专英原文+翻译Unit10概要1.doc

Unit 10 数字音频压缩 Unit 10-1 第一部分：MP3 With the advent of the Internet, there is a desire for more and more information to be transmitted across phone lines. Audio information is one form that is increasingly downloaded, be it a sampler for a band’s album, a radio program, or sound as part of a video.As bandwidth in a telephone wire is limited, this has led to a need for information (including audio) to be compressed. 随着互联网时代的到来，希望通过电话线传输越来越多的信息。音频信息是一种愈来愈多被下载的（多媒体）形式，无论是乐队的唱片选曲，无线电节目，还是视频伴音。因为电话线的带宽有限，所以需要对信息（包括音频）进行压缩。 The traditional method of storing digital audio, used in CDs and digital TV, samples the amplitude of the sound a set number of times per second, and records this. 用于CD和数字电视中存储数字音频的传统方法是每秒抽取并记录一定次数的声音幅度值。 The precision of the amplitude is determined by the number of bits used to store the amplitude. So the bandwidth (or memory) consumed by the audio signal is dependent on three factors: the number of samples taken per second (Frequency), the number of bits used to store the amplitude (Bit Depth) and the length of the signal (Time). 幅度值的精度是由用于存储幅度的比特位数决定的。所以，音频信号消耗的带宽（或者内存）由以下三个因素决定：每秒钟的采样次数（频率），用于存储幅度的比特位数（比特深度）以及信号的长度（时间）。 When we know these three things, the memory used becomes simple to calculate: Memory = Frequency *Bit Depth *Time. Additionally, if the signal is in stereo, then this must be doubled as two signals are in fact used. 当这三个参数已知时，很容易的计算出所用内存： 内存＝频率(比特深度(时间 此外，如果信号是立体声的，内存就乘以二，因为立体声实际上用了两个信号。 This equation can be used to demonstrate why transmitting high-quality audio across the Internet requires compression. CD audio uses 16-bit stereo sampled at 44,100 Hz. This means that one minute of CD audio uses 44,100?16?60?2 = 84,672,000 bits, or slightly over 10 megabytes. 这个等式能用来说明为什么在互联网上传输高品质音频信号时需要压缩。CD音频采用44,100 Hz采样率的16比特立体声。这就意味着1分钟的音频信号需要使用44,100(16(60(2 = 84,672,000比特，或略超过10兆字节。 A standard 56 kbps modem would take 84,672,000/57344 = 1477 seconds or about 25 minutes! 25 minutes is a long time to wa