1.3.1 Logic Gates
Logic circuit
Draw a logic circuit from the following logic expression
Complete the truth table from the above logic expression
Draw a logic circuit from the following logic expression
Complete the truth table from the above logic expression
Logic circuit using problem statements
Practice more
More problems
More problems
Logic circuit minimization
Some circuits can be constructed with fewer gate to produce the same output
We can simplify (minimize) the circuit by applying logical identities. Since the example
states that A is true when B is false or the other way around. In terms of logical
gates, inequality simply means an XOR gate (exclusive or)
A
0
0
1
1
B
1
0
1
0
output
1.3.2 Computer Architecture and the fetch-execute cycle
Von Neumann model
Von Neumann introduced the idea of the stored program. Previously data and
programs were stored in separate memories.von Neumann realized that data and
programs are indistinguishable and can therefore use the same memory.
Von Neumann architecture uses a single processor. It follows a linear sequence of
fetch-decode-execute operations for sequence of instructions which makeup the
program. In order to do this a processor has to use some special registers.
A register is simply a location that can store extremely limited amount of
instruction or data only immediately before or after processing. The registers are
outside the immediate access store and consequently allow faster access to the
data they store.
Processor uses a group of special purpose registers to execute a program
 Program counter (PC): Program counter is sometimes called sequence
control register. It stores the address of the next instruction to be fetched.
 Memory address register (MAR): When the next instruction is needed, its
address is copied from the PC and placed in the memory address register.
 Memory data registers (MDR): It is sometime called the memory buffer
register (MBR) because it acts like a buffer, temporarily storing a data value
before passing it on to e.g. CIR.
 Index register (IR): An index register in a computer's CPU is a processor
register used for modifying operand addresses during the run of a program,
typically for doing vector/array operations. If the address is an indexed
address, add the address to the contents of the index register to form the
address of actual data.
 Current instruction register (CIR): Holds the instruction that is about to be
executed.
 Status register: A status register, flag register, or condition code register is a
collection of status flag bits for a processor. The status register is a hardware
register which contains information about the state of the processor.
Role of arithmetic logic unit (ALU), control unit (CU) and system clock
Arithmetic logic unit
In digital electronics, an arithmetic logic unit (ALU) is a digital circuit that
performs arithmetic and bitwise logical operations on integer binary numbers. It is
a fundamental building block of the central processing unit (CPU) found in many
computers. An ALU performs basic arithmetic and logic operations. Examples of
arithmetic operations are addition, subtraction, multiplication, and division.
Examples of logic operations are comparisons of values such as NOT, AND, and OR.
Control unit
The control
unit is
a
component
of
a
computer's central processing unit (CPU) that
directs operation of the processor. It tells the
computer's memory, arithmetic/logic unit and
input and output devices how to respond to a
program's instructions.
Control unit: The Control Unit makes
decisions and sends the appropriate
signal down its lines to other parts of the
computer. It controls the timing of
operations in the computer and controls
the instructions sent to the processor and
the peripheral devices.
It directs the operation of the other units by providing timing and control
signals. All computer resources are managed by the CU (Control Unit).It directs the
flow of data between the Central Processing Unit (CPU) and the other devices.
The fetch-execute cycle
An instruction
cycle (sometimes
called fetch-and-execute
cycle, fetchdecode-execute cycle, or FDX) is the basic operation cycle of a computer. It
is the process by which a computer retrieves a program instruction from
its memory, determines what actions the instruction requires, and carries out
those actions. This cycle is repeated continuously by the central processing
unit (CPU), from bootup to when the computer is shut down.

Program counter (PC) - an incrementing counter that keeps track of the
memory address of the instruction that is to be executed next.

Memory address register (MAR) - holds the address of a memory block to be
read from or written to.

Memory data register (MDR) - a two-way register that holds data fetched
from memory (and ready for the CPU to process) or data waiting to be stored in
memory.

Current Instruction register (IR) - a temporary holding ground for the
instruction that has just been fetched from memory.

Control unit (CU) - decodes the program instruction in the IR, selecting
machine resources such as a data source register and a particular arithmetic
operation, and coordinates activation of those resources.

Arithmetic logic unit (ALU) - performs mathematical and logical operations.
Description in the form of flowchart
Register transfer notation
1. [MAR]  [PC]
2. PC  [PC] + 1
3. MDR  [[MAR]]
4. CIR  [MDR]
5. DECODE
6. EXECUTE
7. GO TO STEP 1
Step by step description of fetch-execute-cycle
1. Copy the address that is in program counter(pc)into the memory address
register (MAR)
2. Increment the PC (ready for next fetch)
3. Load the instruction that is in the memory address given by the MAR into the
memory data register (MDR)
After increment
4. The MBR loads the Current Instruction Register with the instruction to be executed.
1.3.3 Input Devices
2D scanners
Scanners can be used to convert images or text on paper into a digital format that
can be used by the computer. A scanner works by shining a beam of light onto the
surface of the object that you are scanning. This light is then reflected back onto a
sensor that detects the colour of the light. This is then used to build up the digital
image. Items that are scanned are usually stored in an image format. However,
special software - Optical Character Recognition - can be used to convert text on
the paper into text which can be edited with a word processor. However, the text
doesn't always get converted very well and
you could end up with a lot of mistakes.
There are three types of scanner:



Flatbed scanners
Handheld scanners
Specialist scanners
The most popular type is the flatbed
scanner. This is shown in the picture which
has its lid raised to show the glass surface
where you place the item to be scanned.
This is probably the one that you use at school. They can
scan larger images and are more accurate than handheld
scanners.
Handheld scanners are usually only a few inches wide and
are held in the hand whilst they are rolled across the
document to be scanned. The images produced are generally
not as large or as high quality as those captured with a
flatbed scanner.
Advantages of scanners
Disadvantages of scanners
Flatbed scanners are very accurate Images produced by the scanner
and can produce reasonably high can take up a lot of memory
quality images.
space.
Any image which is digitised by
the scanner can then be included
on electronic documents.
Images lose some quality in the
scanning and digitising process.
Images once digitised can be
enhanced with a graphics
application.
The quality of the final image is
dependent on the quality of the
original image.
3D Scanners
A 3D scanner is a device that analyses a real-world object or environment to collect
data on its shape and possibly its appearance (e.g. colour). The collected data can then
be used to construct digital three-dimensional models.
Many different technologies can be used to build these 3D-scanning devices; each
technology comes with its own limitations, advantages and costs. Many limitations in
the kind of objects that can be digitised are still present, for example, optical
technologies encounter many difficulties with shiny, mirroring or transparent objects
Collected 3D data is useful for a wide variety of applications. These devices are used
extensively by the entertainment industry in the production of movies and video
games. Other common applications of this technology include industrial design
The purpose of a 3D scanner is usually to create a point cloud of geometric samples on
the surface of the subject. These points can then be used to extrapolate the shape of
the subject (a process called reconstruction). If colour information is collected at each
point, then the colours on the surface of the subject can also be determined.
Bar code reader
Barcodes are used to represent information about products. A barcode is normally a pattern
of thick and thin dark and light lines. A form of scanner called a barcode reader is used to
read them. Some are handheld and some are built into the EPOS terminal. Like a flatbed
scanner, it consists of a light source, a lens and a light sensor translating optical impulses into electrical
ones. The different thicknesses of lines are converted by computer into a number.
They are used in all kinds of shops, wherever the details of products represented b code
numbers need to be input. In libraries they are used to input the book number and library
card number.
They are faster than typing the number in using a keypad and more accurate method of
data entry compared with typing the number in.
Digital cameras
A digital camera (or digicam) is a camera that encodes digital images and videos
digitally and stores them for later reproduction. Most cameras sold today are
digital, and digital cameras are incorporated into many devices ranging from PDAs
and mobile phones (called camera phones) to vehicles.
Digital and film cameras share an optical system, typically using a lens with a
variable diaphragm to focus light onto an image pickup device. The diaphragm and
shutter admit the correct amount of light to the imager, just as with film but the
image pickup device is electronic rather than chemical. However, unlike film
cameras, digital cameras can display images on a screen immediately after being
recorded, and store and delete images from memory. Many digital cameras can also
record moving videos with sound. Some digital cameras can crop and stitch pictures
and performs other elementary image editing.
Keyboard
The most common input device it is used for inputting text,numbers,punctuation
marks, symbols and instructions using a number of software applications. By
pressing down a key completes an electrical circuit. This circuit then transmits a
binary signal (commonly using ASCII) to the computer to represent the key
pressed.
Ergonomic or ergonometric keyboards are shaped so that the user is less likely to
suffer from RSI (repetitive strain injury).
Overlay or concept keyboards consist of an A4-sized tray that can have an overlay
sheet in it. It is used by people who may have disabilities as it is easier to press a
large symbol than to press the key on the keyboard.
Laser mouse
An optical computer mouse uses a light source, typically a light-emitting diode, and
a light detector, such as an array of photodiodes or an image sensor, to detect
movement relative to a surface. It is an alternative to the mechanical mouse, which
uses moving parts to perform the same function.
When a mouse is connected with the computer, a red LED starts glowing. This light
is focused on the surface beneath the mouse by a special type of lens (HDNS2100). The reflected light falls back to the camera integrated in Optical Mouse
Sensor. The camera takes images of the surface with a frequency in the range of
1500 to 6000 images per second to calculate the position of mouse. These images
are processed by Digital Signal Processor (DSP) and resultant co-ordinates are sent
to the PC. The driver of the mouse installed in the computer receives the coordinates and makes the cursor move on the screen accordingly.
Touch screens
A touch screen is the only device which works as both
an input and an output device. You view the options
available to you on the screen (output) and you then
use your finger to touch the option that you have
chosen (input).
Touch screens work particularly well with a menu
driven interface. For example, a cashpoint (ATM) at a
bank would first of all ask which service you want. You
touch the option 'withdraw cash' on the screen. You
are then presented with another choice, 'how much
cash do you want to withdraw?’ Once you have chosen
how much, you then get other options such as 'do you
want a receipt?’
Another major use of touch screens are on smart
phones and modern tablet computers. Each 'app' is
accessed by an icon on the touch screen.
Touch screens are easy to use and are often found in
public places such as cashpoints at banks, ticket
collection
terminals
at
theatres
or
airports,
information centres at museums.
Touch screens are not commonly used to input large
amounts of data because they are not very accurate and they would be tiring on
the hands to use for long periods of time.
Capacitive
Infra-red
 Made up of many layers
of glass that act like a
capacitor,
creating
electric fields between
the glass plated in
layers
 The
electric
current
changes when the top
glass layer is touched
which is determined by
a microprocessor.
Heat sensitive
 It uses glass
as screen
material
which needs
a warm
object e.g
finger to
carry out
input
operation
Advantages
 Screen visibility is good
even in strong sunlight
 Permits multi-touch
capability.
 Screen is very durable
 It is a medium cost
technology.
Advantages
 Allow multi-touch capability
 The optical system allows
bare finger, gloved or stylus
for input.
 Both have good screen
durability, it takes a major
impact to break the glass
 Both have fairly good visibility
in strong sunlight.
Disadvantages
 Relatively expensive
technology
 Heat-sensitive only
allow bare fingers to be
used for input.
Disadvantages
 It allows only bare
fingers as input but
latest screens
permit a special
stylus to be used
resistive
Optical
 It uses an
array of
sensors in
the form of
grid
 The point
of contact
is based on
which
coordinated
is touched.
 Consists of upper layer of
polyester and bottom layer
of glass.
 When top layer is touched,
the top and bottom layer
complete a circuit.
 Signals are sent out and
interpreted by a
microprocessor; the
calculations determine the
coordinates of where the
screen was touched.
 Advantages
 Relatively inexpensive
technology
 Can used bare finger,
gloved finger or a stylus to
carry out an input
operation
Disadvantages
 Screen visibility is poor in
strong sunlight.
 Doesn’t permit multitouch capability.
 The screen durability is
only fair and vulnerable t
scratches and wears out
through time.
Microphones
Microphones are used to input sound, which can then be stored and manipulated.
The sound input to the microphone is converted into an analogue electrical signal,
which has to be converted in turn into a digital signal so that the computer can
process it. This is usually done by the computer’s sound card, which acts as an
analogue to digital converter.
Sensor:
A sensor is a device that detects events or changes in quantities and provides a
corresponding output, generally as an electrical or optical signal. They can produce
a stream of input data automatically without any human intervention. Usually this is
an analogue signal so it needs to be converted into digital data for the computer to
process. This is done using by an Analogue-to-Digital Converter (ADC). Sensors are
used extensively in monitoring / measuring / data logging systems, and also in
computer control systems. Following is the list of commonly used sensors:
Temperature
Magnetic Field
Gas
Pressure
Moisture
Humidity
Ph/Acidity/Alkalinity
Motion/ Infra-Red
Advantages of using sensors to collect data are:
1. They can collect data far more frequently than a person.
2. They are more reliable than a person, who may forget to take readings.
3. They are more accurate than a person who may misread the signal.
4. They can collect data from places where it is not possible for a person to go such
as inside a chemical or nuclear reaction vessel.
The disadvantage of using sensors is that they may need a power supply to work
and may need regular calibration to check their accuracy.
Temperature Sensor:
A temperature sensor produces a signal that depends on the temperature of its
surroundings. The computer process the digitize signal to display a measurement or
to control an appliance.
We can use temperature sensor in many appliance such:
 Automatic washing machine
 Digital thermometer
 Controlling heating system in buildings, including greenhouses and chemical
reaction vessels.
Pressure sensor:
A pressure sensor produces a signal that depends on the pressure to which it is
exposed. Pressure sensor can be used in many appliances such as automatic blood
pressure monitor. Pressure sensor can also control the pressure of gases or liquids
in chemical reaction vessel.
Magnetic Field:
The Magnetic Field Sensor can be used to study the field around permanent
magnets, coils, and electrical devices. This sensor uses a Hall effect transducer, and
measures a vector component of the magnetic field near the sensor tip. It has two
ranges, allowing for measurement of relatively strong magnetic fields around
permanent magnets and electromagnets, as well as measurement of weak fields
such as the Earth’s magnetic field. The articulated sensor tip allows you to measure
both transverse and longitudinal magnetic fields.
Gas:
A gas sensor produces a signal depending on the concentration of a particular gas
or vapor. We can use gas sensor for an inflammable gas to monitor the atmosphere
and sound an alarm if there is a leakage. We can use gas sensor in other
applications such as:
 Breathalyzer, which measure the concentration of alcohol vapour in a sample
of breath and estimate the concentration of alcohol in blood.
 Process control in chemical industry.
 Environment monitoring of air pollution
Moisture/ Humidity sensor
A moisture sensor produces a signal that depends on the concentration of water
vapors in the atmosphere. A moisture sensor can control an irrigation system more
efficiently only allow water when soil is dry. We can use moisture sensor in many
other application including:
 Controlling a heating system and air conditioning system.
 Maintaining sufficient humidity in the air in a greenhouse.
 Measuring humidity for meteorological record and forecasting in a weather
station.
PH/acidity/alkalinity sensor:
.1 Input, output and storage devices
PH Sensor measures the pH of aqueous solutions in industrial and municipal
process applications. It is designed to perform in the harshest of environments,
including applications that poison conventional pH sensors.
Typical activities using our pH sensor include:
 Acid-base titrations
 Studies of household acids and bases
 Monitoring pH change during chemical reactions or in an aquarium as a result
of photosynthesis
 Investigations of acid rain and buffering
 Analysis of water quality in streams and lakes
Infrared sensor:
An infra (IR) sensor produces a signal that depends on the level of invisible IR
radiation falling on it.
All objects (unless they are extremely cold) emit significant IR radiation. Security
camera equips with lens and grid of IR sensors uses this IR radiation to form a
detector for a person.
Sensor
Measures
Where Used
Heat
Temperature
Living room for central heating
system
Humidity
Water vapour in the air
Swimming pool, greenhouse
Infra-red
Infra-red radiation e.g. body heat
Security alarm systems
Light
Light levels
External security lights
PH
Acid/alkali levels e.g. pH of soil
Environmental experiments, river
pollution
Pressure
Pressure
Burglar alarm systems, automatic
doors
Smoke
Smoke in the atmosphere
Offices
Sound
Levels of sound
Security alarm systems
Tilt
Angle of tilt
Windows in security alarm system
Touch
Detects if one object bumps into
another
Computer controlled robots
1.3.4 Output devices
Printer
A printer is a peripheral which makes a persistent human-readable representation
of graphics or text on paper or similar physical media.
Impact printers rely on a forcible impact to transfer ink to the media. The impact
printer uses a print head that either hits the surface of the ink ribbon, pressing the
ink ribbon against the paper (similar to the action of a typewriter), or hits the back
of the paper, pressing the paper against the ink ribbon (the IBM 1403 for example)
Non-impact is a type of printer that does not operate by striking a head against a
ribbon. Examples of nonimpact printers include laser and ink-jet printers.
Ink-jet printer
Inkjet printing is a type of computer printing that recreates a digital image by
propelling droplets of ink onto paper, plastic, or other substrates. It works by
spraying ionized ink at a sheet of paper. Magnetized plates in the ink's path direct
the ink onto the paper in the desired shapes. Ink-jet printers are capable of
producing high quality print.
Laser printer
This type of printer produces
very high quality hard copy. It
consists of drum that is
electrically charged. A laser is
used to change the charge on
the drum for each dot of the
output
to
be
produced.
Electrically charged toner is
then attracted to the oppositely
charged
dots.
The
paper
presses
against
the
toner
coated drum and is output with the pattern of dots required which is then heated in
order to permanently fuse the text/imagery. The page cannot be printed until the
whole document has been stored in a large buffer.
Speakers
It is a device which produces sound as output. The digital data from the computer
is converted into analogue signals by a digital to analogue converter and are then
amplified and output using the speakers. At the front of a loudspeaker, there is a
fabric, plastic, paper, or lightweight metal cone. The inner part is fixed to
an iron coil (sometimes called the voice coil) that sits just in front of a permanent
magnet (sometimes called the field magnet). When electric current is passed
through coil; it becomes a temporary
magnet
(electromagnet).
As
the electricity flows back and forth in
the cables, the electromagnet either
attracts or repels the permanent
magnet. This moves the coil back and
forward, pulling and pushing the
loudspeaker cone which produces
sound.
2D, 3D cutters
A 3d laser cutter can recognize all sides of a substance to make more complex
shapes. A 3d laser cutter can also work with a cube instead of a flat piece of a
material.
A 2d and 3d cutter is a high powered laser that cuts material such as thin metals or
woods, used mostly for industrial purposes. They make precise and clean cuts, it is
also is very fast which makes it efficient. It is first designed through a computer
and then sent through a laser cutter to give you the design.
Headphones
Actuators
Actuators are used to convert computer signals into movement.e.g motors. In
industries motors are used to move robot arms. In computers they are used to
cause moving parts t o work in disk drives. An actuator can also be connected from
the computer to heater and set to switch heater either on or off.
LCD
Short for liquid crystal display, a type of display used in digital watches and
many portable computers. LCD displays utilize two sheets of polarizing material
with a liquid crystal solution between them. An electric current passed through the
liquid causes the crystals to align so that light cannot pass through them. Each
crystal, therefore, is like a shutter, either allowing light to pass through or blocking
the light.
The LCD screen is more energy efficient and can be disposed of more safely than a
CRT. Its low electrical power consumption enables it to be used in batterypowered electronic equipment.
LED
Short for light emitting diode. A flat-screen device in which an array of lightemitting diodes can be selectively activated by processor to display numerical and
alphabetical information ,used esp in pocket calculators ,digital timepieces,
measuring instruments , and in some microcomputers.
DLP Projectors, or digital light processing, uses tiny mirrors to reflect light toward
the screen (an "on" pixel) or away (an "off" pixel). Most models use a color wheel,
literally a spinning wheel with color filters to create sequential color. Some high-end
models use three DLP chips; one each for red, green, and blue.
DLP projectors range in price from a few hundred dollars to tens of thousands, and
more. Most projectors in movie theaters use DLP.
LCD Projectors, which stands for liquid crystal display, is very similar to the
technology found in most TVs. LCD projectors use three liquid crystal panels, each
tasked with creating an image using just one of the primary colors (red, green, and
blue). All three are projected on the screen at once, so you see a full color image.
LED vs. DLP projectors




DLP is significantly more expensive than LCD, but requires less ongoing
maintenance.
LCD projectors are portable and lightweight.
DLP quality is superior to LCD (although LCD is sufficient for most usage).
Film, photography, highly detailed graphics and images, etc., will all appear
crisper and brighter with DLP technology as compared to LCD.
1.3.5 Memory, storage devices and media
Primary memory is computer memory that is accessed directly by the CPU. This
includes several types of memory, such as the processor cache and system ROM.
However, in most cases, primary memory refers to system RAM.
RAM, or random access memory, consists of one or more memory modules that
temporarily store data while a computer is running. RAM is volatile memory,
meaning it is erased when the power is turned off. Therefore, each time you start
up your computer, the operating system must be loaded from secondary
memory (such as a hard drive) into the primary memory, or RAM. Similarly,
whenever you launch an application on your computer, it is loaded into RAM.
The operating system and applications are loaded into primary memory, since RAM
can be accessed much faster than storage devices. In fact, the data can be
transferred between CPU and RAM more than a hundred times faster than between
the CPU and the hard drive. By loading data into RAM, programs can run
significantly faster and are much more responsive than if than constantly accessed
data from secondary memory.
NOTE: Primary memory may be called "primary storage" as well. However, this
term is somewhat more ambiguous since, depending on the context, primary
storage may also refer to internal storage devices, such as internal hard drives.
DVD RAM
DVD RAM is a recent addition to the optical media group. Unlike other CD and DVD
formats, DVD RAMs have several discrete concentric tracks rather than a single
spiral track. This gives them the advantage that writing and reading can occur at
the same time. This makes it possible to watch an already recorded television
program at the same time as different program is being recorded.DVD RAMs can
written to many times.DVD RAMs have a long life estimated to be 30 years and re-
write operation can be carried out over 100,000 times which is 100 times more
than RW formats.
Secondary memory refers to storage devices, such as hard drives and solid state
drives. Unlike primary memory; secondary memory is not accessed directly by
the CPU. Instead, data accessed from secondary memory is first loaded
into RAM and is then sent to the processor. The RAM plays an important
intermediate role, since it provides much faster data access speeds than secondary
memory. By loading software programs and files into primary memory, computers
can process data much more quickly.
While secondary memory is much slower than primary memory, it typically offers
far greater storage capacity. For example, a computer may have a
one terabyte hard drive, but only 16 gigabytes of RAM. Secondary memory is nonvolatile, meaning it retains its data with or without electrical power. RAM, on the
other hand, is erased when a computer is shut down or restarted. Therefore,
secondary memory is used to store "permanent data," such as the operating
system, applications, and user files.
Hard disk
A hard disk is really a set of
stacked "disks," each of
which,
like
phonograph
records, has data recorded
electromagnetically
in
concentric circles or "tracks"
on the disk. A "head” records
(writes)
or
reads
the
information on the tracks.
Two heads, one on each side
of a disk, read or write the data as the disk spins. Each read or write operation
requires that data be located, which is an operation called a "seek." (Data already
in a disk cache, however, will be located more quickly.)
A hard disk/drive unit comes with a set rotation speed varying from 4500 to 7200
rpm. Disk access time is measured in milliseconds. The physical location can be
identified with cylinder, track, and sector locations.
Solid state devices (SSDs)
Solid-state drives actually aren't hard drives in the traditional sense of the term, as
there are no moving parts involved. A traditional hard disk drive (HDD) consists of
a spinning disk with a read/write head on a mechanical arm. An SSD, on the other
hand, has an array of semiconductor memory organized as a disk drive, using
integrated circuits (ICs) rather than magnetic or optical storage media.
Off-line storage
Offline storage is any storage that is not currently online, live or connected to the
computer. The data stored in offline storage remains permanently in the storage
device even if it’s disconnected or unplugged from the computer after the data has
been stored. Offline storage is generally portable in nature and can be used on
different computer systems. Common examples of offline storage include floppy
disks, compact disks and USB sticks.
Solid state memory
Solid state systems control the movements of electrons within the microchip. The
1s and 0s are stored in millions of miniature transistors within the microchip. If a
transistor conducts a current this is equivalent to a 1 otherwise 0.they consequently
have no moving parts, consume much less power and are extremely robust.
Memory sticks/pen drives
It uses solid state technology to store several GBytes of data. It is usually
connected to a computer through the USB port and power to operate them is drawn
from the host computer. They are small and very portable.
Memory sticks and pen drives are used for transporting files between computers or
as a backup store. They are used a security device, a dongle, to prevent software
piracy
Flash memory cards
These are a form of electrically erasable programmable
read only memory (EEPROM) and are another example of
solid state memory. They are used to store photos on
digital cameras. Mobile phones use them as memory
cards. They are also used in MP3 players to store music
files. They are very compact and robust.
Compact Disks (CDs)
The data is stored as a series of pits (equivalent to binary 1) and lands (equivalent
to binary 0) in metallic optical layer. The pits are formed by a laser beam etching
the surface at the manufacturing stage. Only a single track exits which spirals out
from the centre of the disk.
The pits and lands are read by a low-powered laser beam which follows the data
stream and reads from the centre outwards in a spiral. The light reflects differently
off a pit than it does off a land and this interpreted a s1s and 0s.
DVDs
DVDs can hold up to ten times the amount of data that a CD can store.DVD writers
use a shorter wavelength of laser light than CD writers do, so the pit on the disk
surface is a lot smaller and more can be stored in the same space.
CDs and DVDs are cheaper and robust than hard disks but data transfer rates and
access times for both are slower than hard disks.
Blu-ray
These have largest capacity of all optical disks up to 100 GB or more. It uses a
shorter wavelength for its laser beam, using light that is close to the blue/violet
spectra rather than the red light used by other optical devices. They are mostly
used for storing films (movies)