第二章第一篇To say that we live in an age of electronics is an understatement. From the omnipresent integrated circuit to the equally omnipresent digital computer, we encounter electronic devices and systems on a daily basis. In every aspect of our increasingly technological society— whether it is science, engineering, medicine, music, maintenance, or even espionage—the role of electronics is large, and it is growing.谈论关于我们生活在一个电子学时代的论调是一种空泛的论调。
从无处不在的集成电路到同样无处不在的数字计算机，我们在日常活动中总会遇到电子设备和电子系统。
在我们日益发展的科技社会的方方面面——无论是在科学、工程、医药、音乐、维修方面甚至是在谍报方面——电子学的作用是巨大的，而且还将不断增强。
In general, all of the tasks with which we shall be concerned can be classified as "signal-processing“tasks. Let us explore the meaning of this term一般说来，我们将要涉及到的工作被归结为“信号——处理”工作，让我们来探究这个术语的含义吧。
A signal is any physical variable whose magnitude or variation with time contains information. This information might involve speech and music, as in radio broadcasting, a physical quantity such as the temperature of the air in a room, or numerical data, such as the record of stock market transactions. The physical variables that can carry information in an electrical system are voltage and current. When we speak of "signals", therefore, we refer implicitly to voltages or currents. However, most of the concepts we discuss can be applied directly to systems with different information-carrying variables. Thus, the behavior of a mechanical system (in which force and velocity are the variables) or a hydraulic system (in which pressure and flow rate are the variables) can often be modeled or represented by an equivalent electrical system. An understanding of the behavior of electrical systems, therefore, provides a basis for understanding a much broader range of phenomena. 信号就是其与时间有关的量值或变化包含信息的任何物理变量。
这种信息或许像无线电广播的演讲和音乐，或许是像室内温度的物理量，或许像股市交易记录的数字数据。
在电气系统中能够载有信息的物理变量是电压和电流。
因此当我们谈到“信号”，我们不言而喻指的是电压和电流，然而，我们要讨论的大多数概念是可以被直接应用于载有不同信息的变量的系统，因此，一个机械系统（在这个系统中力和速度是其变量）或者液压系统（在这个系统中压力和流速是其变量）的性能通常可以用一个等效的电气系统来模拟或表示。
因此，我们对于电气系统性能的理解为理解更宽领域的现象打下了一个基础。
A signal can carry information in two different forms. In an analog signal the continuous variation of the voltage or current with time carries the information. An example, in Fig.2-l, is the voltage produced by a thermocouple pair when the two junctions are at different temperatures. As the temperature difference between the two junctions varies, the magnitude of the voltage across the thermocouple pair also varies. The voltage thus provides an analog representation of the temperature difference.一个信号可以以两种形式来承载信息。
在一个模拟信号中电压或电流随时间而产生的连续变化载有信息。
在图2-1中，当一对热电偶的接头处于不同的温度时由热电偶所产生的电压就是一个例子。
当两个接头之间的温度差改变时，一对热电偶两端的电压也将改变。
于是电压就提供了温度差的模拟表现形式The other kind of signal is a digital signal. A digital signal is one that can take on values within two discrete ranges. Such signals are used to represent ON-OFF or YES-NO information. An ordinary household thermostat delivers a digital signal tocontrol the furnace. When theroom temperature drops below a preset value, the thermostat switch closes turning on the furnace. Once the room temperature rises high enough, the switch opens turning off the furnace. The current through the switch provides a digital representation of the temperature variation: ON equals "too cold" while OFF equals "not too cold".另一种的信号是数字信号。
数字信号是在两个离散的范围内能够呈现一定数值的信号。
这种信号常用以表示“开—关”或“是—不是”信息。
一个普通的家用恒温器传递一种数字信号来控制炉子当房间的温度下降到预定温度以下时，恒温器的开关合上使炉子开始加热；一旦房间的温度上升到足够高，开关就断开使炉子关闭。
流过开关的电流提供了温度变化的数字表示：ON即为“太冷”而OFF即为“不太冷”A signal-processing system is an interconnection of components and devices that can accept an input signal or a group of input signals, operate on the signals in some fashion either to extract or improve the quality of the information, and present the information as an output in the proper form at the proper time.一个信号处理系统是某些元件或设备之间的相互连接，这些元件和设备能够接收一个输入信号或一组输入信号，信号处理系统以某种方式来处理这些信号即提取这些信号或提高这些信号的品质，然后在适当的时间以适当的形式把这个信号表示为输出量。
Fig.2-2 illustrates the components in such a system. The central circles represent the two types of signal processing (digital and analog), while theblock between the two signal- processing blocks represents the conversion of an analog signal to equivalent digital form (A/D=Analog-to-Digital) and the reverse conversion of a digital signal to the corresponding analog form (D/A=Digital-to-Analog). The remaining blocks involve inputs and outputs— getting signals into and out of the processing system.图2-2显示了这样一个系统的组成部分。