%==================================================
% Name: caus.m
% This program is used to obviouse the relationship
% between a non-causal signal and a causal signal.
% and make non-causal signal in to causal signal 
% by multiplying the unit step function .
%==================================================
clear;
t=-4:0.01:4;
t0=input('輸入第一個t0=');
t1=input('輸入第一個t0=');
t2=input('輸入第一個t0=');
x1=exp(-abs(t));
x2=exp(-abs(t)).*u(t-t0);
x3=exp(-abs(t)).*u(t-t1);
x4=exp(-abs(t)).*u(t-t2);
clf;
subplot(4,1,1);
plot(t,x1);axis([-4,4,-0.3,1.2]);grid on;
subplot(4,1,2);
plot(t,x2);axis([-4,4,-0.3,1.2]);grid on;
subplot(4,1,3);
plot(t,x3);axis([-4,4,-0.3,1.2]);grid on;
subplot(4,1,4);
plot(t,x4);axis([-4,4,-0.3,1.2]);grid on;
%==================================================