



They do interpolate - it's not linear interpolation though. You are correct
that the frequency content is not altered (more specifically, the baseband
spectrum is preserved, and so is every Nth image spectrum. The other
(N-1) in N image spectra are removed, where N is the oversampling rate)


Ideally the filter has a gain of 1 from 0Hz to 22050Hz, and a gain of 0
from 22050 to the new Nyquist frequency. In practice a finite transition
band is required, and there is also a certain amount of pass-band ripple
and stop-band leakage. With a high order (eg 200 taps) digital filter,
a very good approximation can be easily achieved.

 <<< BIT DELETED ABOUT OBSERVATIONS OF INTERPOLATION FILTER AT WORK >>>


No, they actually use a digital FIR just like yours, but built in hardware,
on custom VLSI chips for (mainly) economic reasons.



I guess that the answer is somewhere between the two.

As an interesting (?) aside, some of you out there may be aware of a scheme
used by Pioneer and Wadia, which is called (by Pioneer, at least) Legato
Link. I haven't heard one of these players, but by reading advertising
blurb, and a couple of pseudo-technical articles I deduced that all they
do is build a lousy digital oversampling filter, and let a load of the
first image (ie aliassed) spectrum through. Talking to Bob Stuart (of
Boothroyd Stuart, aka Meridian) confirmed my suspicion. He said that it
sounded awful, but then he would, wouldn't he...

Christopher