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Unit 6 Electronic Communications Unit 6 Electronic Communications Passage A Introduction to Electronic Communications Passage B The Principle of PCM Passage C Information Sources Unit 6 Electronic Communications Passage A Introduction to Electronic Communications1.Historical PerspectiveThe fundamental purpose of an electronic communication system is to transfer information from one place to another.1 Thus,electronic communications can be summarized as the transmission,reception,and processing of information between two or more locations using electronic circuits.The original source information can be in analog(continuous)form,such as the human voice or music,or in digital(discrete)form,such as binary-coded numbers or alphanumeric codes.2 All forms of information,however,must be converted to electromagnetic energy before being propagated through an electronic communications system.Unit 6 Electronic Communications Samuel Morse developed the first electronic communications system in 1837.Morse used electromagnetic induction to transfer information in the form of dots,dashes,and spaces between a simple transmitter and receiver using a transmission line consisting of a length of metallic wire.He called his invention the telegraph.In 1876 Alexander Graham Bell and Thomas A.Watson were the first successfully transfer human conversation over a crude metallic wire communications system,they called the telephone.Unit 6 Electronic Communications Guglielmo Marconi successfully transmitted the first wireless radio signals through Earth s atmosphere in 1894,and in 1908 Lee Deforest invented the triode vacuum tube which provided the first practical means of amplifying electrical signals.Commercial radio began in 1920 when radio stations began broadcasting amplitude-modulated(AM)signals,and in 1933,Major Edwin Howard Armstrong invented frequency modulation(FM).Commercial broadcasting of FM began in 1936.Unit 6 Electronic Communications Although the fundamental concepts and principles of electronic communications have changed little since their inception,the methods and circuits used to implement them have undergone considerable change.In recent years,transistors and linear integrated circuits have simplified the design of electronic communications circuits,thus allowing for miniaturization,improved performance and reliability,and reduced overall costs.In recent years,there has been an overwhelming need for more and more people to communicate with each other.This tremendous need has stimulated a monumental growth in the electronic communications industry.Modern electronic communications systems include metallic cable systems,microwave and satellite radio systems,and optical fiber systems.Unit 6 Electronic Communications Unit 6 Electronic Communications Unit 6 Electronic Communications Unit 6 Electronic Communications A time chart showing the historical development of communications is given in Table 6.1.The reader is encouraged to spend some time studying this table to obtain appreciation for the chronology of communications.Note that although the telephone was developed late in the nineteenth century,the first transatlantic telephone cable was not completed until 1954.Previous to this date,transatlantic calls were handled via shortwave radio.Similarly,although the British began television broadcasting in 1936,transatlantic television relay was not possible until 1962 when the Telstar I satellite was placed into orbit.Digital transmission system embodied by telegraph systems-were developed in the 1850s before analog systems-the telephone-in the twentieth century.3Unit 6 Electronic Communications Unit 6 Electronic Communications Unit 6 Electronic Communications Unit 6 Electronic Communications 2.Electronic Communications SystemsCommunications Systems may be described by the block diagram shown in Figure 6.1.Regardless of the particular application,all communications systems involve three main subsystems:the transmitter,the channel,and the receiver.The message from the source is represented by the information input waveform m(t).The message delivered to the sink is denoted by.The“”indicates that the message received may not be the same as that transmitted.That is the message at the sink may be corrupted by noise in the channel or there may be other impairments in the system,such as undesired filtering or undesired nonlinearities.Unit 6 Electronic Communications The message information may be in analog or digital form,depending on the particular system,and it may represent audio,video,or some other type information.In multiplexed systems,there may be multiple input and output message sources and sinks.The spectra(or frequencies)of m(t)and are concentrated about f=0;consequently,they are said to be baseband signals.Unit 6 Electronic Communications Figure 6.1 Communication SystemUnit 6 Electronic Communications The signal-processing block at the transmitter conditions the source for more efficient transmission.4 For example,in an analog system,the signal processor may be an analog low-pass filter that is used to restrict the bandwidth of m(t).In a hybrid system,the signal processor may be an analog-to-digital converter(ADC).This produces a“digital word”that represents samples of the analog input signal.In this case,the ADC in the signal processor is providing source coding of the input signal.In addition,the signal processor may also add parity bits to the digital word to provide channel coding so that error detection and correction can be used by the signal processor in the receiver to reduce or eliminate bit errors that are caused by noise in the channel.5 The signal at the output of the transmitter signal processor is a baseband signal because it has concentrated nearf=0.Unit 6 Electronic Communications The transmitter carrier circuit converts the processed baseband signal into a frequency band that is appropriate for the transmission medium of the channel.For example,if the channel consists of a fiber optic cable,the carrier circuits convert the baseband input to light frequencies,and the transmitted signal s(t)is light.If the channel propagates baseband signals,no carrier circuits are needed,and s(t)can be output of the processing circuit at the transmitter.Carrier circuits are needed when the transmission channel is located in a band of frequencies around fc0.In this case,s(t)is said to be a bandpass because it is designed to have frequencies located in a band about fc.For example,an amplitude modulated(AM)broadcasting station with an assigned frequency of 850 kHz has a carrier frequency of fc=850 kHz.The mapping of the baseband input information waveform m(t)into the bandpass signal s(t)is called modulation.Unit 6 Electronic Communications 3.The Electromagnetic SpectrumIn communication systems that use the atmosphere for the transmission channel,interference and propagation conditions are strongly dependent on the transmission frequency.Theoretically,any type of modulation(e.g.,amplitude modulation,frequency modulation,single sideband,phase-shift keying,frequency-shift keying,etc.)could be used at any transmission frequency.However,to provide some semblance of order and for political reasons,government regulations specify the modulation type,bandwidth,and type of information that can be transmitted over designed frequency bands.Unit 6 Electronic Communications On an international basis,frequency assignments and technical standards are set by the International Telecommunications Union(ITU).The ITU is a specialized agency of the United Nations,and the ITU administrative headquarters are located in Geneva,Switzerland,with a staff of 700 persons.This staff is responsible for administering the agreements that have been ratified by the 184 member nations of the ITU.In 1992,the ITU was restructured into three sectors.The Radio Communication Sector(ITU-R)provides frequency assignments and is concerned with efficient use of the radio frequency spectrum.The Telecommunications Standardization Section(ITU-T)examines technical,operating,Unit 6 Electronic Communications and tariff questions.It recommends worldwide standards for the public telecommunications network(PTN)and related radio systems.The Telecommunication Development Sector(ITU-D)provides technical assistance,especially for developing countries.This encourages a full array of telecommunication services to be economically provided and integrated into the worldwide telecommunication system.Before 1992,the ITU was organized into two main sectors:the International Telegraph and Telephone Consultative Committee(CCITT)and the International Radio Consultative Committee(CCIR).Unit 6 Electronic Communications Each member nation of the ITU retains sovereignty over the spectral usages and standards adopted in its territory.However,each nation is expected to abide by the overall frequency plan and standards that are adopted by the ITU.Usually,each nation establishes an agency that is responsible for administration of the radio frequency assignments within its borders.In the United States,the FCC regulates and licenses radio systems for the general public,and state and local government.In addition,the National Telecommunication and Information Administration(NTIA)is responsible for U.S.government and U.S.military frequency assignments.The international frequency assignments are divided into subbands by the FCC to accommodate 70 categories of services and 9 million transmitters.Table 6.2 gives a general listing of frequency bands.Their common designations,typical propagation conditions,and typical services assigned to these bands.Unit 6 Electronic Communications Unit 6 Electronic Communications Unit 6 Electronic Communications 4.Bandwidth and Information CapacityThe two most significant limitations on the performance of a communications system are noise and bandwidth.Noise will be discussed later.The bandwidth of an information signal is simply the difference between the highest and lowest frequencies contained in the information,and the bandwidth of a communications channel is the difference between the highest and lowest frequencies that the channel will allow to pass through it(i.e.,its passband).The bandwidth of a communications channel must be large(wide)enough to pass all significant information frequencies.In other words,the bandwidth of the communications channel must be equal to or greater than the bandwidth of information.For example,voice frequencies contain signals between 300Hz and 3000 Hz.Unit 6 Electronic Communications Therefore,a voice-frequency channel must have a bandwidth equal to or greater than 2700 Hz.If a cable television transmission system has a passband from 500 kHz to 5000 kHz,it has a bandwidth of 4500 kHz.As a general rule,a communications channel cannot propagate a signal that contains a frequency that is changing at a rate greater than the bandwidth of the channel.Unit 6 Electronic Communications Information theory is a highly theoretical study of the efficient use of bandwidth to propagate information through electronic communications systems.Information theory can be used to determine the information capacity of a communications system.Information capacity is a measure of how much information can be transferred through a communications system in a given period of time.The amount of information that can be propagated through a transmission system is a function of system bandwidth and transmission time.In 1920,R.Hartley of Bell Telephone Laboratories developed the relationship among bandwidth,Unit 6 Electronic Communications transmission time,and information capacity.Hartleys law simply states that the wider the bandwidth and the longer the time of transmission,the more information that can be conveyed through the system.Mathematically,Hartleys law is stated as IBt,where I=information capacity,B=system bandwidth(Hz),t=transmission time(second).This equation shows that information capacity is a linear function and directly proportional to both system bandwidth and transmission time.If the bandwidth of a communications channel doubles,the amount of information can carry also doubles.If the transmission time increase or decrease,there is a proportional change in the amount of information that can be transferred through the system.Unit 6 Electronic Communications In general,the more complex the information signal,the more bandwidth required to transport it in a given period of time.Approximately 3 kHz of bandwidth is required to transmit voice-quality telephone signals.In contrast 200 kHz of bandwidth is allocated for commercial FM transmission of high-fidelity music,and almost 6 MHz of bandwidth is required for broadcast-quality television signals.Unit 6 Electronic Communications In 1948,C.E.Shannon(also of Bell Telephone Laboratories)published a paper in the Bell System Technical Journal relating the information capacity of a communications channel in bits-per-second(bps)to bandwidth and signal-to-noise ratio.Mathematically stated,the Shannon limit for information capacity iswhere I=information capacity(bps),B=bandwidth(Hz),S/N=signal-to-noise power ratio(unit-less).For a standard voice-band communications channel with a signal-to-noise power ratio of 1000(30 dB)and a bandwidth of 2.7 kHz,the Shannon limit for information capacity is I=26.9 kbps.Unit 6 Electronic Communications Shannons formula is often misunderstood.The results of the preceding example indicate that 26.9 kbps can be transferred through a 2.7 kHz channel.This may be true,but it cannot be done with a binary system.To achieve an information transmission rate of 26.9 kbps through a 2.7 kHz channel,each symbol transmitted must contain more than one bit of information.Therefore,to achieve the Shannon limit for information capacity,digital transmission system that have more than two output conditions(symbols)must be used.Unit 6 Electronic Communications Notes1 The fundamental purpose of an electronic communication system is to transfer information from one place to another.电子通信系统的基本作用是把信息从此地传送到彼地。information,intelligence,message 三词在通信技术中可能混用,都可译为信息、消息、情报等,但 information 似乎用得更普遍。例如,信息技术就只能用 information technology(IT)。Unit 6 Electronic Communications 2 The original source information can be in analog(continuous)form,such as the human voice or music,or in digital(discrete)form,such as binary-coded numbers or alphanumeric codes.原始的信息源既可以是模拟(连续)的,例如语音或音乐;也可以是数字(离散)的,例如二进制编码数或字符码。analog signal:时间和幅值皆连续分布的信号。discrete signal:时间离散分布的信号。digital signal:时间和幅值皆离散分布的信号。Unit 6 Electronic Communications 3 Digital transmission system-embodied by telegraph systems-were developed in the 1850s before analog systems-the telephone-in the twentieth century.以电报为代表的数字传输系统研发于19世纪50年代,早于以电话为代表的模拟系统,它是20世纪才开发的。4 The signal-processing block at the transmitter conditions the source for more efficient transmission.发射机中的信号处理单元对信源进行调理,以便更有效地传送。condition 是动词,作谓语,而非名词。for 引导目的状语。Unit 6 Electronic Communications 5 In addition,the signal processor may also add parity bits to the digital word to provide channel coding so that error detection and correction can be used by the signal processor in the receiver to reduce or eliminate bit errors that are caused by noise in the channel.此外,信号处理器还可以给数码字加入奇偶校验位,即提供信道编码,使得接收机的信号处理器可以进行误码检测和纠错,以减少或消除由信道噪声所引起的误码。parity bit 奇偶校验位。error detection and correction 误码检测和纠错。Unit 6 Electronic Communications Exercises1.Translate the following phrases into Chinese.(1)binary-coded number (2)Hi-Fi(3)electromagnetic induction (4)ADC(5)VLSI (6)block diagram (7)PSK (8)discrete signal(9)PTN Unit 6 Electronic Communications(10)a full array(11)amplitude modulation(AM)(12)negative-feedback amplifier(13)PCM(Pulse-code Modulation)(14)time-division multiplexing(TDM)(15)adaptive equalization (16)random access memory(RAM)(17)ISDN(Integrated Services Digital Network)(18)Los propagation Unit 6 Electronic Communications 2.Translate the following phrases into English.(1)真空三极管 (2)纠错编码(3)视线传播 (4)电离层反射(5)多路通信系统 (6)奇偶校验位(7)频率配置 (8)频率的整体规划(9)扩频系统 (10)高清晰度电视(11)带通信号 (12)单边带(13)国际电信联盟 (14)业余无线电(15)信噪比Unit 6 Electronic Communications 3.Answer the following questions.(1)When and by whom was the first electronic communications system developed?(2)What is the first amplifying device for electrical signals?(3)What systems are included in modern electronic communications systems?(4)Whats the difference between analog signal,discrete signal and digital signal?(5)Could you state Hartleys law and Shannons formula?Unit 6 Electronic Communications Passage B The Principle of PCMPCM is dependent on three separate operations:sampling,quantizing,and coding.Many different schemes for performing these three functions have evolved during recent years,and we shall describe the main ones.1 In these descriptions we shall see how a speech channel of telephone quality maybe conveyed as a series of amplitude values,each value being represented,that is,coded,as a sequence of 8 binary digits.2 Furthermore,we shall prove that a minimum theoretical sampling frequency of order 6.8 kilohertz(kHz)is required to convey a voice channel occupying the range 300 Hz to 3.4 kHz.3 Practical equipments,however,normally use a sampling rate of 8 kHz,and if 8-digits per sample value are used,the voice channel becomes represented by a stream of pulses with a repetition rate of 64 kHz.Figure 6.2 illustrates the sampling,quantizing,and coding processes.Unit 6 Electronic Communications Figure 6.2 The Sampling and Coding Processes,and the Resultant PCM SignalUnit 6 Electronic Communications The Sampling and Coding Processes,and the Resultant PCM SignalFigure 6.2Unit 6 Electronic Communications Figure 6.2 The Sampling and Coding Processes,and the Resultant PCM SignalUnit 6 Electronic Communications Reexamination of our simple example shows us that the speech signal of maximum frequency 3.4 kHz has been represented by a signal of frequency 64 kHz.However,if only 4 digits per sample value had been used,the quality of transmission would drop,and the repetition rate of the pulses would be reduced to 32 kHz.Thus the quality of transmission is dependent on the pulse repetition rate,and for digital communication systems these two variables may be interchanged most efficiently.Unit 6 Electronic Communications Digital transmission provides a powerful method for overcoming noisy environments.Noise can be introduced into transmission path in many different ways;perhaps via a nearby lightning strike,the sparking of a car ignition system,or the thermal low-level noise within the communication equipment itself.It is the relationship of the true signal to the noise signal,known as the signal-to-noise ratio,which is of most interest to the communication engineer.Basically,if the signal is very large compared to the noise level,then a perfect message can take place;however,this is not always the case.For example,the signal received from a satellite,located in far outer space,is very weak and is at a level only slightly above that of the noise.4 Alternative examples may be found within terrestrial systems where,although the message signal is strong,so is the noise power.Unit 6 Electronic Communications If we consider binary transmission,the complete information about a particular message will always be obtained by simply detecting the presence or absence of the pulse.By comparison,most other forms of transmission systems convey the message information using the shape,or level of the transmitted signal;parameters that are most easily affected by the noise and attenuation introduced by the transmission path.5 Consequently there is an inherent advantage for overcoming noisy environments by choosing digita1 transmission.Unit 6 Electronic Communications So far in this discussion we have assumed that each voice channel has a separate coder,the unit that converts sampled amplitude values to a set of pulses;and decoder,the unit that performs the reverse operation.This need not be so,and systems are in operation
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