Matlab , upsampling sin waves

1-sin.jpg

2-upsample_2.jpg

3-upsample_4.jpg

4-upsample_8.jpg

5-duplicate_4.jpg

upsample.m

%-- 03/15/2013 04:50:02 PM --%
fs = 11025;
r = 1/fs;
x = [0:r:1];
y = sin(2 * pi * 400 * x);
NFFT = 2^nextpow2(11025);
Y = fft(y,NFFT)/11025;
f = fs/2*linspace(0,1,NFFT/2+1);

plot(f,2*abs(Y(1:NFFT/2+1)))
u2 = upsample(y, 2);
U2 = fft(u2,NFFT)/11025;
plot(f,2*abs(U2(1:NFFT/2+1)))
u4 = upsample(y, 4);
U4 = fft(u4,NFFT)/11025;
plot(f,2*abs(U4(1:NFFT/2+1)))
u8 = upsample(y, 8);
U8 = fft(u8,NFFT)/11025;
plot(f,2*abs(U8(1:NFFT/2+1)))