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% dsb1.m % Matlab demonstration script for DSB-AM modulation. The message signal % is +1 for 0 < t < t0/3, -2 for t0/3 < t < 2t0/3 and zero otherwise. echo on t0=.15; % signal duration ts=0.001; % sampling interval fc=250; % carrier frequency snr=20; % SNR in dB (logarithmic) fs=1/ts; % sampling frequency df=0.3; % desired freq. resolution t=[0:ts:t0]; % time vector snr_lin=10^(snr/10); % linear SNR % message signal m=[ones(1,t0/(3*ts)),-2*ones(1,t0/(3*ts)),zeros(1,t0/(3*ts)+1)]; c=cos(2*pi*fc.*t); % carrier signal u=m.*c; % modulated signal [M,m,df1]=fftseq(m,ts,df); % Fourier transform M=M/fs; % scaling [U,u,df1]=fftseq(u,ts,df); % Fourier transform U=U/fs; % scaling [C,c,df1]=fftseq(c,ts,df); % Fourier transform f=[0:df1:df1*(length(m)-1)]-fs/2; % freq. vector signal_power=spower(u(1:length(t))); % power in modulated signal noise_power=signal_power/snr_lin; % compute noise power noise_std=sqrt(noise_power); % compute noise standard deviation noise=noise_std*randn(1,length(u)); % generate noise r=u+noise; % add noise to the modulated signal [R,r,df1]=fftseq(r,ts,df); % spectrum of the signal+noise R=R/fs; % scaling pause % Press a key to show the modulated signal power signal_power pause % Press any key to see a plot of the message clf subplot(2,2,1) plot(t,m(1:length(t))) xlabel('Time') title('The message signal') pause % Press any key to see a plot of the carrier subplot(2,2,2) plot(t,c(1:length(t))) xlabel('Time') title('The carrier') pause % Press any key to see a plot of the modulated signal subplot(2,2,3) plot(t,u(1:length(t))) xlabel('Time') title('The modulated signal') pause % Press any key to see a plots of the magnitude of the message and the % modulated signal in the frequency domain. subplot(2,1,1) plot(f,abs(fftshift(M))) xlabel('Frequency') title('Spectrum of the message signal') subplot(2,1,2) plot(f,abs(fftshift(U))) title('Spectrum of the modulated signal') xlabel('Frequency') pause % Press a key to see a noise sample subplot(2,1,1) plot(t,noise(1:length(t))) title('noise sample') xlabel('Time') pause % Press a key to see the modulated signal and noise subplot(2,1,2) plot(t,r(1:length(t))) title('Signal and noise') xlabel('Time') pause % Press a key to see the modulated signal and noise in freq. domain subplot(2,1,1) plot(f,abs(fftshift(U))) title('Signal spectrum') xlabel('Frequency') subplot(2,1,2) plot(f,abs(fftshift(R))) title('Signal and noise spectrum') xlabel('Frequency')