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"通信英语The_Principle_of_PCM译文汇编.pdf" PCM（Pulse Code Modulation，脉冲编码调制）是一种常用的数字信号处理技术，广泛应用于通信领域。PCM 的工作原理基于三个基本操作：采样、量化和编码。采样是PCM 的第一步骤，采样率是指每秒钟采样次数。 PCM 采样率的选择取决于信号的频率范围。在本文中，我们看到PCM 的采样率为 8 kHz，这是因为人类听觉系统最敏感的频率范围为 300 Hz 到 3.4 kHz。量化是 PCM 的第二步骤，量化是指将连续信号转换成离散信号的过程。在 PCM 中，量化是通过将信号的幅值转换成数字信号来实现的。量化的精度取决于量化的位数，在本文中，我们看到每个样值使用 8 位二进制数来表示。编码是 PCM 的第三步骤，编码是指将量化后的数字信号转换成二进制数字信号的过程。在 PCM 中，编码是通过对信号的幅值进行量化和编码来实现的。在 PCM 中，采样、量化和编码三个步骤是紧密相连的。采样率的选择取决于信号的频率范围，而量化的精度取决于量化的位数。编码的结果取决于采样率和量化的精度。在数字通信系统中，PCM 是一种非常重要的技术，它提供了一种强大的方法来克服噪音环境的干扰。 PCM 的应用非常广泛，包括电话通信、数据传输、音频压缩等领域。在 PCM 中，脉冲重复率是一个非常重要的参数，它决定了传输质量的高低。脉冲重复率越高，传输质量越高。因此，在设计 PCM 系统时，需要选择合适的采样率、量化精度和脉冲重复率以确保传输质量。此外，PCM 还有很多其他的应用领域，包括图像压缩、语音识别、数据压缩等。这些应用领域都需要对 PCM 的原理和技术进行深入的了解和应用。 PCM 是一种非常重要的数字信号处理技术，它在通信领域有着广泛的应用。对 PCM 的原理和技术的了解和应用是非常必要的，以便更好地设计和实现数字通信系统。

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UNIT 1

The Principle of PCM

PCM 原理

Pcm is dependent on three separate operations, sampling, quantizing, and coding. Many

different schemes for performing these three functions have evolved during recent years, and we

shall describe the main ones. In these descriptions we shall see how a speech channel of telephone

quality may be conveyed as a series of amplitude values, each value being represented, that is,

coded as a sequence of 8 binary digits. Furthermore, we shall prove that a minimum theoretical

sampling frequency of order 6.8 kilohertz(khz) is required to convey a voice channel occupying

the range 300 HZ to 3.4 Khz. Practical equipments, however, normally use3 a sampling rate of 8

khz, and if 8-digits per sample value are used, the voice channel becomes represented by a stream

of pulses with a repetition rate of 64khz. Fig .1-1 illustrates the sampling, quantizing, and coding

processes.

PCM 的构成依赖于三个环节，即采样、量化和编码。近年来，人们对这三个环节的实

现提出了许多不同的方案，我们将对其中的一些主要的方案进行讨论。在这些讨论中，我们

会看到话路中的语声信号是如何转换成幅值序列的，而每个幅值又被编码，即以 8 位二进制

数的序列表示。而且，我们将证明，为了转换频率范围为 300HZ—3.4KHZ 的话路信号，理

论上最小采样频率须为 6.8khz。但是，实际设备通常用 8khz 的采样速率，而如果每个样值

用 8 位码的话，则话路是由一个重复速率为 64khz 的脉冲流来表示的。图 1-1 表示了采样、

量化、编码的过程。

Reexamination of our simple example shows us that the speech signal of maximum

frequency 3.4khz has been represented by a signal of frequency 64khz. However, if only 4-digits

per sample value had been used, the quality of transmission would drop, and the repetition rate of

the pulses would be reduced to 32khz. Thus the quality of transmission is dependent on the pulse

repetition rate, and for digital communication systems these two variables may be interchanged

most efficiently.

让我们再研究一下上面提到的简单例子。可以看出，最高频率为 3.4khz 的话音信号适

用 64khz 的（脉冲流）信号来表示的。但是，如果每个样值中用 4 位（码）表示，则传输质

量会下降，而脉冲的重复速率也将减小到 32khz。因而传输质量是取决于脉冲重复速率的。

对于数字通信系统，这两个量之间极明显的互相影响着。

Digital transmission provides a powerful method for overcoming noisy environments. Noise

can be introduced into transmission patch in many different ways : perhaps via a nearby lightning

strike, the sparking of a car ignition system, or the thermal low-level noise within the

communication equipment itself. It is the relationship of the true signal to the noise signal, known

as the signal-to-noise ratio, which is of the most interest to the communication engineer. Basically,

if the signal is very large compared to the noise level, then a perfect message can take place;

however, this is not always the case. For example, the signal received from a satellite, located in

far outer space, is very weak and is at a level only slightly above that of the noise. Alternative

examples may be found within terrestrial systems where, although the message signal is strong, so

is the noise power.

数字传输对于克服噪声环境的影响提供了一个强有力的手段。噪声可以以多种不同方式

进入传输信道，比如说因为附近的闪电、汽车点火系统的打火或因通信设备本身低电平的热

噪声所致。正是这种被称为信噪比的东西，即真实信号与噪声的关系引起了通信工程师的极

大的兴趣。从本质上讲，如果信号比噪声电平大得多，则信息的传输是完美的。但是，实际

情况并不总是这样，例如，从位于遥远太空中的卫星接收到的信号极其微弱，其电平仅比噪

声稍高一点。地面系统则是另一类例子，尽管信号很强，噪声也很强。

If we consider binary transmission, the complete information about a particular message will

always be obtained by simply detecting the presence or absence of the pulse. By comparison, most

other forms of transmission systems convey the message information using the shape, or level of

the transmitted signal; parameters that are most easily affected by the noise and attenuation

introduced by the transmission path. Consequently there is an inherent advantage for overcoming

noisy environments by choosing digital transmission.

研究二进制信号的传输可见，只要简单的区判别脉冲的有和无，完美就获得了一条消息

的全部信息。相比之下，许多其他形式的传输系统是利用被传信号的波形或电平的高低来传

送信息的，而这些参数又极易受到传输途径中的噪声和衰耗的影响。因此选择数字传输系统

在克服噪声环境的影响方面有其固有的优势。

So far in this discussion we have assumed that each voice channel has a separate coder, the

unit that converts sampled amplitude values to a set of pulses; and decoder, the unit that performs

the reverse operation. This need not be so, and systems are in operation where a single codec (i.e.,

coder and its associated decoder) is shared between 24, 30, or even 120 separate channels. A

high-sped electronic switch is used to present the analog information signal of each channel, taken

in turn, to the codec. The codec is then arranged to sequentially sample the amplitude value, and

code this value into the 8-digit sequence. Thus the output to the codec may be seen as a sequence

of 8 pulses relating to channel 1, then channel 2, and so on. This unit is called a time division

multiplexer (TDM), and is illustrated in fig. 1-2. The multiplexing principle that is used is known

as work interleaving. Since the words, or 8-digit sequences, are interleaved in time.

到目前为止，在这个讨论中，我们一直假定每个话路各有一个编码器和解码器。前者是

将幅度采样值变换成脉冲，而后者则施行相反的变换，这种设置并非必须。在实际运行的

PCM 系统中，一个编、译码器为 24 路、30 路，甚至 120 路所共用（注：在当代的 PCM 设

备中，编、译码器系分路设备，即每个话路各有一套。）一个高速的电子开关被用来将每一

话路的模拟信号依次的送往编、译码器。然后编、译码器再顺序采样幅值并把这个幅值编成

8 位码序列。这样，编解码器输出的 8 位码序列就分别对应于话路 1、话路 2，等等。这种

设备称为时分复用（TDM）,如图 1-2 所示。由于 8 位码的码字序列按时间顺序插接在一起，

所以上面所用的复用原则称为码字插接。

At the receive terminal a demultiplexer is arranged to separate the 8-digit sequences into the

appropriate channels. The reader may ask, how does the demultiplexer know which group of

8-digits relates to channel 1,2, and so on? Clearly this is important! The problem is easily

overcome by specifying a frame format, where at the start of each frame a unique sequence of

pulses called the frame code, or synchronization word, is placed so at to identify the start of the

frame. A circuit of the demultiplexer is arranged to detect the synchronization word, and thereby it

knows that the next group of 8-digits corresponds to channel 1. The synchronization word

reoccurs once again after the last channel has been received.

接收端设置了分路设备将 8 位码序列分配到相应的话路中。读者也许会问，分路设备怎

么会知道哪一组 8 位码对应于第 1 路、第 2 路及其他各路呢？显然这是很重要的。这个问题

是很容易解决的。我们只要制定一个帧格式，即在每一帧的开始放置一个被称作帧码或同步

字的独特码序列以标志每帧的起始，而用分路设备的一个电路去检测同步字，从而就知道下

一个 8 位码组对应于话路 1。当最后一个话路的码字收到之后，同步码又再次出现。

UNIT 2

Asynchronous Serial Data Transmission

异步串行数据传输

By far the most popular serial interface between a computer and its CRT terminal is the

asynchronous serial interface. This interface is so called because the transmitted data and the

received data are not synchronized over any extended period and therefore no special means of

synchronizing the clocks at the transmitter and receiver is necessary. In fact ,the asynchronous

serial data link is a very old form of data transmission system and has its origin in the era of

teleprinter.

在计算机及其显示器之间最为常见的串行接口是异步串行接口。这个接口之所以如此称

呼，是因为无论在多长的时间区间里发送的数据和接收的数据是不同步的，因而没有必要采

用特殊的手段使发送器和接收器的时钟同步。实际上，异步串行数据链路是一种古老的数据

传输方式，它起源于电传打字机的时代。

Serial data transmission systems have been around for a long time and are found in the

telephone (human speech), Morse code, semaphore, and even the smoke signals one used by

native Americans. The fundamental problem encountered by all serial data transmission systems is

how to split the incoming data steam into individual units (i.e., bits) and how to group these units

into characters. For example, in Morse code the dots and dashes of a character are separated by an

intersymbol space, while the individual characters are separated by an intercharacter space, which

is three times the duration of an intersymbol space.

串行数据传输系统已有很长的历史了，电话（人类语音）、莫尔斯电码、旗语，甚至土

著美洲人从前用过的烟火信号都可以视为传些数据传输。所有的串行数据传输系统面临的首

要的问题都是如何把如数的数据流分开为单独的码元（即比特），以及怎样把这些码元组合

成字符。例如，在莫尔斯电码中，字的点、划是由符号间的空格来分开的，而字符之间又是

由字符间的空格分开的，字符间空格的时长是点、划间空格的三倍。

First we examine how the data stream is divided into individual bits and the bits grouped into

characters in an asynchronous serial data link. The key to the operation of this type of link is both

simple and ingenious. Fig.2-1 gives the format of data transmitted over such a link.

首先我们研究一下在异步串行数据链路中数据流是怎样分成单独码元的，以及码元是如

何组成字符的。这类系统运行的核心原理既简单有精巧。图 2-1 绘出了在这个链路中传送的

数据格式。

An asynchronous serial data link is said to be character-oriented, as information is

transmitted in the form of groups of bits called characters. These characters are invariable units

comprising 7 or 8 bits of “information” plus 2 to 4 control bits and frequently correspond to

ASCII-encoded characters. Initially ,when no information is being transmitted, the line is in an

idle state. Traditionally, the idle state is referred to as the mark level. By convention this

corresponds to a logical 1 level.

异步串行数据链路被称为面向字符的，因为信息是以被称作字符的比特组的形式传送

的。这些字符是一些故此能够的单元，每个单元都包含 7 或 8 个信息比特加上 2-4 个控制比

特，并通常与 ASCII 码的字符一致。当传输开始，无信息发送时，线路处于空闲状态，而

空闲状态习惯上被称为信号电平。通常它对应于逻辑 1 电平。

When the transmitter wishes to send data, it first places the line in a space level (i.e., the

complement of a mark ) for one element period. This element is called the start bit and has a

duration of T seconds. The transmitter then sends the character, 1 bit at a time, by placing each

successive bit on the line for a duration of T seconds, until all bits have been transmitted. Then a

single parity bit is calculated by the transmitter and sent after the data bits. Finally, the transmitter

sends a stop bit at a mark level (i.e., the same level at the idle state) for one or two bit periods.

Now the transmitter may send another character whenever it wishes.

当发送器想要发送数据时，它首先将线路置成空号电平（即信号的反码），此电平持续

一个单元（码元）的间隔时间。此（空号）单元称为起始位，持续时间为 T 秒。然后发送

器发送字符，一次一个比特的将相继的码元送上线路。每个码元持续 T 秒，直到所有码元

发完为止。此后发送器计算得出一个奇偶校验位并将它在数据码元之后发出。最后，发送器

送出一个停止位，其电平为信号电平（于空闲状态电平相同），时长为 1 个或 2 个比特宽度。

如果发送器需要，它又可发送另一个字符。

At the receiving end of an asynchronous serial data link, the receiver continually monitors the

link looking for a start bit. Once the start bit has been detected, the receiver waits until the end of

the start bit and then samples the next N bits at their centers, using a clock generated locally by the

receiver. As each incoming bit is sampled, it is used to construct a new character. When the

received character has been assembled, its parity is calculated and compared with the received

parity bit following the character. If they are not equal, a parity error flag is set to indicate a

transmission error.

在异步串行数据链路系统的接收端，接收器持续监视着线路。搜索着起始位。一旦检测

到起始位并等到它结束，接收器就对随后的 N 个码元抽样，抽样点选择这些码元的中心处。

抽样所用的时钟是由接收器本地产生的。当每一个输入的码元被抽样后，就用这些样值构成

一个新的字符。当接收到的字符汇齐后，它的奇偶校验位就由计算得出并与接收到的奇偶校

验位进行比较。如果它们不等，则将奇偶校验错误标志置位，以标明传输错误。

The most critical aspect of the system is the receiver timing. The falling edge of the start bit

triggers the receiver’s local clock, which samples each incoming bit at its nominal center. Suppose

the receiver clock waits T/2 seconds from the falling edge of the start bit and samples the

incoming data every T seconds thereafter until the stop bit has been sampled. Fig .2-2 shows this

situation. As the receiver’s clock is not synchronized with the transmitter clock, the sampling is

not exact.

对系统来说，最关键的问题是接收器的定时。接收器的本地时钟由起始位的下降沿启动，

然后在码元的标称中心处对每个输入比特进行抽样。接收器的时钟自起始位的下降沿开始等

待 T/2，而后每隔 T 秒抽样输入数据，直至抽样到停止位。图 2-2 标明的这种情况。如果接

收器时钟与发送器时钟不同步，抽样则是不准确的。

The most obvious disadvantage of asynchronous data transmission is the need for a start,

parity, and stop bit for each transmitted character. If 7 bit characters are used, the overall

efficiency is only 70%. A less obvious disadvantage is due to the character-oriented nature of the

data link. Whenever the data link connects a CRT terminal to a computer, few problems arise, as

the terminal is itself character oriented. However, if the data link is being used to, say, dump

binary data to a magnetic tape ,problems arise.

对于每一个传送的字符，异步数据传输都需要起始位、奇偶校验位和停止位，这是它最

明显的缺点。如果采用 7 比特字符，则总效率仅为 70％。一个不太明显的缺点是由于数据

链路面向字符的特性造成的。在数据链路中无论何时将 CRT 终端连接到计算机上都不会出

现什么问题，因为终端本身也是面向字符的。但是如果数据链路由于别处，比方说，将大量

二进制数据转储到磁带上，则会产生麻烦。

UNIT 3

The ISO Networking Standards

ISO 联网标准

The early development of LANs, MANs, and WANs was chaotic in many ways. The early

1980s saw tremendous increases in the numbers and sizes of networks. As companies realized

they could save money and gain productivity by using networking technology, they added

networks and expanded existing networks almost as rapidly as new network technologies and

products could be introduced.

局域网、城域网和广域网的早期发展，在许多方面是混乱无序的。80 年代初期，网络

的数量和规模发展极快。由于公司意识到，利用联网技术，它们可以省钱和提高生产率，因

而只要能引入新的网络技术和产品，它们就要扩充现有的网络和添加网络。

By the mid-1980s, these companies began to experience growing pains from all the

expansions they made. It became harder for networks that used different specifications and

implementations to communicate with each other. They realized that they needed to move away

from proprietary networking systems.

到了 80 年代中期，这些公司开始经历由于他们扩张网络所带来的不断的麻烦。采用不

同规范和实现的网络不能够相互通信。他们认识到，需要离开专用的联网系统。

Proprietary systems are privately developed, owned, and controlled. In the computer industry,

proprietary is the opposite of open. Proprietary means that one or a small group of companies

controls all usage and evolution of the technology. Open means that free usage of the technology

is available to the public.

专用系统是指那些被私人开发、拥有和控制的网络。在计算机行业，专用是开放的反义

词。专用意味着一个公司或一小群公司完全控制着技术的适用和发展。而开放却意味着技术

可为整个公众所利用。

To address the problem of different network systems being incompatible and incapable of

communicating with each other, the International Organization for Standardization (ISO)

researched network schemes, such as DECnet, SNA, and TCP/IP, to find a set of rules. As a result

of this research, the ISO created a network model that could help vendors create networks that

would be compatible with, and interoperate with, other networks.

为了弄清不同网络系统不能兼容和不能互相通信的问题，国际标准化组织（ISO）研究

了网络体系，比如 DEC 网、SNA 和 TCP/IP，以便寻找一些规律。结果，国际标准化组织创

造了一个网络模型，它能帮助供应商创建可以与其他网络互相兼容和互操作的网络。

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