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1、,ENGLISH ON TELECOMMUNICATIONS,课程名称：通信专业外语,UNIT 1 The Principle of PCM,1,2,3,4,UNIT 5 Introduction to Optical Fiber Communication,UNIT 4 Internet,UNIT 2 Asynchronous Serial Data Transmission,5,6,7,UNIT 10 GSM,UNIT 8 Cellular Mobile Telephone System,UNIT 7 WDM,UNIT 11 Circuit Switching and Packet Swi
2、tching,11,UNIT 16 Current Situation and the Future In the Telecommunication World,16,1,The Principle of PCM,Unit 1 The Principle of PCM NEW WORDS AND PHRASESprinciple n. 原理 be dependent on 依赖，取决于 sample v. 采样 quantize v. 量化，分层 code v. 编码 n.码 scheme n. 方案，设计，安排describe vt叙述，描述 description n叙述，描述ampli
3、tude n幅，幅度binary a二进制的minimum n最小值，最小量,1,The Principle of PCM,Unit 1 The Principle of PCM NEW WORDS AND PHRASEStheoretical a理论上的repetition n重复，反复reexamination n再审查，重考maximum n最大值reduce vt；vi减少，缩小interchange y互换，转换，相互影响method n方式方法，手段 overcome vt克服，打败，征服environment n环境，周围情况lightning n电光，闪电，雷电strike v
4、击，敲，打spark v发火花，打火，闪光,1,The Principle of PCM,Unit 1 The Principle of PCM NEW WORDS AND PHRASESignition n点火，点火装置signal-to-noise ratio 信噪比satellite n卫星terrestrial a地球的，地面的，大地的by comparison 比较起来，相对之下parameter prmit n参数，系数 technical parameter 技术参数 parameter estimation 参数估计 key parameter 关键参数,1,The Princ
5、iple of PCM,Unit 1 The Principle of PCM NEW WORDS AND PHRASESattenuation n衰减，衰耗attenuation coefficient 衰减系数attenuation distortion 衰减失真inherent a固有的，内在的 assume vt假设，假定decoder n解(译)码器codec n编译码器,1,The Principle of PCM,Unit 1 The Principle of PCM NEW WORDS AND PHRASESinterleave vt交插，交错，插接appropriate a适
6、当的，合适的unique a惟一的，独特的 unique solution 唯一解 In some ways, each of us is like ervey person; and in some ways, each of us is unique.reoccur vi再发生，再次发生,1,PCM is dependent on three separate operations, sampling, quantizing, and coding. Many different schemes for performing these three functions have evolv
7、ed during recent years, and we shall describe the main ones. In these descriptions we shall see how a speech channel of telephone quality may be conveyed as a series of amplitude values, each value being represented, that is, coded,as a sequence of 8 binary digits.,PASSAGE,1,The Principle of PCM,1,F
8、urthermore, 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. Practical equipments, however, normally use a sampling rate of 8 kHz, and if 8-digits per sample value are used, the voic
9、e channel becomes represented by a stream of pulses with a repetition rate of 64 kHz. Fig. 1-1 illustrates the sampling, quantizing, and codingprocesses.,PASSAGE,1,The Principle of PCM,1,Reexamination of our simple example shows us that the speech signal of maximum frequency 3.4 kHz has been represe
10、nted 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 commun
11、ication systems these two variables may be interchanged most efficiently.,PASSAGE,2,The Principle of PCM,1,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,
12、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 the most interest to the communicationengineer.,PASSAGE,3,The Principle of P
13、CM,1,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. A
14、lternative examples may be found within terrestrial systems where, although the message signal is strong, so isthe noise power .,PASSAGE,3,The Principle of PCM,1,If we consider binary transmission, the complete information about a particular message will always be obtained by simply detecting the pr
15、esence 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. Consequently there is
16、 an inherent advantage for overcoming noisy environments by choosing digitaltransmission.,PASSAGE,4,The Principle of PCM,1,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
17、 that performs the reverse operation. This need not be so, and systems are in operation where a single codec (i. e., coder and its associated decoder) is shared between 24, 30, or even 120 separate channels. A high-speed electronic switch is used to present the analog information signal of each channel, taken in turn, to the codec.,
