《通信原理实验》课程电子教案（PPT讲稿）MATLAB与通信仿真（英文）Chapter 3 Analog-to-Digital Conversion（Pulse Amplitude Modulation、Pulse Code Modulation）

Analog-to-Digital Conversion PAM(Pulse Amplitude Modulation) PCM(Pulse Code Modulation)

Analog-to-Digital Conversion PAM(Pulse Amplitude Modulation) PCM(Pulse Code Modulation)

PAM(Pulse Amplitude Modulation) ■ Conversion of analog signal to a pulse type signal where the amplitude of signal denotes the analog information Two class of PAM signals Natural sampling (gating) ■Easier to generate a Instantaneous sampling Flat-top pulse More useful to conversion to PCM

PAM(Pulse Amplitude Modulation) ◼ Conversion of analog signal to a pulse type signal where the amplitude of signal denotes the analog information ◼ Two class of PAM signals ◼ Natural sampling (gating) ◼ Easier to generate ◼ Instantaneous sampling ◼ Flat-top pulse ◼ More useful to conversion to PCM

PAM with natural sampling W(t) .Ws(t) S(t) Analog bilateral switch W(t) Ws(t) =W(t)S(t) Duty Cycle D=t/Ts=1/3 S(t)

PAM with natural sampling W(t) t S(t) t Ts  Duty Cycle D=/Ts=1/3 Ws(t) t W(t) S(t) Ws(t) =W(t)S(t) Analog bilateral switch

Spectrum of PAM with natural sampling Spectrum of input analog signal Iw(f)I ■Spectrum of PAM ▣D=13,fs=4B ■B=3fs=12B -BB IWs(f) D=1/3 D sinπtf πrf -3fs -2fs -fs-B B fs 2fs 3fs

Spectrum of PAM with natural sampling ◼ Spectrum of input analog signal ◼ Spectrum of PAM ◼ D=1/3, fs=4B ◼ BT= 3fs = 12B |W(f)| -B B f 1 |Ws(f)| -3fs -2fs -fs -B B fs 2fs 3fs D=1/3 sin f D f   sin ( )s n f D W f nf f    =−  −

PAM with flat-top sampling W(t) .Ws(t) S(t) Ts Sample and Hold t

PAM with flat-top sampling W(t) t S(t) t Ws(t) t Ts  Sample and Hold

Spectrum of PAM with flat-top sampling ■Spectrum of Input Iw(f)I ■Spectrum of PAM ▣xTs=1/3,fs=4B f ■B=3fs=12B -BB IWs(f) 1/72mU- D=1/3 -3fs -2fs -fs -BB fs 2fs 3fs

Spectrum of PAM with flat-top sampling ◼ Spectrum of Input ◼ Spectrum of PAM ◼ /Ts=1/3, fs=4B ◼ BT= 3fs = 12B |W(f)| -B B f 1 |Ws(f)| -3fs -2fs -fs -B B fs 2fs 3fs D=1/3 sin s f T f    1 ( ) ( )s s n H f W f nf T  =−  −

Summary of PAM Require very wide bandwidth Bad noise performance Not good for long distance transmission Provide means for converting a analog signal to PCM signal Provide means for TDM(Time Division Multiplexing) Information from different source can be interleaved to transmit all of the information over a single channel

Summary of PAM ◼ Require very wide bandwidth ◼ Bad noise performance ◼ Not good for long distance transmission ◼ Provide means for converting a analog signal to PCM signal ◼ Provide means for TDM(Time Division Multiplexing) ◼ Information from different source can be interleaved to transmit all of the information over a single channel

PCM(Pulse Code Modulation) ■ Definition PCM is essentially analog to digital conversion of a signal type where the information contained in the instantaneous samples of an analog signal is represented by digital words in a serial bit stream Analog signal is first sampled at a rate higher than Nyquist rate,and then samples are quantized Uniform PCM:Equal quantization interval Nonuniform PCM:Unequal quantization interval

PCM(Pulse Code Modulation) ◼ Definition ◼ PCM is essentially analog to digital conversion of a signal type where the information contained in the instantaneous samples of an analog signal is represented by digital words in a serial bit stream ◼ Analog signal is first sampled at a rate higher than Nyquist rate, and then samples are quantized ◼ Uniform PCM : Equal quantization interval ◼ Nonuniform PCM : Unequal quantization interval

Why PCM is so popular PCM requires much wider bandwidth ▣But, Inexpensive digital circuitry PCM signal from analog sources(audio,video,etc.)may be merged with data signals(from digital computer)and transmitted over a common high-speed digital communication system(This is TDM) Regeneration of clean PCM waveform using repeater. But,noise at the input may cause bit errors in regenerated PCM output signal The noise performance is superior than that of analog system. Further enhanced by using appropriate coding techniques

Why PCM is so popular ? ◼ PCM requires much wider bandwidth ◼ But, ◼ Inexpensive digital circuitry ◼ PCM signal from analog sources(audio, video, etc.) may be merged with data signals(from digital computer) and transmitted over a common high-speed digital communication system (This is TDM) ◼ Regeneration of clean PCM waveform using repeater. ◼ But, noise at the input may cause bit errors in regenerated PCM output signal ◼ The noise performance is superior than that of analog system. ◼ Further enhanced by using appropriate coding techniques

PCM transmitter/receiver Bandlimited Flat-top Analog LPF Analog signal Sampler PAM signal signal BW=B Hold Encoder Quantizer PCM Quantized No.of levels=M signal PAM signal Channel,Telephone lines with regenerative repeater Reconstruction Decoder PCM Quantized LPF signal PAM signal Analog Signal output

PCM transmitter/receiver LPF BW=B Sampler & Hold Quantizer No. of levels=M Encoder Analog signal Bandlimited Analog signal Flat-top PAM signal Quantized PAM signal PCM signal Channel, Telephone lines with regenerative repeater Decoder PCM signal Quantized PAM signal Reconstruction LPF Analog Signal output