1、 These low frequencies are called audio frequencies because the human ear can respond to corresponding sound frequencies in the same range. 通过将信息叠加在携带过程称为调制。 在无线电的情况下广播信息或消息一般由在20HZ到20000赫兹的范围内的低频率。 这些低频率被称为音频频率，因为人耳可以在同一范围内，以相应的声音频率响应。 Audio frequencies be themselves cannot travel long distance but
2、 when superimposed on the carrier frequency, they can cover the same distance as the carrier wave itself. A modulated wave is like an aeroplaneerplen carrying passengers who could not have reached their destination without the help of the aeroplane. In the case of television broadcasts the modulatio
3、n frequencies are called video frequencies which correspond to the visual information in the picture to be transmitted. 音频频率是本身不能长途跋涉，但是，从载波频率叠加时，它们可以覆盖相同的距离作为载波本身。 调制波是像飞机erplen运载乘客谁也无法达到他们的目的，而不飞机的帮助。 在电视的情况下，广播的调制频率被称为其在画面对应于视觉信息的视频频率要发送 For modulating a radio wave, the two important characterist
4、ics of the radio wave that can be varied are the amplitude and the frequency of the carrier wave. When the amplitude of the carrier is varied in accordance with the variation in the amplitude of the modulating signal （audio frequency）, the modulation is amplitude modulation （AM）. 用于调制的无线电波，无线电波的两个重要
5、特征是可以变化的幅度和载波的频率。 当载体的幅度而变化，根据在调制信号（音频）的振幅的变化中，调制是幅度调制（AM）。 If, however, the frequency of the carrier is varied in accordance with the variation in the frequency of modulating signal, the modulation is called frequency modulation （FM）. Both Amplitude Modulation and Frequency Modulation are used to r
6、adio and television broadcasts. In television the picture transmission are used amplitude modulation but sound transmission employs frequency modulation. 然而，如果所述载波的频率是变化的，根据在调制信号的频率的变化中，调制被称为频率调制（FM）。 振幅调制和频率调制用于广播和电视节目。在电视画面传输均采用调幅，但声音传输采用调频v Fading Fading is the name given to the phenomenon due to
7、 which the strength of the radio signal received at a point fluctuatesflktuet with time. The period of these variations in signal strength may be short or long. Fading is specially noticeable at night for frequencies between 500 and 1000KHZ and at distances of 160 to 1600km from the transmitter. 衰退
8、衰落是给由于它在点接收到的无线信号的强度波动flktuet随时间的现象的名称。 这些变体中的信号强度的期间可以是短或长。 衰落是特别明显的，晚上的500和1000 kHz之间的，并在160至1600公里的传输距离的频率。 Fading may be due to various causes but one of the most common causes is that the direct ray and indirect ray may reach a particular receiving point in opposite phase due to different dista
9、nces traveled by these rays. If the two rays are of comparable strength, they will tend to cancel each other out resulting in fading. 衰落可能是由于各种原因，但最常见的原因之一是，直接射线和间接光线可能达到以相反的相位的特定接收点由于由这些光线行进不同距离。 如果两个光线是相当的强度，它们将趋向于相互抵消导致褪色。 Selective fading results when the components of a modulated wave fade in a
10、 manner in which each is independent of the other. This results in distortion of a received signal as its waveform is not the same as that of the transmitted signal. Special methods of transmission and reception have to be adopted to avoid fading. 选择性衰落的结果时的调制波的衰落的方式的组件，其中每个都独立于其它的。 这导致所接收的信号的失真作为其波
11、形是不一样的是，发射的信号的。 已被采用的发送和接收的特殊方法，以避免褪色v Communication System In essence, electronic communications is the transmission, reception, and processing of information with the use of electronic circuits . The basic concepts involved in electronic communications have not changed much since their inception开始
12、, 开端, although the methods by which these concepts are implemented have undergone dramatic changes. 通信系统 从本质上说，电子通信是传输，接收，和处理信息与使用电子电路。 涉及电子通信的基本概念自成立以来开始，开端没有多大变化，虽然这些概念实现的方法已经发生了戏剧性的变化。 The simplest communications system comprises three primary sections: a source （transmitter）, a destination （rece
13、iver）, and a transmission medium （wire pair, coaxialkksl, 同轴 cable, fiber link 光纤链路, or free space）. 最简单的通信系统包括三个主要部分：一个源（发射机），目的地（接收机），和传输介质（线对，同轴电缆kksl，同轴电缆，光纤链路光纤链路，或游离空间）。 The information that is propagated through a communications system can be analog （proportional）, such as the human voice, vi
14、deo picture information, or music, or it can be digital pulses, such as binary-coded numbers, alphanumericlfnju:merk, 文字数字的 codes, graphic symbols, microprocessor op-codes, or data base information. However, very often the source information is unsuitable for transmission in its original form and th
15、erefore must be converted to a more suitable form prior to transmission. 即通过通信系统传播的信息可以是模拟的（比例），如人的声音，视频图像信息，或听音乐，或者它可以是数字脉冲，如二进制编码数字，字母数字lfnju：merk，文字数字的码，图形符号，微处理器操作码，或数据库信息。 然而，很多时候的源信息是不适合用于其原来的形式传输，因此必须被转换成在传输之前一个更合适的形式。 For example, with digital communications systems, analog information is co
16、nverted to digital form prior to transmission, and analog communications systems, digital data are converted to analog signals prior to transmission. This book is concerned with the basic concepts of conventional analog radio communications and the concepts of digital radio communications. 例如，对于数字通信
17、系统中，模拟信息转换成数字形式传输之前，和模拟通信系统，数字数据被转换为模拟信号传输之前。 这本书关注的是传统的模拟无线电通信的基本概念和数字无线电通信的概念。v Modulation and Demodulation For reasons they are explained in Chapter 10, it is impractical to propagate low-frequency electromagnetic energy through the earths atmosphere. Therefore, with radio communications, it is n
18、ecessary to superimpose a relatively low-frequency intelligence signal onto a relatively high-frequency signal for transmission. 调制和解调 至于原因，他们在第10章中解释，这是不切实际的，通过地球大气层传播的低频电磁能量。 因此，利用无线电通信，有必要叠加相对低频情报信号到一个相对高的频率的信号进行传输。 In electronic communications systems, the source information （intelligence signal
19、） acts upon or modulates a single-frequency sinusoidal snsdl signal. Modulate simply means to vary or change. Therefore, the source information is called the modulating signal, the signal that is acted upon （modulated） is called the carrier, and the resultant signal is called the modulated wave. In
20、essence, the source information is transported through the system on the carrier. 在电子通信系统中，源信息（情报信号）作用于或调制一个单频正弦snsdl信号。 调制只是意味着改变或变化。因此，源信息称为调制信号，即采取行动（调制）被称为载波的信号，并将所得信号被称为调制波。 在本质上，源信息是通过该系统的载波上传送。 With analog communications systems, modulation is the process of changing some property of an analo
21、g carrier in accordance with the original source information and then transmitting the modulate carrier. Conversely, demodulation is the process of converting the changes in the analog carrier back to the original source information. 与模拟通信系统中，调制是变化的根据原始源信息的模拟载体的某些属性，然后发送该调制的载波的过程。 相反地，解调是转换中的变化模拟载波回
22、原始源信息的处理。 The total or composite information signal that modulates the main carrier is called baseband. The baseband is converted from its original frequency band to a band more suitable for transmission through the communications system. Baseband signals are up-converted at the transmitter and down
23、-converted at the receiver. Frequency translation is the process of converting a frequency or band of frequencies to another location in the total frequency spectrum. 该调制主载波的总或复合信息信号称为基带。 基带从其原始频带转换为频带更适合通过通信系统中的传输。 基带信号被上变频在发射器和降频转换的接收器。 频率变换是在总频谱的频率的一个频率或带转换到另一个位置的过程。 A device that performs modula
24、tion is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator （sometimes detector or demod）. A device that can do both operations is a modem （from modulatordemodulator）. The aim of digital modulation is to transfer a digital bit stream over an
25、analog bandpass channel, for example over the public switched telephone network （where a bandpass filter limits the frequency range to between 300 and 3400 Hz）, or over a limited radio frequency band. 执行调制的装置被称为一个调制器，并且执行调制的逆操作称为一个解调器（有时检测器或解调）的装置。 一种设备，可以做两种操作是（来自“调制解调器”）调制解调器。 数字调制的目的是要传送一个数字位流在模拟
26、带通信道，例如通过公共交换电话网（其中一个带通滤波器限制的频率范围3400之间300和赫兹），或在有限的无线电频带。 The aim of analog modulation is to transfer an analog baseband （or lowpass） signal, for example an audio signal or TV signal, over an analog bandpass channel at a different frequency, for example over a limited radio frequency band or a cabl
27、e TV network channel. Analog and digital modulation facilitate frequency division multiplexing （FDM）, where several low pass information signals are transferred simultaneously over the same shared physical medium, using separate passband channels （several different carrier frequencies）. 模拟调制的目的是要以不同
28、的频率在一个有限的无线电频带或有线电视网络信道传送一个模拟基带（或低通）信号，例如音频信号或电视信号，通过一个模拟带通信道，例如。 模拟和数字调制促进频分复用（FDM），其中几个低通信息信号同时传输在相同的共享的物理介质，使用单独的通带信道（几个不同的载波频率）。 The aim of digital baseband modulation methods, also known as line coding, is to transfer a digital bit stream over a baseband channel, typically a non-filtered copper
29、 wire such as a serial bus or a wired local area network. The aim of pulse modulation methods is to transfer a narrowband analog signal, for example a phone call over a wideband baseband channel or, in some of the schemes, as a bit stream over another digital transmission system. 的数字基带调制方法，也被称为线路编码，其目的是在一个基带信道，典型地一个非过滤的铜导线传输的数字位流诸如串行总线或有线局域网。 的脉冲调制方法的目的是要传送一个窄频带的模拟信号，例如电话呼叫在宽带基带信道，或在某些方案中，由于对另一数字传输系统的比特流。 In music synthe
