用fft进行谱分析

n=8;n=0:n-1;
x4n=cos(pi*n/4);
x5n=cos(pi*n/4)+cos(pi*n/8);
x4k8=fft(x4n,8);
x5k8=fft(x5n,8);
N=16;n=0:N-1;
x4n=cos(pi*n/4);
x5n=cos(pi*n/4)+cos(pi*n/8);
x4k16=fft(x4n,16);
x5k16=fft(x5n,16);
figure(3)
subplot(2,2,1);mstem(x4k8);
title('x4a 8点FFT');xlabel('w/pi');ylabel('幅度');
axis([0,2,0,1.2*max(abs(x4k8))]);
subplot(2,2,2);mstem(x4k16);
title('x4a 16点FFT');xlabel('w/pi');ylabel('幅度');
axis([0,2,0,1.2*max(abs(x4k16))]);
subplot(2,2,3);mstem(x5k8);
title('x5a 8点FFT');xlabel('w/pi');ylabel('幅度');
axis([0,2,0,1.2*max(abs(x5k8))]);
subplot(2,2,4);mstem(x5k16);
title('x5a 16点FFT');xlabel('w/pi');ylabel('幅度');
axis([0,2,0,1.2*max(abs(x5k16))]);

function mstem(xk)
m=length(xk);
k=0:m-1;wk=2*k/m;
stem(wk,abs(xk),'.');
box on;
xlabel('w/pi');ylabel('fudu');