sin

clear;
clc;
Fs = 50000;         %sampling freq
f1 = 1000;          %signal freq

N = Fs / f1;        % samples
n = 0:N-1;          % sample number

x = sin( 2*pi*(f1/Fs)*n );

%subplot(2,1,1); plot( n/Fs, x, '-*');  % n/Fs - signal freq
%subplot(2,1,2); plot( n, x, '-*');

for i = n+1

    if ( n(i) < 30 )
        x = sin( 2*pi*(f1/Fs)*n );
    end

    if ( n(i) > 30 )
        x(i) = 1;
    end

    if ( n(i) > 40 )
        x(i) = 0;
    end

end

plot(n,x);