Raymond Toy wrote:
>>>>>>"Tim" == Tim Wescott <[email protected]> writes:
>
>
> Tim> Raymond Toy wrote:
>
> >>>>>>> "Tim" == Tim Wescott <[email protected]> writes:
> Tim> I'm trying to think of examples where using an anti-alias
> >> filter is a
> Tim> bad idea, or must at least be approached with extreme caution. I
> Tim> already know about control systems and video applications, and I
> Tim> believe that this is a big issue with EKG machines.
> >> Isn't there always some kind of anti-alias filter, whether you want
> >> it
> >> or not? Or are you saying that aliasing is better than whatever
> >> artifacts an anti-aliasing filter would produce?
> >> Just curious,
> >> Ray
>
> Tim> There's always some sort of filtering going on, because no system has
> Tim> infinite bandwidth. I _am_ saying, however, that in many instances
> Tim> (such as EKG strips, video, or control systems) aliasing is better
> Tim> than the artifacts that you'd get from a filter that you could
> Tim> honestly call "anti-aliasing".
>
> I think I understand what you're saying. Basically, you have some
> kind of a priori knowledge that the signal is bandlimited and you're
> sampling at a high enough frequency that aliasing isn't there. No
> problem with that.
>
> But that does bring up the question of how you got the a priori
> knowledge.
>
> But unless you do the experiment, it's kind of hard to tell if
> anti-aliasing filter effects are worse than aliasing effects. Maybe
> those EKG strips are mostly aliasing artifacts. :-)
>
> Ray
I would contend that you need a priori knowledge about what you're going
to process, no matter what the problem space is. The less a priori
knowledge you have about your signal, the more capable -- and big, and
power hungry, and expensive, your signal processing equipment is going
to have to be.
So you always start with some a priori knowledge or at least
assumptions. If you're doing basic research you'll be inclined to get
the biggest bestest fastest equipment, but eventually you'll know enough
that you'll be wanting to build light, inexpensive stuff (itty bitty
battery powered EKG recorders, for example).
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
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