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1、1,Unit 14 Pulse-Code Modulation,脉冲编码器,2,一 NEW WORDS,1. pulse n. 脉冲 2. modulation n. 调制 3. sequence n. 顺序，序列 4. impair v. 削弱 5. represent vt. 代表 6. rectangular adj. 矩形的 7. code v. 编码 n. 代码 8. discrete adj. 不连续的，离散的 9. amplitude n. 振幅 10. sample vt. 取样，采样 11. quantize v. 量化 12. encode v. 编码 13. transm
2、itter n. 发射机 14. reconstruction n. 重建，改造 15. distortion n. 失真 16. route n. 线路，路程 17. decode vt. 解码，译解 18. filter vt. 过滤,3,1. Pulse-Code Modulatio（PCM） 脉冲编码调制 2. digital pulse 数字脉冲 3. analog-to-digital (A/D) converter 模/数转换器 4. rectangular pulse 矩形脉冲 5. cutoff frequency 截止频率 6. low-pass-filter 低通滤波器,
3、二 Technical terms:,4,三 Expressions:,a sequence of 一系列的 a train of 一连串的 lie in 在于 by means of 借助于 a chain of 一连串的 be equal to 等于，与相等,5,四. Questions for text discussion:,1. what is “PCM” short for ? 2. what are the basic operational performed in the transmitter of PCM? 3. what are the basic operations
4、 performed in the PCM?,6,The first paragraph: Pulse-Code Modulation (PCM) is the most basic form of digital pulse modulation. In pulse-code modulation, a message signal is represented by a sequence of coded pulses, which is accomplished the signal in discrete form in both time and amplitude. The bas
5、ic operation performed in the transmitter (Fig. 16) of PCM are sampling, quantizing, and encoding, analog-to-digital converter. The basic operations in the receiver (Fig. 17) are regeneration of impaired signals, decoding, and reconstruction of the train of quantized samples. In what follows, we des
6、cribe the various operation that constitute a basic PCM system. 译：脉冲编码调制（PCM）是数字脉冲调制的最基本形式。在脉冲编码调 制中，消息信号用一串编码脉冲来表示，这些编码脉冲由在时间上和幅度上都离散的信号组成。PCM发射机（图16）中的基本操作有取样、量化和编码，量化和编码通常在同一电路中完成，该电路称为模/数转换器。,五 Learn the text:,7,接收机（图17）中的基本操作包括衰减信号的再生、译码和量化了的取样脉冲的还原，下面让我们来描述一下组成基本PCM系统的不同操作。,8,接收机（图17）中的基本操作包括衰
7、减信号的再生、译码和量化了的取样脉冲的还原，下面让我们来描述一下组成基本PCM系统的不同操作,9,Sample The incoming message signal is sampled with a train of narrow rectangular pulses. Quantization The sampled version of the message signal is then quantized, thereby providing a new representation of the signal that is discrete in both time and a
8、mplitude. 译：取样。输入的信息信号由一组窄的矩形脉冲进行取样。 量化。消息信号取样后进而被量化，因此产生了新的信号形式，即时间上和幅度上都离散的信号,10,Encoding Any plan for representing each of this discrete set of sample values as a group of particular binary numbers is called edcoding. Regeneration The most important feature of PCM systems lies in the ability to c
9、ontrol the effects of distortion and noise produced by transmitting a PCM signal by means of a chain of regenerative repeaters located sufficiently close spacing along the transmission route.,注： 这里的 to control the effects a channel 为动词不定式短语作状语修饰 the ability ， 而不定式中的 produced by为分词短语作定语修饰前面的 distorti
10、on and noise。,11,译：编码。将一系列分立的取样值表示成一组特定的二进制代码的方法称为编码 再生。PCM系统最重要的特点在于它能够控制在信道内传送PCM信号时产生的失真和干扰噪声的影响。这种功能的实现是通过系列再生中继器重组PCM信号，这些再生中继器位于传输路径上，间隔紧凑。,Decoding The decoding process involves generating a pulse, the amplitude of which is the linear sum of all the pulses in the code word, with each pulse be
11、ing weighted by its place value (20，21,2R-1) in the code, where R is the number of bits per sample.,12,Filtering The final operational in the receiver is to recover the message signal by passing the decoder output through a low-pass filter whose cutoff frequency is equal to the message bandwidth W. 译：译码。译码过程中涉及产生一个脉冲，其幅度是所有编码脉冲的线性总和，每一个脉冲可由码中位权值(20，21,2R-1) 进行加权得到。这里R是每样一次的比特数。,注：这里的 with each pulse being weighted by its place value (20，21,2R-1) in the code 为with 的独立主格结构，在句中作状语表示一种伴随状况。,13,译： 滤波。接收机的最后一个处理是消息信号经过译码其输出后将其通过一个低通滤波器还原出消息信号，低通滤波器的截止频率与信号的带宽W相等。,
