英文外文翻译数字通信系统.doc

英文资料及中文翻译 Digital Communication System For modern people, if you do not know “Digital Communication” or “Digital Signal”, it is seem to be outdated, even would be laughed as “a ignorant men” . well then ,what is the “Digital Communication” and what is the “Digital Signal” ? This text is to do an generalize in the way of brief and plain. 1、Why Digital? Why are communication system，military and commercial alike，“going digital”？The primary advantage is the ease with which digital signals，compared with analog signals，are regenerated. Figure 1 illustrates an ideal binary digital pulse propagating along a transmission line. The shape of the waveform is affected by two basic mechanisms: (1) as all transmission lines and circuits have some nonideal frequence transfer function，there is a distorting effect on the ideal and pulse; and (2) unwanted electrical noise or other interference further distorts the pulse waveform. Both of these mechanisms cause the pulse shape to degrade as a function of line lenge，as shown in Figure 1. During the time that the transmitted pulse can still be reliably identified before it is degraded to an ambiguous state，the pulse is amplified by a digital amplifier that recovers its original ideal shape. The pulse is thus “rebor” or regenerated. Circuits that perform this function at regular intervals along a transmission system are called regenerative repeaters. Distance 1 Distance 2 Dance 3 stance 4 Distance 5 Original Some signal Degraded Signal is badly Amplification dstortiuon dstortiuon dstortiuon degraded regenerate pulse Propagation distance Figure 1 Pulse degradation and regeneration Digital circuit are less subject to distortion and interfernce than analog circuit . Becanse binary circuits opoerate in one of two states fully on fully of —to be meaningful，a disturbance must be large enough to change the circuit ooperating point from one state to the other. Such