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Chapter 18
Section 1 Signals and
Telecommunication
Signals and Codes
• A signal is anything that serves to direct, guide, or
warn.
• Signals can be sent in the form of gestures, flags, lights,
shapes, colors, or even electric current.
• Codes are used to send signals.
• A code is a set of rules used to interpret data.
• Signals are sent in many different forms.
• Both electricity and electromagnetic waves offer excellent
ways to send signals that can travel long distances.
• A transducer converts signals.
• A speaker converts an incoming electrical signal into
sound.
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Chapter 18
Section 1 Signals and
Telecommunication
Transducers
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Chapter 18
Section 1 Signals and
Telecommunication
Signals and Codes
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Chapter 18
Section 1 Signals and
Telecommunication
Telecommunication
• Telecommunication is the sending of visible or
audible information by electromagnetic means.
• An analog signal varies continuously within a range.
• An analog signal is a signal whose properties,
such as amplitude and frequency, can change
continuously in a given range.
• Analog signals consisting of radio waves can be
used to transmit picture, sound, and telephone
messages.
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Chapter 18
Section 1 Signals and
Telecommunication
Telecommunication, continued
• Digital signals consist of separate bits of information.
• A digital signal is a signal that can be
represented as a sequence of discrete values.
• A binary digital signal consists of a series of zeros
and ones.
• Each binary digit is called a bit.
• In electrical form, 0 and 1 are represented by the
two states of an electric current: off (no current
present) and on (current present).
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Chapter 18
Section 1 Signals and
Telecommunication
Binary Code
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Chapter 18
Section 1 Signals and
Telecommunication
Telecommunication, continued
• Sound can be stored digitally.
• Sound can be described by noting the air pressure
changes.
• The air pressure is measured in numbers and
represented in binary digits.
• Digital signals can be sent quickly and accurately.
• Digital signals have many advantages over analog
signals.
• Noise and static have less effect on digital
transmissions.
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Chapter 18
Section 1 Signals and
Telecommunication
Telecommunication Today
• Optical fibers are more efficient than metal wires.
• An optical fiber is a transparent thread of plastic
or glass that transmits light.
• These fibers carry signals that are represented by
pulses of light emitted by a laser.
• The optical-fiber system is lighter and smaller than
the wire-cable system.
• A single optical fiber can carry 11 000
conversations at once using the present coding
system.
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Chapter 18
Section 1 Signals and
Telecommunication
Telecommunication Today, continued
• Relay systems make it possible to send messages
across the world.
• Microwave towers should be tall.
• Communications satellites receive and transmit
electromagnetic waves.
• A satellite receives a microwave signal, called an
uplink, from a ground station on Earth.
• The uplink signal has frequency of around 6 GHz.
• The satellite then processes and transmits a
downlink signal to another ground station.
• The downlink signal typically has a lower frequency of
about 4 GHz.
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Chapter 18
Section 1 Signals and
Telecommunication
Telecommunication Today, continued
• Many communications satellites have geostationary
orbits.
• These satellites orbit Earth every 24 hours, the
same amount of time it takes for Earth to rotate
once.
• The position of the satellite relative to the ground
doesn’t change.
• The orbit of this type of satellite is called a
geostationary orbit, or a geosynchronous orbit.
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Chapter 18
Section 2 Telephone, Radio, and
Television
Telephones
• The electret microphone vibrates with sound waves,
creating an analog signal.
• In an electret microphone, an electrically charged
membrane is mounted over an electret, which is a
material that has a constant electric charge.
• The electrical signal that is created is transmitted
as variations in an electric current between your
telephone and the telephone of the person to
whom you are talking.
• The movement of the speaker cone converts the
analog signal back into sound waves.
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Chapter 18
Section 2 Telephone, Radio, and
Television
Telephone
The sound waves from
your voice are transformed
by the microphone into an
analog electrical signal.
A speaker converts the
analog electrical signal
back to sound waves.
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Chapter 18
Section 2 Telephone, Radio, and
Television
Telephones, continued
• Telephone messages are sent through a medium in
physical transmission.
• Sometimes telephone conversations travel a short
distance by wire and then are carried by light
through fiber-optic cables.
• The electrical signal is converted into a light or
optical signal by a laser diode.
• Transmission of signals by wires or optical fibers is
called physical transmission.
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Chapter 18
Section 2 Telephone, Radio, and
Television
Telephones, continued
• Messages traveling longer distances are sent by
atmospheric transmission.
• Atmospheric transmission is the passage of an
electromagnetic wave signal through the
atmosphere between a transmitter and a receiver.
• Computers help route calls.
• Cellular phones transmit messages in the form of
electromagnetic waves.
• A cellular phone is a small radio
transmitter/receiver, or transceiver.
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Chapter 18
Section 2 Telephone, Radio, and
Television
Radio and Television
• Sound waves are converted to electromagnetic waves
for radio broadcast.
• An electronic device called an amplifier increases
the power of the weak signal produced by a
microphone.
• The oscillator in the transmitter produces a carrier,
which is a signal of constant frequency and
amplitude.
• A carrier is a wave that can be modulated to
send a signal.
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Chapter 18
Section 2 Telephone, Radio, and
Television
Radio and Television, continued
• In a specialized circuit in the transmitter the audio
signal and the carrier signal combine, and the audio
signal changes, or modulates, the carrier wave.
• Modulate means to change a wave’s amplitude or
frequency in order to send a signal.
• The result is a signal of constant frequency with an
amplitude that is shaped by the audio signal.
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Chapter 18
Section 2 Telephone, Radio, and
Television
Radio and Television, continued
• Modulation can be either AM or FM.
• Most broadcast carrier waves are modulated either
by amplitude modulation (AM) or by frequency
modulation (FM).
• In amplitude modulation, the audio signal
increases and decreases the amplitude of the
carrier wave in a pattern that matches the audio
signal.
• In frequency modulation, the audio signal affects
the frequency of the carrier wave, changing it in
a pattern that matches the audio signal
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Chapter 18
Section 2 Telephone, Radio, and
Television
Radio and Television, continued
• Higher frequency transmissions can follow only a
simple straight line called line-of-sight transmission.
• AM frequencies between 540 and 1700 kHz can
travel as ground waves, which can follow the
curvature of the Earth for some distance, unlike
line-of-sight transmissions.
• AM radio stations use sky waves to broadcast long
distances.
• Radio receivers convert electromagnetic waves back
into sound.
• The antenna of your radio receiver works as a
transducer.
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Chapter 18
Section 2 Telephone, Radio, and
Television
Radio
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Chapter 18
Section 2 Telephone, Radio, and
Television
Radio and Television, continued
• Television sets convert electromagnetic waves back
into images and sound.
• The carrier wave is passed to a detector that
separates the audio and video electrical signals
from the carrier.
• The picture tube of a black-and-white television is a
large cathode- ray tube or CRT.
• A cathode-ray tube is a tube that uses an electron
beam to create a display on a phosphorescent
screen.
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Chapter 18
Section 2 Telephone, Radio, and
Television
Radio and Television, continued
• Color picture tubes produce electron beams.
• Color picture tubes in some televisions produce
three electron beams, one for each of the primary
colors of light: red, blue, and green.
• Each group of three dots is a pixel, which is the
smallest element of a display image
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Chapter 18
Section 2 Telephone, Radio, and
Television
Television
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Chapter 18
Section 3 Computers and
the Internet
Computers
• A computer is an electronic device that can accept
data and instructions, follow the instructions, and
output the results.
• Computers have been changing greatly since the
1940s.
• The first electronic computer was the Electronic
Numerical Integrator and Computer (ENIAC).
• Computers carry out four functions.
• Digital computers perform four basic functions:
input, storage, processing, and output.
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Chapter 18
Section 3 Computers and
the Internet
Computers, continued
• Computer input is in the form of binary code.
• Computers process binary data, including
numbers, letters, and other symbols, in groups
of eight bits.
• Each bit can have only one of two values, usually
represented as 1 and 0.
• A group of eight bits is called a byte.
• Computers must have a means of storing data.
• Both hard drives and floppy drives are referred
to as magnetic media because they use disks
coated with a magnetizable substance.
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Chapter 18
Section 3 Computers and
the Internet
Computers, continued
• Random-access memory is used for short-term
storage of data and instructions.
• Random-access memory is a storage device that
allows a computer user to write and read data; it is
the amount of data that the memory chips can
hold at one time (abbreviation, RAM)
• Read-only memory is for long-term storage of
operating instructions.
• Read-only memory is a memory device that
contains data that can be read but cannot be
changed (abbreviation, ROM)
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Chapter 18
Section 3 Computers and
the Internet
CPU, RAM, and ROM
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Chapter 18
Section 3 Computers and
the Internet
Computers, continued
• Optical storage devices can be more permanent than
magnetic disks.
• Compact discs (CDs) and digital versatile discs (DVDs) are
called optical media because the information on them is read
by a laser light.
• Operating systems control hardware.
• The hardware is the parts or pieces of equipment that make
up a computer.
• The software is a set of instructions or commands that tells
a computer what to do; a computer program.
• The operating system is the software that controls a
computer’s activities.
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Chapter 18
Section 3 Computers and
the Internet
Computers, continued
• The processing function is the primary operation of
a computer.
• Computing or data processing is carried out by the
central processing unit, or CPU.
• Chips have many components.
• This chip, or microprocessor, consists of millions of tiny
electronic parts, including resistors, transistors, and
capacitors.
• Logic circuits in the CPU make decisions.
• The heart of the CPU is an arithmetic/logic unit, or ALU,
which performs calculations and logic decisions.
• The CPU also contains temporary data storage units,
called registers.
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Chapter 18
Section 3 Computers and
the Internet
Computers, continued
• The CPU’s logic gates can be built up to evaluate
data and make decisions.
• A logic gate can open or close a circuit depending
on the condition of two inputs.
• One kind of logic gate is called an AND gate.
• An AND gate closes the circuit and allows current to pass
only when both inputs are in the “on” position.
• Another type of logic gate is called an OR gate.
• An OR gate closes the circuit and allows current to pass
when one of the input is in the “on” position.
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Chapter 18
Section 3 Computers and
the Internet
A Logic System
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Chapter 18
Section 3 Computers and
the Internet
Computer Networks and the Internet
• In local area networks, or LANs, all PCs are
connected by cables to a central computer called a
server.
• A server consists of a computer with lots of
memory and several hard-disk drives for storing
huge amounts of information.
• The Internet is a worldwide network of computers.
• The Internet is a large computer network that
connects many local and smaller networks all over
the world.
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Chapter 18
Section 3 Computers and
the Internet
Computer Networks and the Internet,
continued
• You need three things to use the Internet.
• You need a computer with a modem to connect
the computer to a telephone line.
• The word modem is short for
modulator/demodulator.
• You need a software program called an Internet,
or Web, browser.
• You need a connection to an Internet service
provider, or ISP.
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