请大家帮我看看这个MATLAB程序怎么修改，谢谢！-经管之家官网！

这是程序原文：
function z = bigshow (varargin)
%function z = bigshow (a,b)
%function z = bigshow (a,b,tsim,timpulse)
% This function takes a ARMA process of the form:
%
% a*y_t = b*w_t
%
%and graphs a simulation, the impulse response,
%and the spectrum.
%e.g., for
%y_t = .9 y_{t-1} + w_t
%the input vectors would be:
%
%a=[1 -.9]; and b=1;
%
%The function would then be called:
%
%>> bigshow (a,b)
%a=[1 -.99]; b=1;%near random walk
%a=[1 0 0 0 -.8]; b=1;%
%a=[1 -.8]; b=[1 -.7];% muth process
%a=[1 -1.3 +.7]; b=1; %business cycle process
% Set up the input variables.
if nargin < 2 | nargin >4
error ('Error.Wrong number of input arguments.')
else
if nargin == 4
timpulse = varargin{4};
else
timpulse = 26;
end;
if nargin >= 3;
tsim = varargin{3};
else
tsim = 70;
end
a = varargin{1};
b = varargin{2};
end;
% first generate the simulation (Gaussian errors)
nn=100; % number of observations to be discarded
u=randn(tsim+nn,1);
y=filter(b,a,u);
subplot(2,2,1),
axis ([0 tsim -Inf Inf]);
plot(y(nn+1:length(y)));
title ('Simulation');
%Now generate the impulse response
subplot(2,2,2);
nn=timpulse;
y2=dimpulse(b,a,nn);
xx=[0:nn-1];% fix the x axis to start at zero
y2=y2(1:nn);
%axis ([0 timpulse -Inf Inf]);
plot(xx',y2)
axis ([0 timpulse -Inf Inf]);
title ('Impulse Response')
%Now generate the spectrum
n=256;
subplot(2,2,3);
%axis ([0 pi -1 1]);
[spect2,f2]=shownew(b,a,n);
title ('Log Spectrum')
运行后显示：??? Error using ==> bigshow
Error.Wrong number of input arguments.
请高人帮我看看吧，真的非常感谢