sat=zeros(1,N);
sat(4501:6502)=1;

sbt=zeros(1,N);
sbt(4501:5501)=1;
sbt(5502:6502)=-1;

figure
plot(t,sat)
axis([-5 5 -1.2 1.2])
xlabel('t')
ylabel('x')
title('s_a(1,t)')

figure
plot(t,-sat)
axis([-5 5 -1.2 1.2])
xlabel('t')
ylabel('x')
title('s_a(0,t)')

figure
plot(t,sbt)
axis([-5 5 -1.2 1.2])
xlabel('t')
ylabel('x')
title('s_b(1,t)')

figure
plot(t,-sbt)
axis([-5 5 -1.2 1.2])
xlabel('t')
ylabel('x')
title('s_b(0,t)')
