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System For modern people, if you do not know “Digital Communication” or “Digital Signal”, it is seem to be outdated, even would be laughed as “a ignorant men” .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 5Original Some signal Degraded Signal is badly Amplification dstortiuon dstortiuon dstortiuon degraded regenerate pulse Propagation distance Figure 1 Pulse degradation and regenerationDigital 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 two-state operation facilitates signal regeneration and thus prevents noise and other disturbances from accumulating in transmission. Analog signals，however，are not two-state signals; they can take an infinite variety of shapes. With analog circuits，even a small disturbance can render the reproduced waveform unacceptably distorted. Once the analog signal is distorted，the distortion cannot be removed by amplification. Because accumulated noise is irrevocably bound to analog signals，they cannot be perfectly regenerated. With digital techniques，extremely low error rates produres high signal fidelity are possible through error detection and correction but similar procedures are not available with analog.There are other important advantages to digital communications. Digital circuit are more reliable and can be produced at a lower cost than analog circuit . Also, digital hardwave lends itself to more flexble implementation than analog hardwave (e.g., microprocessor, digital switching, and large-scale integrated (LSI) circuit). The combing of digital signals using time-division multiplexing (TDM) is simpler than the combing of analog signals using frequcency-division multiplexing (FDM). Different types of digital signals (data, telegraph, telephone, television) can be treated as identical signals in transmission and switching -a bit is a bit. Also, for convenient switching, digital messages can be handled in autonomous groups called packets. Digital techniques lend themselves naturally to signal processing functions that project against interferene and jamming,or that provide encryption and privacy. Also much data communication is from computer to computer, or from digital instruments or terminal to computer. Such digital terminations are naturally best served by digital communication links.What are the costs associated with the beneficial attributes of digital cimmunication system? Digital system tend to be very signal-processing intensive compared with analog. Also, digital system need to allocate a significant share of their resources to the task of synchroniztion at various levels . With analog system , on the other hand , synchroniztion often is accomplished more easily. One disadvantage of a digital communication system is non-graceful degradation. When the signal-to-noise ratio drops below a certain threshold, the quality of service can change suddenly from very poor. In cintrast, most communication aystem degrade more gracefully.2、Typical Blook Diagram and Transformations The function block diagram shown in Figure 2 illustrates the signal flow and the signal-processing steps through a typical digital communication system (DCS). This figure can serve as a kind of road map, guiding the reader through the chapter. The upper blocks - format, source encode, encrypt, channel encode, multiplex ,pulse modulate, bandpass modulate, frequency spread, and multiple access-denote signal transformations from t