一个matlab里面用的很好用的小工具 是用来算出den,num以及一切zplane的

				% Program 2_4
				% Signal Smoothing by a Moving-Average Filter
				%
				R = 50;
				d = rand(R,1)-0.5;
				m = 0:1:R-1;
				s = 2*m.*(0.9.^m);
				x  = s + d';
				plot(m,d,'r-',m,s,'b--',m,x,'g:')
				xlabel('Time index n'); ylabel('Amplitude')
				legend('d[n]','s[n]','x[n]');
				pause
				M = input('Number of input samples = ');
				b = ones(M,1)/M;
				y = filter(b,1,x);
				plot(m,s,'r-',m,y,'b--')
				legend('s[n]','y[n]');
				xlabel ('Time index n');ylabel('Amplitude')