reading 1(1)

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Reading 1

(1)The telecommunications age is upon us. In our private lives, we listen to the radio, watch television, and talk on the telephone regularly. To enhance our television viewing, we may have cable TV or a satellite dish in the yard. Further, we may have a mobile telephone in the car, and we may connect our personal computer with other computers over telephone lines using a modem. The list of communication services available to us is seemingly endless and growing almost daily, and the demand for expanded communication services continues to be high.

电信时代正在向我们走来。在我们的私人生活，我们定期的听收音机，看电视，打电话。为了提高我们的电视画面，我们可能有有线电视，或在院子里接一个卫星天线。此外，我们可以在车上使用移动电话，并且，我们可以通过使用调制解调器，用电话线将个人电脑和其它计算机连接起来。提供给我们的通信服务的列表每天是无休止的生长的，并且用于扩展通信服务的需求仍然很高。

(2)The field "communications" involves many disciplines, from signal processing, detection theory, estimation theory, information theory, and coding theory to switching, transmission and terminal design. Indeed, it is this great diversity of disciplines that makes working in the communications field both challenging and exciting.

在这个“通信”领域涉及许多学科，从信号处理，检测理论，估计理论，信息论，编码理论到交换，传输和终端设计。事实上，正是这种巨大的差异使得在通信领域的工作变成同时具有令人兴奋和挑战性的工作。

Fig. 2.1

(3)Here，We focus on the very general communication system block diagram shown in Fig. 2.1. The source block represents messages to be transmitted, such as a speech signal, a color television signal, a stream of binary ones and zeros, or even an electrocardiogram signal. We assume that the source output is an electrical signal of some kind, so the transducer that changes these quantities into an electrical waveform, such as a microphone or cameras, is included within the source block. 在这里，我们专注于在图中所示的非常一般的通信系统框图。2.1。源块表示要传送的消息，如一个话音信号，一个彩色电视信号，二进制一和零，或甚至一个心电图信号流。我们假定源输出是某种类型的电信号，以便改变这些量转换成电波形，诸如麦克风或摄像机传感器，包括源块内。

(4)The transmitter operates on the source output and prepares it for propagation over the channel. The channel may consist of a pair of wires, coaxial cable, optical fibers, a mobile radio channel, among many other possibilities. The receiver attempts to retrieve the source output from the channel output and presents the resulting signal to the user, which may be the human eye or ear, a remote controlled device, or a computer.．

发射机运行在源输出并准备将其传播通过的通道。该通道可包括一对导线，同轴电缆，光纤，一个移动无线电信道，以及许多其他的可能性的。接收机试图检索从信道输出的输出源，并

提出将所得信号提供给用户，这可能是人的眼睛或耳朵，远程控制设备，或计算机..

(5) The input to the user is assumed to be an electrical signal, and therefore the user block may also contain a transducer, such as a speaker or display unit, so that the received electrical signals can be returned to its “original” physical form.

输入到用户被假定为电信号，并且因此用户块还可以包含一个换能器，诸如扬声器或显示单元，从而使所接收的电信号可被返回到其“原始”的物理形式..

(6) To carry our development a little further, we can expand the block diagram in Fig.2.1 as shown in Fig.2.2. Each block can represent many operations and every communication system does not have all of the blocks. However, the source, channel, and user blocks are always present.

要执行我们的发展远一点，我们可以扩展如图图2.2图2.1中的框图。每个块可以代表许多操作和各通信系统不具有所有块。然而，源极，信道和用户块是始终存在。

Transmitter Receiver

Fig. 2.2

(7) Although communication systems can be classified in a wide variety of ways, a communication system is usually classified as analog or digital, depending on whether the signal transmitted over the channel is analog or discrete, respectively. An analog signal can take an infinite number of possible amplitudes in a fixed range, while a discrete signal can take only a finite number of amplitudes in a fixed range. This distinction becomes imprecise for those communication systems that transmit discrete amplitude signals using analog modulation methods.

虽然通信系统可归类于多种方式，一个通信系统通常分类为模拟或数字的，这取决于在该信道上发送的信号是模拟的或离散的，分别。模拟信号可采取的可能的振幅无限数目在一个固定的范围内，而离散信号可以在一个固定的范围内的振幅只有有限数目。这种区分变得不精确对于传输使用模拟调制方法离散幅度信号的通信系统。

(8) Speech and images, are analog signals. However, for numerous applications there may be advantages to changing these signals into digital form prior to transmission. The source encoder block represents such an operation. A source encoder can also be used to encode an existing digital signal more efficiently.

语音和图像，是模拟信号。然而，对于许多应用，可能是有利的改变这些信号转换成数字形式传输之前。源编码块表示这样的操作。源编码器，也可以用于现有的数字信号更有效地进行编码。

(9) The source decoder block returns the source signal to its original form as nearly as is possible. An example of a system that uses source encoding is the medium-to long- distance transmission of telephone speech via pulse code modulation (PCM). Most of the communication systems contain the modulator and demodulator blocks but do not include the source encoder, channel encoder, and source decoder blocks.

源解码器块返回源信号到其原始形式，几乎是可能的。使用源编码系统的一个例子是中等至通过脉冲编码调制（PCM ）长电话语音的传输距离。大多数的通信系统的包含在调制器和解调器的块，但不包括源编码器，信道编码器和信源解码器块。

(10) The modulator changes the signal into a form suitable for transmission over the channel,