Signals and Codes
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Transcript of Signals and Codes
Copyright © by Holt, Rinehart and Winston. All rights reserved.
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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.
Section 1 Signals andTelecommunicationChapter 18
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Transducers
Section 1 Signals andTelecommunicationChapter 18
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Signals and Codes
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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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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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Binary Code
Section 1 Signals andTelecommunicationChapter 18
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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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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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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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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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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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Telephone
Section 2 Telephone, Radio, and Television
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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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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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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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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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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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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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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Radio
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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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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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Television
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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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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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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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CPU, RAM, and ROM
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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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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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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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A Logic System
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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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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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