如果信号是立体声的，内存就 如果信号是立体声的， 乘以二， 乘以二，因为立体声实际上用 了两个信号
8
3
This equation can be used to demonstrate why transmitting highhigh-quality audio across the Internet requires compression. CD audio uses 16-bit stereo sampled at 44,100 Hz. This means 16that one minute of CD audio uses 44,100×16×60×2 = 44,100×16×60× 84,672,000 bits, or slightly over 10 megabytes.
13
6
This replaces all bitcodes with unique bitcodes of varying length according to the frequency of the pattern occurrence.4 For instance, the most commonly occurred bit pattern would be encoded as ‘01’, while the next common would be ‘010’ and the next would be ‘011’, and so on.
9
3
A standard 56 kbps modem would take 84,672,000/57344 = 1477 seconds or about 25 minutes! 25 minutes is a long time to wait for one minute of audio, so an alternative is imperative. imperative. That alternative was MPEG Audio Layer 3, or MP3.
11
5
A psycho-acoustic model is then applied to the sound. If a psychohighhigh-pitched sound is played, then the decibel threshold for sounds of lower frequencies to be made audible is increased.3 The psycho-acoustic model removes any and all sounds that psychowill be ‘masked’ in this way.
5
2
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.2
3
New Words
multitude 多数，大众 多数， unsigned band 未签约的乐队 rip 撕，拉，劈 clout 权力，影响，力量 权力，影响， advocate 提倡者，拥护者 提倡者， rival 对手 a multitude of …种种的，众多的 …种种的 种种的， promote 推销，促进 推销， royalty 版税，庄严，王权 版税，庄严， spell 拼写，迷住 拼写， stem 阻止，堵住 阻止， lax 宽松的，不严格的 宽松的，
4
1
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.1 As bandwidth in a telephone wire is limited, this has led to a need for information (including audio) to be compressed. 音频信息是一种愈来愈多被下载的 多媒体）形式， （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。 选曲，无线电节目，还是视频伴音。
信息科学与电子工程专业英语
Technical English
For Information Science and Electronic Engineering
信息科学与电子工程专业英语
Unit 10
Digital Audio Compression
信息科学与电子工程专业英语
Part I
MPEG Audio Layer 3
16
7
Some mainstream acts have also taken to distributing some free tracks in order to promote albums. There is, unfortunately, a darker side to this revolution. Illegal sites distribute tracks illegally ‘ripped’ from the albums of established artists, who subsequently lose out on royalties.6
等待1分钟的音频要25分钟这么长的 等待1分钟的音频要25分钟这么长的 时间， 时间，因此必须有另一个选择
10
4 The codec
The human ear can only hear a limited range of frequencies. The codec therefore removes all sounds outside this range, as they will not be heard anyway.
在播放音调高的声音时，要提高 在播放音调高的声音时， 使低频声能被听到的临界分贝数 分贝阈值）。 （分贝阈值）。 Nhomakorabea12
6
The next stage is Joint Stereo. The human brain is unable to place the directions of sounds at low frequencies, so sounds below this threshold are encoded in mono. If some sections of mono. the signal are still above the required bitrate, then the quality of these sections will be decreased. Finally, Huffman encoding is applied. 人脑无法估计低频声音的方向， 人脑无法估计低频声音的方向， 所以这个阈值之下的声音都用单 声道编码
7
2
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. used.
New Words
advent 到来，来临 到来， album 照相簿，歌曲集 照相簿， precision 精度，精确的 精度， megabyte 兆字节 imperative 势在必行的 pitch 音调，程度 音调， mono 单一，单声道 单一， Huffman encoding Huffman编码 Huffman编码 underestimate 低估 mainstream 主流 sampler 样品 amplitude 幅度，广阔 幅度， stereo 立体声 modem 调制解调器 psychopsycho-acoustic 心理声学的 threshold 门限，阈值 门限， bitrate 比特率 bitcode 比特字，比特码 比特字， minority 少数 genre 流派，类型 流派，
用于CD和数字电视中存储数字音频 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。 次数的声音幅度值。
6
2
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). 每秒钟的采样次数（频率），用于 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） 存储幅度的比特位数（比特深度） 以及信号的长度（时间） 以及信号的长度（时间）