N=1024;
gauss=randn(N,1);
fgauss=filtr(b,a,gauss);
subplot(2,1,1);
plot(fgauss);
subplot(2,1,2);
fftg=abs(fft(fgauss));
wf=(1:1024)/512;
plot(wf,20*log10(fftg));