Interpolation — increase sample rate by integer factor

Interpolation increases the original sample rate of a sequence to a higher rate. It is
the opposite of decimation. `interp`

inserts zeros into the original
signal and then applies a lowpass interpolating filter to the expanded sequence. The
function uses the lowpass interpolation algorithm 8.1 described in [1]:

Expand the input vector to the correct length by inserting 0s between the original data values.

Design a special symmetric FIR filter that allows the original data to pass through unchanged and interpolates to minimize the mean-square error between the interpolated points and their ideal values. The filter used by

`interp`

is the same as the filter returned by`intfilt`

.Apply the filter to the expanded input vector to produce the output.

[1] Digital Signal Processing Committee of the IEEE Acoustics,
Speech, and Signal Processing Society, eds. *Programs for Digital Signal
Processing*. New York: IEEE Press, 1979.

[2] Oetken, G., Thomas W. Parks, and H. W. Schüssler. “New results in
the design of digital interpolators.” *IEEE ^{®} Transactions on Acoustics, Speech, and Signal Processing*.
Vol. ASSP-23, No. 3, June 1975, pp. 301–309.