通信工程文献翻译——对于现代人来说，如果不知道“数字通信”或“数字信号”，那就显得很落伍了，甚至会被人讥笑“孤陋寡闻”的。那么何为“数字信号”，何为“数字通信”呢？本文就来做一个简单浅显易懂的概括说明。

一、为什么要进行数字化

在现今，无论是军用还是商用，为什么通信系统都在进行“数字化”呢？这有许多原因，其中最重要的原因是，数字信号与模拟信号相比，更易于再生。如图1所示，是沿传输线传输的一种理想二进制数字脉冲。其波形的形状要受到两个基本因素的影响：第一，所有传输线和电路都具有非理想的频率传递函数，因而使理想脉冲产生了失真；第二，电子噪声或其他干扰进一步使脉冲波形产生失真。这两种失真机理实际线路长度的函数，都会引起脉冲形状发生畸变。还来看图1，在传输脉冲恶化到模糊状态之前，传输脉冲仍可被可靠的识别，由数字放大器将脉冲放大并恢复最初的理想的形状，脉冲就这样“再生”了。在传输系统中，在规定时间间隔内执行这种功能的电路成为“再生中继器”。

图1  脉冲编码再生

与模拟电路相比，数字电路不易产生失真和干扰。因为二进制数字电路工作在全通或全断的开关状态下才有意义，所以干扰必须足够大才能使电路从一种状态变到另一种状态。这两种工作状态有助于信号的再生，因而能在传输中有效地抑制噪声和其他累积干扰。然而模拟信号不是双态信号，它的波形有无限多个。在模拟电路中，即使很小的干扰也能导致信号产生难以接受的失真。失真一旦产生，就无法通过放大器去驱除。由于模拟信号和累积噪声密不可分，所以不能完全再生。若采用数字技术，通过检错和纠错可获得极低的差错概率，从而高保真信号，而模拟系统没有这样类似的技术。

数字通信系统还有其他的优点：数字电路比模拟电路更可靠，且其生产成本比模拟电路低；数字硬件比模拟硬件更具灵活性，比如微处理器、数字开关、大规模集成电路等；时分复用（TDM）信号比频分复用（FDM）信号的模拟信号更简单；不同类型的数字信号（数据、电报、电话、电视等）在传输和交换中都被看成是相同的信号——比特信号；为方便交换，还可将数字信号以数据包（packet）的形式进行处理。数字技术本身借助与信号出处理，具有抗人为干扰和自然干扰的功能，还能够提供加密和隐私处理。计算机与计算机之间、数字设备或终端与计算机之间的数据通信需求越来越多，这些数字终端可以通过数字通信链路获得最好的服务。

数字通信系统获得这些优点的代价是什么呢？与模拟系统相比，数字系统侧重于信号处理技术，并在通信的各个阶段，都需要分配一部分共享的资源用于实现同步。而在模拟系统中，同步相对比较容易。数字通信系统的另一个缺点是具有门限效应。即当信噪比下降到一定限度......

For modern people, if you do not know “Digital Communication” or “Digital Signal”, it is seem to be outdated, even would be laughed as “a ignorant men” .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 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......