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; 通 信 英 语;1、通信专业基础知识
2、基础英语知识;通信英语学习的三个要点:
; 重要提示:
1、网上作业提交
2、综合练习
3、课后练习
; The Principle of PCM
专业背景介绍
; 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. ;amplitude n. 幅度
represent v. 表示,代表
sequence n. 序列
binary adj. 二进制的
;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 use 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 64 kHz. ;Reexamination of our simple example shows us that the speech signal of maximum frequency 3.4 kHz has been represented by a signal of frequency 64 kHz. ;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 32 kHz. ; 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.;Digital transmission provides a powerful method for overcoming noisy environments. ;Noise can be introduced into transmission path 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. ;strike n. 打,打击
spark v.闪烁,打火球
ignition n. 点火
thermal adj. 热的
level n. 电平;It is the relationship of the true signal to the noise signal, known as the signal-to-noise
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