通信与电子信息工程专业英语第2单元.ppt

PCM 脉冲编码调制 Unit 2 原文：Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals, which was invented by Alec Reeves in 1937. It is the standard from digital audio in computers and various Blue-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems. 译文：1937年，Alec Reeves发明了一种用数字信号表示被抽样的模拟信号的方式，即脉冲编码调制（PCM）。对于数字电话、DVD格式以及计算机的各种蓝光压缩光盘盒数字音频而言，PCM是一种标准形式。 Unit 2 原文：A PCM stream is a digital representation of an analog signal, in which the magnitude of the analogue signal is sampled regularly at uniform intervals, with each sample being quantized to the nearest value within a range of digital steps. 译文：PCM数据流是模拟信号的数字表示，模拟信号的幅度被间隔规则抽样，每个样值被量化为数字量阶内最近的值。 Unit 2 原文：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 a sequence of 8 binary digits. 译文：PCM依赖于抽样，量化和编码三种单独的操作。 在二十年内演变出了许多不同的实现能这三种功能的方法，我们将描述最主要的方法。我们将会了解到语音信道的电话质量可以由一系列每个的幅度值来表示，如一串编码的8位二进制。 Unit 2 原文：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. 译文：进一步说，我可以证明6.8kHz序列的最小理论抽样频率能表示语音信道需占用300Hz到3.4kHz。 Unit 2 原文：Practical equipment, however, normally use a sampling rate of 8 kHz, and if 8-digit per sample value are used, the voice channel becomes represented by a stream of pulse with a repetition rate of 64 kHz. 译文：然而，实用设备通常使用的抽样频率是8kHz，如果每次抽样值是8位，语音信道将变成由速率重复的64kHz的脉冲流表示。 Unit 2 原文：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 of 64 kHz. However, if only 4-digits per sample value had been used