Signals and Codes

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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



Transducers




Signals and Codes




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.




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).




Binary Code




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.




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.




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.




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.




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.

Section 2 Telephone, Radio, andTelevisionChapter 18



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.

Chapter 18



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.




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.




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.




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.





• 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





• 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.




Radio

Section 2 Telephone, Radio, and TelevisionChapter 18




• 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.





• 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




Television

Section 2 Telephone, Radio, and TelevisionChapter 18



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.

Section 3 Computers andthe InternetChapter 18



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.





• 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)




CPU, RAM, and ROM





• 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.





• 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.





• 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.




A Logic System




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.




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.


Signals and Codes

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