Whoa ... reset. I had thought I had not considered Harold's point below,
but I previously had (honest!). Even if one is not going to be
reconstructing the original signal from its associated samples, one
should still include a low pass filter if only to help limit noise which
might enter the system.

Mario

Mario Thomaidis wrote:
>
> Harold's point is a good one. I will just add that the explicit reason
> why one may choose not to implement an anti-aliasing filter in such a
> case is because one would have no intention of reconstructing the
> orginal signal. After all, the reconstruction of the original signal was
> the context in which Nyquist devised the sampling criterion.
>
> Mario
>
> Harold M Hallikainen wrote:
> >
> > On Wed, 22 Sep 1999 02:07:23 -0400 Mario Thomaidis
> > <mario.thomaidis@UTORONTO.CA> writes:
> >
> > >
> > >*** Exception to 2x rule *** (see below)
> > >
> > >The frequencies that one would want to remove in Harold's system would
> > >be those create from the turning of the pots - probably at most upto a
> > >hundred Hz (instantaneously) (i.e. at what frequency can you swing the
> > >wiper value from rail to rail?).
> > >
> >
> >         And, since the sampling rage is much higher than several hundred
> > per second, it appears that aliasing is not a problem (no need to filter
> > out stuff above Nyquist frequency, since it's not there to begin with).
> > Further, what is the effect of aliasing when one is merely reading the
> > position of a pot?  The A/D value is still correct (corresponding to the
> > pot position), but we do not know what happened between the samples.  The
> > pot COULD have been moved up and down a bunch of times between samples,
> > and we only caught it being moved down (aliasing the true high frequency
> > down into a lower frequency due to sampling), but... who cares?  We just
> > want to know where the pot is every 20ms or so.
> >         So, in my opinion, when just using an A/D to read the position of
> > a pot, no anti-alias filter is required.
> >
> > Harold
> >
> > Harold Hallikainen
> > harold@hallikainen.com
> > Hallikainen & Friends, Inc.
> > See the FCC Rules at http://hallikainen.com/FccRules and comments filed
> > in LPFM proceeding at http://hallikainen.com/lpfm
> >
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