[PD] Combat aliasing!

[Charles Henry]

> The interpolation, since it cannot be an ideal interpolation, may introduce
> other noises or artifacts, not aliasing as far as I can see.

There's two parts to it, aliasing (stopband) and non-flat frequency
response (passband).  Since interpolation of uniform samples is
linear, what we see in interpolation is the introduction of other
frequencies.

The intermediate stage between sampling and playback is a dirac-delta
comb which takes our original spectrum and copies it centered at n*fs
for all n.  It's an infinitely long spectrum.  The interpolation is a
linear convolution operator on the dirac-delta comb.  The distortion
we observe comes from non-flat frequency response in the passband (0
to Nyquist) and from the copied spectra above the Nyquist frequency.

Now, we hardly realize its there, because we don't represent the
intermediate stages.  We only need to get the output at a series of
discrete points, so we only need to evaluate the convolution at those
discrete points.

Chuck