1、New words and phrases1 analog(=analogue)n.模拟 2 quantization n.量化3 volt n.伏特4 discrete adj.离散的，不连续的 5 quantum n.量，量子,量子论 6 unipolar adj.单极的,单极性的7 polar adj.极性的,有两种相反性质的8 onoff adj.开关的,通断的 9 encoding n.编码10 encoder n.编码器11 decoder n.解码器12 likewise adv.同样地,照样地,又,也 PulseCode Modulation FundamentalsNew w
2、ords and phrases13 amplitude n.振幅14 margin n.余量,容限15 trinary adj.三倍的,三重的 16 immune adj.免疫的17 hereafter adv.今后,从此以后 18 unavoidable adj.不能避免的19 reconstructed adj.重建的,改造的20 evaluation n.计算,计值 21 span vt.横跨,跨越22 interval n.间隔,距离23 companding n.压缩扩展，压扩24 compandor n.压扩器,压缩扩展器New words and phrases25 unifo
3、rmly adv.均匀地26 compress vt.压缩27 compressor n.压缩器 (compression ratio压缩比)28 quantizer n.量化器 (uniform quantizer均匀量化器)29 equivalent adj.相等的,相当的 30 serially adv.连续地,串行地 31 reshape vt.再成形,改造 32 distortion n.失真,变形33 interconnect vt.使互相连接34 filter vt.滤波 35 entropy n.熵，平均信息量36 transition n.转变,转换 New words an
4、d phrases1 PCM(pulse code modulation)脉(冲编)码调制2 PAM(pulse amplitude modulation)(冲振)幅调制3 PDM(pulse duration modulation)宽调制4 PPM(pulse phase modulation)（冲相）位调制5 pulse train 脉冲序列 6 amount to 等于,实际上是，总计7 round off 舍入，用四舍五入化为整数8 step size 步长 quantum step size量化步长，量化级大小9 positive pulse 正脉冲 10 refer to as 称
5、作New words and phrases11 quantization error 量化误差12 peak magnitude 峰值,最高值13 overall performance 总性能 14 crest factor 振幅（波峰）因数(振幅与有效值之比)15 rms(rootmeansquare)均方根(值)16 DPCM(Differential Pulse Code Modulation)差分脉冲编码调制 ADPCM(adaptive DPCM)自适应差分脉冲编码调制17 predictive coding 预测编码18 correspond to 相应,符合19 in pra
6、ctice 在实践中,实际上20 in an attempt to 力图,试图PulseCode Modulation Fundamentals One of the most valuable modem modulation types based on transmitting samples of a message is Pulse Code Modulation(PCM).PCM is a widely different form of modulation compared with the analog pulse types such as PAM,PDM，and PPM.
7、These types require that variation of some parameter of the pulse train be a linear function of the message sample.In PCM no such welldefined relationship exists.Instead,the presence or absence(or,alternatively,the sign)of pulses in a group of pulses is made to depend,in a somewhat arbitrary manner,
8、on message samples.The characteristics of the pulse group are related to the message sample through operations called quantization and coding.Quantization and Coding Fig.3.1(a)is useful in understanding quantization.Assume some message is known to always fall in the range 0 to 7 volts as shown.As us
9、ual,samples are taken Ts seconds apart;four exact sample values are 2.1,4.7,5.2,and 2.83 volts in the figure.Quantization amounts to selecting a set of discrete voltagelevels(say eight in the example,1 volt apart)and rounding off the exact samples to the nearest discrete or quantum level.The quantum
10、 levels are separated by thequantum step size(1 volt in the example).The four quantized samples are then 2,4,5,and 3 volts.A simple form of coding is to assign a binary code to represent the quantized sample value.A binary code is a finite sequence of zeros and ones.A number N having a three digit c
11、ode k2,k1,k0may be represented by N=k2*22+k1*21+k0*20.The ks are either zero or one.Numbers between 0 and 7 may be represented by a three digit code.An example of a three digit code is shown in Fig.31(b).Each digit in the code is PulseCode Modulation FundamentalsPulseCode Modulation Fundamentalscall
12、ed a bit(after binary digit).All bits of a code taken together form a codeword.The coding process is completed by assigning a pulse to a code 1 and no pulse to a code 0.This results in unipolar PCM(on off signaling).Alternatively,a positive pulse can be assigned to a code 1 and a negative pulse to a
13、 code 0 to give polar PCM.Fig.3.1(b)illustrates unipolar and polar PCM pulses.If the various pulses are run together PulseCode Modulation Fundamentalssuch that they have no gaps,the waveform is said to be synchronous.A synchronous PCM signal may be either unipolar or polar.All these coding processes
14、 are referred to as encoding of the message.The circuit that generates the coded pulse train from quantized message samples is called an encoder.Please translate the following words and phrases into Chinese.1.overall performance 2.crest factor 3.nonlinear operation 4.inverse operation5.rms 6.PAM 7.P
15、DM 8.PPM 9.maximum magnitude 10.error intervals11.Entropy12.round off13.quantum level 14.DPCMExercises总性能总性能振幅振幅(波峰波峰)因数因数(振幅与有效值之比振幅与有效值之比)非线性运算非线性运算逆操作逆操作均方根均方根(值值)脉幅调制脉幅调制(pulse amplitude modulation)脉宽调制脉宽调制(pulse duration modulation)脉脉(冲相冲相)位调制位调制(pulse phase modulation)最大幅值最大幅值误差间隔误差间隔熵熵,平均信息量平
16、均信息量舍入，用四舍五入化为整数舍入，用四舍五入化为整数量化电平量化电平差分脉冲编码调制差分脉冲编码调制(differential pulse code modulation)Exercises.Please translate the following words and phrases into English.1.正脉冲2.脉冲编码调制3.解码器4.编码器5.量化步长6.峰值7.线性函数8.脉冲序列9.均匀量化器10.预测编码11.压扩器12.压缩比 positive pulsepulse code modulation(PCM)decoderencoderquantum step s
17、izepeak magnitudeslinear functionpulse trainuniform quantizerpredictive codingCompandorcompression ratioExercises.Fill in the blanks with the missing word(s).1.PCM is a widely different form of modulation compared with the analog pulse types such PAM,PDM,and PPM.2.The characteristics of the pulse gr
18、oup are related the message sample through operations called quantization and coding.3.The term symbol coder distingnishes this coding operation the overall source encoding process.In most cases,a variablelength code is used to represent the mapped and quantized data set.It assigns the shortest code
19、words the most frequently (occurring,occure,occurred)output values and thus reduces coding redundancy.4.Thus,a quantization error exists PCM and,as will be seen,is the Basic limitation in performance.5.Quantization and sampling produce the same result sampling and quantizing.astofromtooccuringinasEx
20、ercises6.For equal peak magnitudes,signals with large crest factors give poorer performance those with small Kcr.7.In the receiver the inverse operation is implemented an expandor restore the original message.8.The digital data is fed serially the decoder.9.the rate drops,samples are made less frequ
21、ently and step sizes increase.10.The actual compression ratio resulting this method is no more than an order of 4 1.11.In an attempt to （reduce，reducing）the number of codes sent by a PCM system,a slight variance sampling method is used by a system known Differential Pulse Code Modulation(DPCM).thanb
22、ytointoasfromreduceonasExercises.Answer the following questions according to the text.1.Why is PCM a widely different form of modulation compared with the analog pulse types such as PAM,PDM,and PPM？2.Is binary PCM the most commonly used form?And why?PCM is a widely different form of modulation compa
23、red with the analog pulsetypes such as PAM,PDM,and PPM.These types require that variation of some parameter of the pulse train be a linear function of the message sample.In PCM no such well defined relationship exists.Instead,the presence or absence(or,alternatively,the sign)of pulses in a group of
24、pulses is made to depend,in a somewhat arbitrary manner,on message samples.The characteristics of the pulse group are related to the message sample through operations called quantization and coding.Yes,binary is the most commonly used form of PCM.The most important reasons for this choice are:(1)bin
25、ary PCM is the most noise immune form and(2)binary PCM can be processed quite easily using modern digitalcircuits.Exercises3.Is the quantization error avoidable?Does it affect the performance of the PCM system?Please tell us your reasons.4.Why do we use the companding in PCM?For equal peak magnitude
26、s,signals with large crest factors give poorer performance than those with small Kcr.A process called companding can be used to lower the crest factor of a waveform and produce better performance.The action of thecompressor is to increase the rms signal value for a given peak magnitudeThe process of
27、 quantization leads to unavoidable error.Clearly,informationis lost in the process of rounding off exact sample values to the nearest quantum level.Perfect recovery of the message by the receiver is therefore impossible in PCM,even if noise is absent.Thus,a quantization error exists in PCM and is th
28、e basic limitation in performance.Exercises5.Please briefly describe the process of the PCM decoding.6.How can we reduce the number of codes sent by a PCM system?The PCM decoder reverses the process in the encoder.The digital data is fed serially into the decoder.Each one of the data bits are reshap
29、ed to remove distortions caused by the transfer along the interconnecting medium used.After shaping,they are fed into a digital to analog converter to produce the quantized samples they represent.These samples are held and filtered to recreate the original signal,f(t).In an attempt to reduce the num
30、ber of codes sent by a PCM system,a slight variance on sampling method is used by a system known asdifferential pulse code modulation(DPCM).The binary codes in this technique represent the difference between two samples rather than the samples themselves.Another way to reduce the size of binary codes is to employ predictive coding.With this method,the value of the next sample is predicted based on the value of the current sample called the predictor.
