Tim Wescott wrote:
> I'm trying to think of examples where using an anti-alias filter is a
> bad idea, or must at least be approached with extreme caution. I
> already know about control systems and video applications, and I believe
> that this is a big issue with EKG machines.
>
> Does anyone else have any examples that they can share?
>

I believe that this would be a consideration any time transient or
impulsive phenomena are being digitized. However, Nyquist still holds,
and if the anti-aliasing filter is interfering with your recording,
it's an indication that you aren't sampling fast enough and the filter
bandwidth needs to be wider.

Some people try to use overly-sharp (eg: Butterworth or Chebyshev)
analog AA filters to push the cutoff closer to Nyquist. These filters
have a tendency to ring, even when implemented at low orders--not good
for sampling transients.

Why would EKG recorders be especially problematic? If they are, then
maybe EEG and polygraph recorders too?

One trick that I used in process control instrumentation was to
oversample by a factor of several hundred, do the main anti-aliasing
digitally and then decimate to a more appropriate sample rate. This
means that the analog anti-aliasing filter can be very cheap and when
the sampling can be synchronized to a multiple of the power line
frequency, you can get a double whammy.