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1、Sound is a physical phenomenon produced by the vibration of matter, such as a violin string, or a block of wood. As the matter vibrates, pressure variations are created in the air surrounding it. This alteration of high and low pressure is propagated through the air in a wave-like motion. When a wav
2、e reaches the human ear, a sound is heard.,声音是一种由物体振动引发的物理现象，如小提琴的弦声等。物体的振动使其四周空气的压强产生变化，这种忽强忽弱变化以波的形式向四周传播，当被人耳所接收时，我们就听见了声音。,Sound / Audio,声音和音频技术涉及的主要是声波(声学信号)的处理。编码、录音机或数字音频带的存储、音乐及语音处理是这一领域的主要课题。 Sound methodology and audio techniques engage in processing these sound waves (acoustic signals). I
3、mportant topics in this area are coding, storage on recorders or digital audio tapes, music and speed processing.,声音是由物体的振动产生的，这种振动引起了周围空气压强的振荡，我们称这种振荡的函数表现形式为波形. Sound is produced by the vibration of matter. During the vibration, pressure variations are created in the air surrounding it. The patter
4、n of the oscillation is called a waveform,Basic sound Concepts,Figure 3.1:Oscillation of an air pressure wave.,Amplitude,Time,Air Pressure,One Period,+,声音的频率是周期的倒数，它表示的是声音在1秒钟内的周期数，单位是赫兹(Hz)。千赫(kHz),即1000Hz,表示每秒振动1000次Boo87。声音按频率可作如下划分: The frequency of a sound is the reciprocal value of the period;
5、 it represents the number periods in a second and is measured in hertz (Hz) or cycles per second (cps). A convenient abbreviation, kHz (kilohertz), is used to indicate thousands of oscillation per second: 1 kHz equals 1000 Hz Boo87. The frequency range is divided into:,Frequency,次声 020Hz 人耳能听见的声音 20
6、Hz20KHz 超声 20KHz1GHz 特超声 1GHz10THz Infra-sound from 0 to 20 Hz Human hearing frequency range from 20Hz to 20kHz Ultrasound from 20kHz to 1GHz Hypersound from 1GHz to 10 THz,多媒体系统中使用的声音一般只限于人耳能听到的范围，我们把人耳所能听见的声音为音频(audio),将这个范围内的声波称为声学信号(acoustic signal)。例如语音是由人发出的声学信号，乐器的频率范围在20Hz之间。我们将语音和音乐之外的音频信号统
7、称为噪音。 Multimedia systems typically make use of sound only within the frequency range of human hearing. We will call sound within the human hearing range audio and the waves in this frequency range acoustic signals Boo87. For example, speech is an acoustic signal produced by humans; music signals hav
8、e a frequency range between 20Hz and 20khz. Besides speech and music, we denote any other audio signal as noise.,声音有振幅，振幅的主观感觉是声音的大小。声音的振幅大小取决于空气压力波距平均值(也称平衡态)的最大偏移量。 A sound also has an amplitude, a property subjectively heard as loudness. The amplitude of a sound is the measure of the displacement
9、 of the air pressure wave from its mean, or quiescent state.,Amplitude,计算机并不直接使用连续平滑的波形来表示声音，它是每隔固定的时间对波形的幅值进行采样，用得到的一系列数字量来表示声音。图3.2是经过数字采样的波形示意图。 The smooth, continuous curve of a sound waveform is not directly represented in a computer. A computer measures the amplitude of the waveform at regular
10、 time interval to produce a series of numbers. Each of these measurements is a sample. Figure 3.2 illustrates one period of a digitally sampled waveform.,Computer Representation of Sound,Time,Sample,Sample height,+,0,Figure 3.2:Sampled waveform.,使用模-数转换器(ADC)可以将音频信号转换成数字采样值，使用数-模转换器(DAC)则可以将数字信号转换为模
11、拟信号。AM79C30A芯片是一种ADC，SPARC工作站将其用于音频输入，而对于音频输出，桌面SPARC系统使用的是内置的扬声器。DAC也是一个标准的UNIX设备，例如SPARC服务器6xx系统没有自带的扬声器但支持外接的话筒和扬声器。 The Mechanism that converts and audio signal into digital samples is the Analog-to-Digital Converter (ADC). The reverse conversion is performed by a Digital-to-Analog Converter (DA
12、C). The AM79C30A Digital Subscriber Controller chip is an example of an ADC and is available on SPARCstationsTM. Desktop SPARCTMsystems include a built-in speaker for audio output. DAC is also available as a standard UNIXTMdevice. For example, SPARCserver 6xx systems do not have an internal speaker,
13、 but support an external microphone and speaker.,采样频率表征的是单位时间内对波形采样44100次，这似乎超出了人耳所能听见声音的频率范围。但根据Nyguist采样定律，要从采样中完全恢复原始信号波形，采样频率必须至少是信号中最高频率的两倍，所以CD标准采样频率至少要是人耳所能听到频率上限20KHz的两倍，这样44100的采样率所能表示的频率上限为22050Hz。 The rate at which a continuous waveform (Figure 3.1) is sampled is called the sampling rate.
14、 Like frequencies, sampling rates are measured in Hz. The CD standard sampling rate of 44100 Hz means that the waveform is sampled 44100 times per second. This seems to be above the frequency range the human ear can hear. However, the bandwidth (which in this case is 20000 Hz-20Hz=19980Hz) that digi
15、tally sampled audio signal can represent, is at most equal to half of the CD standard sampling rate (44100Hz). This is an application of the Nyquist Sampling theorem. (“For lossless digitization, the sampling rate should be at least twice the maximum frequency responses.”) Hence, a sampling rate of
16、44100Hz can only represent frequencies up to 22050Hz, a boundary much closer to that of human hearing.,Sampling Rate,采样是在离散的时间点上进行的，而采样值本身在计算机中也是离散的。采样值的精度取决于它用多少位来表示，这就是量化。例如8位量化可以表示256个不同值，而CD质量的16位量化可以表示65 536个值。图3.3是一个3位量化的示意图，可以看出3位量化只能表示8个值:0.75,0.5,0.25,0,0.25,0.5,0.75和1，因而量化位数越少，波形就越难辨认，还原后的声音质量也就越差(可能除了一片嗡嗡声之外什么都没有) Just as a waveform is sampled at discrete times, the value of the sample is also discrete. The resol
