2. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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 and Telecommunication Chapter 18

3. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Transducers Section 1 Signals and Telecommunication Chapter 18

4. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Signals and Codes Section 1 Signals and Telecommunication Chapter 18

5. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 1 Signals and Telecommunication Chapter 18

6. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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). Section 1 Signals and Telecommunication Chapter 18

7. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Binary Code Section 1 Signals and Telecommunication Chapter 18

8. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 1 Signals and Telecommunication Chapter 18

9. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 1 Signals and Telecommunication Chapter 18

10. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 1 Signals and Telecommunication Chapter 18

11. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 1 Signals and Telecommunication Chapter 18

12. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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, and Television Chapter 18

13. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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

14. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 Telephone, Radio, and Television Chapter 18

15. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 Telephone, Radio, and Television Chapter 18

16. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 Telephone, Radio, and Television Chapter 18

17. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 Telephone, Radio, and Television Chapter 18

18. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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 Section 2 Telephone, Radio, and Television Chapter 18

19. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 Telephone, Radio, and Television Chapter 18

20. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Radio Section 2 Telephone, Radio, and Television Chapter 18

21. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 Telephone, Radio, and Television Chapter 18

22. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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 Section 2 Telephone, Radio, and Television Chapter 18

23. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Television Section 2 Telephone, Radio, and Television Chapter 18

24. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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 and the Internet Chapter 18

25. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 3 Computers and the Internet Chapter 18

26. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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) Section 3 Computers and the Internet Chapter 18

27. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu CPU, RAM, and ROM Section 3 Computers and the Internet Chapter 18

28. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 3 Computers and the Internet Chapter 18

29. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 3 Computers and the Internet Chapter 18

30. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 3 Computers and the Internet Chapter 18

31. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu A Logic System Section 3 Computers and the Internet Chapter 18

32. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 3 Computers and the Internet Chapter 18

33. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 3 Computers and the Internet Chapter 18