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1、声音是一种由物体振动引发的物理现象 Still waters run deep.流静水深流静水深,人静心深人静心深 Where there is life,there is hope。有生命必有希望。有生命必有希望 声声声声音音音音和和和和音音音音频频频频技技技技术术术术涉涉涉涉及及及及的的的的主主主主要要要要是是是是声声声声波波波波(声声声声学学学学信信信信号号号号)的的的的处处处处理理理理。编编编编码码码码、录录录录音音音音机机机机或或或或数数数数字字字字音音音音频频频频带带带带的的的的存存存存储储储储、音音音音乐乐乐乐及及及及语语语语音音音音处处处处理理理理是是是是这一领域的主要课题。这
2、一领域的主要课题。这一领域的主要课题。这一领域的主要课题。Sound Sound methodology methodology and and audio audio techniques techniques engage engage in in processing processing these these sound sound waves waves(acoustic(acoustic signals).signals).Important Important topics topics in in this this area area are are coding,codi
3、ng,storage storage on on recorders recorders or or digital digital audio audio tapes,music and speed processing.tapes,music and speed processing.声声声声音音音音是是是是由由由由物物物物体体体体的的的的振振振振动动动动产产产产生生生生的的的的，这这这这种种种种振振振振动动动动引引引引起起起起了了了了周周周周围围围围空空空空气气气气压压压压强强强强的的的的振振振振荡荡荡荡，我我我我们们们们称称称称这这这这种种种种振振振振荡荡荡荡的的的的函函函函数数数数表
4、表表表现现现现形形形形式式式式为波形为波形为波形为波形.Sound Sound is is produced produced by by the the vibration vibration of of matter.matter.During During the the vibration,vibration,pressure pressure variations variations are are created created in in the the air air surrounding surrounding it.it.The The pattern pattern
5、of of the the oscillation oscillation is is called a waveformcalled a waveformFigure 3.1:Oscillation of an air pressure wave.AmplitudeTimeAir PressureOne Period+如如如如果果果果每每每每隔隔隔隔一一一一定定定定时时时时间间间间波波波波形形形形就就就就重重重重复复复复相相相相同同同同的的的的形形形形状状状状，那那那那么么么么就就就就称称称称这这这这个个个个时时时时间间间间为为为为周周周周期期期期。一一一一般般般般然然然然声声声声音音音音的
6、的的的波波波波形形形形既既既既不不不不会会会会十十十十分分分分光光光光滑滑滑滑也也也也不不不不会会会会有有有有很很很很强强强强的的的的周周周周期期期期性性性性。与与与与非非非非周周周周期期期期性性性性的的的的声声声声音音音音相相相相比比比比，具具具具有有有有周周周周期期期期性性性性的的的的声声声声音音音音听听听听起起起起来来来来要要要要悦悦悦悦耳耳耳耳得得得得多多多多。乐乐乐乐器器器器的的的的演演演演奏奏奏奏、元元元元音音音音、鸟鸟鸟鸟鸣鸣鸣鸣及及及及风风风风声声声声都都都都是是是是周周周周期期期期性性性性的的的的声声声声音音音音，而而而而打打打打击击击击乐乐乐乐器器器器声声声声、咳嗽声、喷嚏
7、声以及流水声则是非周期性的。咳嗽声、喷嚏声以及流水声则是非周期性的。咳嗽声、喷嚏声以及流水声则是非周期性的。咳嗽声、喷嚏声以及流水声则是非周期性的。The The waveform waveform repeats repeats the the same same shape shape at at regular regular intervals intervals and and this this portion portion is is called called a a period.period.Since Since sound sound waves waves occu
8、r occur naturally,naturally,they they are are never never perfectly perfectly smooth smooth or or uniformly uniformly periodic.periodic.However,However,sounds sounds that that display display a a recognizable recognizable periodicity periodicity tend tend to to be be more more musical musical than t
9、han those those that that are are nonperiodic.nonperiodic.Examples Examples of of periodic periodic sound sound sources sources are are musical musical instruments,instruments,vowel vowel sounds,sounds,the the whistling whistling wind wind and and bird bird songs.songs.Nonperiodic Nonperiodic sound
10、sound sources sources include include unpitched unpitched percussion percussion instruments,instruments,coughs and sneezes and rushing water.coughs and sneezes and rushing water.声声声声音音音音的的的的频频频频率率率率是是是是周周周周期期期期的的的的倒倒倒倒数数数数，它它它它表表表表示示示示的的的的是是是是声声声声音音音音在在在在1 1秒秒秒秒钟钟钟钟内内内内的的的的周周周周期期期期数数数数，单单单单位位位位是是是是赫
11、赫赫赫兹兹兹兹(Hz)(Hz)。千千千千赫赫赫赫(kHz),(kHz),即即即即1000Hz,1000Hz,表表表表示示示示每每每每秒振动秒振动秒振动秒振动10001000次次次次Boo87Boo87。声音按频率可作如下划分。声音按频率可作如下划分。声音按频率可作如下划分。声音按频率可作如下划分:The frequency of a sound is the reciprocal value of the The frequency of a sound is the reciprocal value of the period;it represents the number periods
12、 in a second and period;it represents the number periods in a second and is measured in is measured in hertzhertz(Hz)or (Hz)or cycles per secondcycles per second(cps).A (cps).A convenient abbreviation,kHz(kilohertz),is used to convenient abbreviation,kHz(kilohertz),is used to indicate thousands of o
13、scillation per second:1 kHz equals indicate thousands of oscillation per second:1 kHz equals 1000 Hz Boo87.The frequency range is divided into:1000 Hz Boo87.The frequency range is divided into:次声次声次声次声 0 020Hz20Hz 人耳能听见的声音人耳能听见的声音人耳能听见的声音人耳能听见的声音 20Hz 20Hz20KHz20KHz 超声超声超声超声 20KHz 20KHz1GHz1GHz 特超声特
14、超声特超声特超声 1GHz 1GHz10THz10THz Infra-sound from 0 to 20 Hz Infra-sound from 0 to 20 Hz Human hearing frequency range from 20Hz to 20kHzHuman hearing frequency range from 20Hz to 20kHz Ultrasound from 20kHz to 1GHzUltrasound from 20kHz to 1GHz Hypersound from 1GHz to 10 THzHypersound from 1GHz to 10 TH
15、z 多多多多媒媒媒媒体体体体系系系系统统统统中中中中使使使使用用用用的的的的声声声声音音音音一一一一般般般般只只只只限限限限于于于于人人人人耳耳耳耳能能能能听听听听到到到到的的的的范范范范围围围围，我我我我们们们们把把把把人人人人耳耳耳耳所所所所能能能能听听听听见见见见的的的的声声声声音音音音为为为为音音音音频频频频(audio),(audio),将将将将这这这这个个个个范范范范围围围围内内内内的的的的声声声声波波波波称称称称为为为为声声声声学学学学信信信信号号号号(acoustic(acoustic signal)signal)。例例例例如如如如语语语语音音音音是是是是由由由由人人人人发发发
16、发出出出出的的的的声声声声学学学学信信信信号号号号，乐乐乐乐器器器器的的的的频频频频率率率率范范范范围围围围在在在在20Hz20Hz之之之之间间间间。我我我我们们们们将将将将语语语语音音音音和和和和音音音音乐乐乐乐之之之之外外外外的的的的音音音音频频频频信信信信号号号号统统统统称称称称为为为为噪音。噪音。噪音。噪音。Multimedia Multimedia systems systems typically typically make make use use of of sound sound only only within within the the frequency frequ
17、ency range range of of human human hearing.hearing.We We will will call call sound sound within within the the human human hearing hearing range range audioaudio and and the the waves waves in in this this frequency frequency range range acoustic acoustic signalssignals Boo87.Boo87.For For example,e
18、xample,speech speech is is an an acoustic acoustic signal signal produced produced by by humans;humans;music music signals signals have have a a frequency frequency range range between between 20Hz 20Hz and and 20khz.20khz.Besides Besides speech speech and and music,we denote any other audio signal
19、as music,we denote any other audio signal as noise.noise.声声声声音音音音有有有有振振振振幅幅幅幅，振振振振幅幅幅幅的的的的主主主主观观观观感感感感觉觉觉觉是是是是声声声声音音音音的的的的大大大大小小小小。声声声声音音音音的的的的振振振振幅幅幅幅大大大大小小小小取取取取决决决决于于于于空空空空气气气气压压压压力力力力波波波波距距距距平平平平均均均均值值值值(也也也也称称称称平平平平衡衡衡衡态态态态)的的的的最最最最大大大大偏移量。偏移量。偏移量。偏移量。A sound also has an A sound also has an amp
20、litudeamplitude,a property subjectively,a property subjectively heard as loudness.The amplitude of a sound is the heard as loudness.The amplitude of a sound is the measure of the displacement of the air pressure wave measure of the displacement of the air pressure wave from its mean,or quiescent sta
21、te.from its mean,or quiescent state.计算机并不直接使用连续平滑的波形来表示声音，它计算机并不直接使用连续平滑的波形来表示声音，它计算机并不直接使用连续平滑的波形来表示声音，它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，用得到的是每隔固定的时间对波形的幅值进行采样，用得到的是每隔固定的时间对波形的幅值进行采样，用得到的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。图一系列数字量来表示声音。图一系列数字量来表示声音。图一系列数字量来表示声音。图3.23.2是经过数字采样的波是经过数字采样的波是经过数
22、字采样的波是经过数字采样的波形示意图。形示意图。形示意图。形示意图。The smooth,continuous curve of a sound waveform is The smooth,continuous curve of a sound waveform is not directly represented in a computer.A computer not directly represented in a computer.A computer measures the amplitude of the waveform at regular time measures
23、the amplitude of the waveform at regular time interval to produce a series of numbers.Each of these interval to produce a series of numbers.Each of these measurements is a measurements is a samplesample.Figure 3.2 illustrates one.Figure 3.2 illustrates one period of a digitally sampled waveform.peri
24、od of a digitally sampled waveform.TimeSampleSample height+0Figure 3.2:Sampled waveform.使使使使用用用用模模模模-数数数数转转转转换换换换器器器器(ADC)(ADC)可可可可以以以以将将将将音音音音频频频频信信信信号号号号转转转转换换换换成成成成数数数数字字字字采采采采样样样样值值值值，使使使使用用用用数数数数-模模模模转转转转换换换换器器器器(DAC)(DAC)则则则则可可可可以以以以将将将将数数数数字字字字信信信信号号号号转转转转换换换换为为为为模模模模拟拟拟拟信信信信号号号号。AM79C30AAM79
25、C30A芯芯芯芯片片片片是是是是一一一一种种种种ADCADC，SPARCSPARC工工工工作作作作站站站站将将将将其其其其用用用用于于于于音音音音频频频频输输输输入入入入，而而而而对对对对于于于于音音音音频频频频输输输输出出出出，桌桌桌桌面面面面SPARCSPARC系系系系统统统统使使使使用用用用的的的的是是是是内内内内置置置置的的的的扬扬扬扬声声声声器器器器。DACDAC也也也也是是是是一一一一个个个个标标标标准准准准的的的的UNIXUNIX设设设设备备备备，例例例例如如如如SPARCSPARC服服服服务务务务器器器器6xx6xx系系系系统统统统没没没没有有有有自自自自带带带带的的的的扬扬扬
26、扬声声声声器器器器但但但但支支支支持持持持外外外外接接接接的话筒和扬声器。的话筒和扬声器。的话筒和扬声器。的话筒和扬声器。The The Mechanism Mechanism that that converts converts and and audio audio signal signal into into digital digital samples samples is is the the Analog-to-Digital Analog-to-Digital Converter Converter(ADC).(ADC).The The reverse reverse co
27、nversion conversion is is performed performed by by a a Digital-to-Analog Digital-to-Analog Converter Converter(DAC).(DAC).The The AM79C30A AM79C30A Digital Digital Subscriber Subscriber Controller Controller chip chip is is an an example example of of an an ADC ADC and and is is available available
28、 on on SPARCstationsTM.SPARCstationsTM.Desktop Desktop SPARCTMsystems SPARCTMsystems include include a a built-in built-in speaker speaker for for audio audio output.output.DAC DAC is is also also available available as as a a standard standard UNIXTMdevice.UNIXTMdevice.For For example,example,SPARC
29、server SPARCserver 6xx 6xx systems systems do do not not have have an an internal internal speaker,speaker,but support an external microphone and speaker.but support an external microphone and speaker.采采采采样样样样频频频频率率率率表表表表征征征征的的的的是是是是单单单单位位位位时时时时间间间间内内内内对对对对波波波波形形形形采采采采样样样样4410044100次次次次，这这这这似似似似乎乎乎乎
30、超超超超出出出出了了了了人人人人耳耳耳耳所所所所能能能能听听听听见见见见声声声声音音音音的的的的频频频频率率率率范范范范围围围围。但但但但根根根根据据据据NyguistNyguist采采采采样样样样定定定定律律律律，要要要要从从从从采采采采样样样样中中中中完完完完全全全全恢恢恢恢复复复复原原原原始始始始信信信信号号号号波波波波形形形形，采采采采样样样样频频频频率率率率必必必必须须须须至至至至少少少少是是是是信信信信号号号号中中中中最最最最高高高高频频频频率率率率的的的的两两两两倍倍倍倍，所所所所以以以以CDCD标标标标准准准准采采采采样样样样频频频频率率率率至至至至少少少少要要要要是是是是人人
31、人人耳耳耳耳所所所所能能能能听听听听到到到到频频频频率率率率上上上上限限限限20KHz20KHz的的的的两两两两倍倍倍倍，这这这这样样样样4410044100的的的的采样率所能表示的频率上限为采样率所能表示的频率上限为采样率所能表示的频率上限为采样率所能表示的频率上限为22050Hz22050Hz。The rate at which a continuous waveform(Figure 3.1)is sampled is called The rate at which a continuous waveform(Figure 3.1)is sampled is called the sa
32、mpling rate.Like frequencies,sampling rates are measured in Hz.the sampling rate.Like frequencies,sampling rates are measured in Hz.The CD standard sampling rate of 44100 Hz means that the waveform is The CD standard sampling rate of 44100 Hz means that the waveform is sampled 44100 times per second
33、.This seems to be above the frequency 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 range the human ear can hear.However,the bandwidth(which in this case is 20000 Hz-20Hz=19980Hz)that digitally sampled audio
34、signal can is 20000 Hz-20Hz=19980Hz)that digitally sampled audio signal can represent,is at most equal to half of the CD standard sampling rate represent,is at most equal to half of the CD standard sampling rate(44100Hz).This is an application of the Nyquist Sampling theorem.(“For(44100Hz).This is a
35、n application of the Nyquist Sampling theorem.(“For lossless digitization,the sampling rate should be at least twice the lossless digitization,the sampling rate should be at least twice the maximum frequency responses.”)Hence,a sampling rate of 44100Hz can maximum frequency responses.”)Hence,a sampl
36、ing rate of 44100Hz can only represent frequencies up to 22050Hz,a boundary much closer to that only represent frequencies up to 22050Hz,a boundary much closer to that of human hearing.of human hearing.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采样是在离散的时间点上进行的，而采
37、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。例如化。例如化。例如化。例如8 8位量化可以表示位量化可以表示位量化可以表示位量化可以表示256256个不同值，而个不同值，而个不同值，而个不同值，而CDCD质量的质量的质量的质量的1616位量化位量化位量化位量化可以表示可以表示可以表示可以表示65 53665 536个值。图个值。图个值。图个值。图3.33.3是一个是一个是一个是一个3 3位量化的示意图，
38、可以看出位量化的示意图，可以看出位量化的示意图，可以看出位量化的示意图，可以看出3 3位量化只能表示位量化只能表示位量化只能表示位量化只能表示8 8个值个值个值个值:0.75,0.5,0.25,0,0.25,0.5,0.75:0.75,0.5,0.25,0,0.25,0.5,0.75和和和和 11，因而量，因而量，因而量，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化位数越少，波形就越难辨认，还原后的声音质量也就越差化位数越少，波形就越难辨认，还原后的声音质量也就越差化位数越少，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可能除了一片嗡嗡声之外什么都
39、没有可能除了一片嗡嗡声之外什么都没有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 Just as a waveform is sampled at discrete times,the value of the sample is also discrete.The resolution or quantization of a sample is also discrete.The resolution or quantization of a sample value de
40、pends on the number of bits used in measuring sample value depends on the number of bits used in measuring the height of the waveform.An 8-bit quantization yields 256 the height of the waveform.An 8-bit quantization yields 256 possible values;16-bit CD-quality quantization results in over possible v
41、alues;16-bit CD-quality quantization results in over 65536 values.Figure 3.3 presents a 3-bit quantization.The 65536 values.Figure 3.3 presents a 3-bit quantization.The sampled waveform with a 3-bit quantization results in only eight sampled waveform with a 3-bit quantization results in only eight p
42、ossible values:.75,.5,.25,0,-.25,-.5,-.75 and 1.The shape of possible values:.75,.5,.25,0,-.25,-.5,-.75 and 1.The shape of the waveform becomes less discernible with a lowered the waveform becomes less discernible with a lowered quantization,i.e.,the lower the quantization,the lower the quantization
43、,i.e.,the lower the quantization,the lower the quality of the sound(the result might be a buzzing sound).quality of the sound(the result might be a buzzing sound).Sample0.25.5.75-.25-.5-.75Sample HeightFigure 3.3:Three-bit quantization.计计计计算算算算机机机机必必必必须须须须有有有有相相相相应应应应的的的的输输输输入入入入输输输输出出出出设设设设备备备备才才才才
44、能能能能进进进进行行行行声声声声音音音音信信信信号号号号的的的的处处处处理理理理。话话话话筒筒筒筒与与与与内内内内置置置置扬扬扬扬声声声声器器器器分分分分别别别别是是是是与与与与ADCADC和和和和DACDAC相相相相连连连连的音频输入与输出设备。的音频输入与输出设备。的音频输入与输出设备。的音频输入与输出设备。Before sound can be processed,a computer needs Before sound can be processed,a computer needs input/output devices.Microphone jacks and built-i
45、n input/output devices.Microphone jacks and built-in speakers are devices connected to an ADC and DAC,speakers are devices connected to an ADC and DAC,respectively for the input and output of audio.respectively for the input and output of audio.音频的两个重要格式参数是采样频率音频的两个重要格式参数是采样频率音频的两个重要格式参数是采样频率音频的两个重要
46、格式参数是采样频率(如每秒如每秒如每秒如每秒8012.88012.8次次次次)和和和和采样量化位数采样量化位数采样量化位数采样量化位数(如如如如8 8位量化位量化位量化位量化)。The above examples of telephone-quality and CD-quality The above examples of telephone-quality and CD-quality audio indicate that important format parameters for audio indicate that important format parameters f
47、or specification of audio are:sampling rate(e.g.,8012.8 specification of audio are:sampling rate(e.g.,8012.8 samples/second)and sample quantization(e.g.,8-bit samples/second)and sample quantization(e.g.,8-bit quantization).quantization).A MIDI interface has two different components:A MIDI interface
48、has two different components:3.2.1 MIDI3.2.1 MIDI的基本概念的基本概念的基本概念的基本概念 MIDI MIDI接口由两个不同的部分组成接口由两个不同的部分组成接口由两个不同的部分组成接口由两个不同的部分组成:一是与设备相连的硬件标准，它规定了乐器间的物理一是与设备相连的硬件标准，它规定了乐器间的物理一是与设备相连的硬件标准，它规定了乐器间的物理一是与设备相连的硬件标准，它规定了乐器间的物理连接方式，要求乐器必须带有连接方式，要求乐器必须带有连接方式，要求乐器必须带有连接方式，要求乐器必须带有MIDIMIDI端口，它还对连接端口，它还对连接端口，它
49、还对连接端口，它还对连接两个乐器的两个乐器的两个乐器的两个乐器的MIDIMIDI缆线和缆线上传输的电信号作了规定。缆线和缆线上传输的电信号作了规定。缆线和缆线上传输的电信号作了规定。缆线和缆线上传输的电信号作了规定。Hardware connects the equipment.It specifies the Hardware connects the equipment.It specifies the physical connection between musical instruments,physical connection between musical instrument
50、s,stipulates that a MIDI port is built into an instrument,stipulates that a MIDI port is built into an instrument,specifies a Midi cable(which connects two instruments)specifies a Midi cable(which connects two instruments)and deals with electronic signals that are sent over the and deals with electr