usage: y = interp(x, q [, n [, Wc]])

Upsample the signal x by a factor of q, using an order 2*q*n+1 FIR
filter. Note that q must be an integer for this rate change method.
n defaults to 4 and Wc defaults to 0.5.

Example
Generate a signal.
t=0:0.01:2; x=chirp(t,2,.5,10,'quadratic')+sin(2*pi*t*0.4);
y = interp(x(1:4:length(x)),4,4,1);   # interpolate a sub-sample
stem(t(1:121)*1000,x(1:121),"-g;Original;"); hold on;
stem(t(1:121)*1000,y(1:121),"-r;Interpolated;");
stem(t(1:4:121)*1000,x(1:4:121),"-b;Subsampled;"); hold off;

See also: decimate, resample