disk that enables you to write onto it in multiple sessions. One of the
problems with CD-R disks is that you can only write to them once. With CD-RW
drives and disks, you can treat the optical disk just like a floppy or hard
disk, writing data onto it multiple times.
The first CD-RW drives became available in mid-1977. They
can read CD-ROMs and can write onto today’s CD-R disks, but they cannot write
on CD-ROMs. Many experts believe that they will be a popular storage medium.
Digital Versatile Disk
(DVD)-> DVD is a standard format for
the distribution and interchange of digital content in both computer based and
consumer electronic products. The format is designed to store various types of
large multimedia data on a new and improved version of the CD (Compact Disk),
which works on optical disk technology. That is, both DVD-ROM and CD-ROM store
data using pits embossed in the disk surface. However, these pits are about 4 ½
times as dense on the DVD-ROM as on the CD-ROM and can store about seven times
more data per side. (The greater density is due to a more efficient data
modulation scheme and error correction method that packs more data bits into
every bit). Physically, both DVD and CD are identical in both diameter and
total thickness. Currently available DVDs have 4.7 GB capacity per side. They
provide about 135 minutes of video playback at an average data rate of little
less than 5 megabits/second.
DVD-RW:- DVD+RW is the name
of a standard for optical discs: one of several types of DVD, which hold up to
about 4.7 GB
per disc (interpreted as approximately 4.7 × 109 bytes; actually
2295104 sectors of 2048 bytes
each which comes to 4700372992 bytes, 4590208 kilobytes, 4482.625 megabytes, or
4.377563476 gigabytes) and are used for storing films, music or other data.
DVD+RW supports random
write access, which means that data can be added and removed without erasing
the whole disc and starting over (up to about 1000 times). With suitable
support from the operating system, DVD+RW media can thus be treated
like a large floppy disk.
DVD+RW was primarily developed
for holding discrete data sets (which change with time) or as recyclable discs
for backing up collections of files. However, they (and DVD-RW) are less
popular for computer use than DVD-R or DVD+R discs, because they are not suitable for permanent
backup files (because non-rewritable media is significantly cheaper). For
similar reasons, rewritable discs are not as widely used for permanent storage
of home DVD video recorders as DVD-R and DVD+R.
On the other hand, DVD+RW or
DVD-RW make an inexpensive medium for multiple temporary recordings: they can
be used for the daily discs of a backup cycle (which are overwritten after a
number of days or weeks), and became very popular for their convenience and
cheapness as a medium for time-shifting TV (recording programs for a single
later viewing and erasing), DVD+RW discs are now playable in three quarters of
today's DVD players, many of them Hybrid (DVD±RW)
drives.
Flash Memory: Flash
memory is
an electronic non-volatile computer storage medium that can be electrically erased
and reprogrammed. Introduced by Toshiba in
1984, flash memory was developed from EEPROM (electrically
erasable programmable read-only memory). There are two main types of flash
memory, which are named after the NAND and NOR logic
gates. The internal characteristics of the individual flash memory cells
exhibit characteristics similar to those of the corresponding gates.
Whereas EPROMs had to be completely erased before
being rewritten, NAND type flash memory may be written and read in blocks (or
pages) which are generally much smaller than the entire device. NOR type flash
allows a single machine word (byte)
to be written—to an erased location—or read independently.
The NAND
type is primarily used in main memory, memory cards, USB flash drives, solid-state drives, and similar products, for
general storage and transfer of data. The NOR type, which allows true random
access and therefore direct code execution, is used as a replacement for the
older EPROM and as an alternative to certain kinds
of ROM applications,
whereas NOR flash memory may emulate ROM primarily at the machine code level;
many digital designs need ROM (or PLA)
structures for other uses, often at significantly higher speeds than
(economical) flash memory may achieve NAND
or NOR flash memory is also often used to store configuration data in numerous
digital products, a task previously made possible by EEPROM or
battery-powered static RAM. One significant disadvantage of
flash memory is the finite amount of read/write cycles in a specific block.
Example
applications of both types of flash memory include personal computers, PDAs,
digital audio players, digital cameras, mobile phones, synthesizers,
video games, scientific
instrumentation, industrial robotics, medical electronics,
and so on. In addition to being non-volatile, flash memory offers fast read access times, as fast as dynamic RAM, although not as fast as static RAM or
ROM. Its mechanical shock resistance helps explain its popularity over hard disks in
portable devices, as does its high durability, being able to withstand high
pressure, temperature, immersion in water, etc.[1]
Although
flash memory is technically a type of EEPROM, the term "EEPROM" is
generally used to refer specifically to non-flash EEPROM which is erasable in
small blocks, typically bytes. Because erase cycles are slow, the large block
sizes used in flash memory erasing give it a significant speed advantage over
non-flash EEPROM when writing large amounts of data. As of 2013, flash memory
costs much less than byte-programmable EEPROM and has become the dominant
memory type wherever a system requires a significant amount of non-volatile, solid state storage.
Nowadays, most new PCs have
built-in slots for a variety of memory cards; Memory Stick, CompactFlash, SD,
etc. Some digital gadgets support more than one memory card to ensure
compatibility.
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