How can I find the interpolation error when I use INTERP in the Signal Processing Toolbox?

t = 0:0.001:1; % Time vector

x = sin(2*pi*30*t) + sin(2*pi*60*t);

r = 4;

y = interp(x,r);

is to use:

e = x - y(1:r:end);

"e" is the difference between the original data, "x", and the interpolated data, "y", at the original discrete times.