Re: Low Pass Filter Design Help



Jerry Avins wrote:

dbd wrote:
On Aug 15, 1:40 pm, glen herrmannsfeldt <g...@xxxxxxxxxxxxxxxx> wrote:
Phil wrote:
For what it's worth, a moving average *is* an FIR filter.
Is it still considered FIR when implemented with a delayed
differentiator (delay sets the window size) followed by an
integrator? I would consider this implementation as IIR.
It is IIR if implemented in floating point and FIR in
fixed point.

-- glen

The only way for the last statement to by true is to violate the basic
principles of floating point comparison.

But it is an interesting response, philosophically. This is one of the
types of responses that triggered that second failed attempt at
creating a human based source of usable information on the web: the
wiki.

Dale B. Dalrymple
http://dbdimages.com

Glen is right, though in a non-obvious way. A floating-point box-car
isn't necessarily completely stable. (It's stable IFF the exponent of
the running sum never changes.) The filter's impulse response becomes
infinite as soon as the implied "empty" value begins its random walk.

Maybe that's true on some other planet for some other problem. But for
the problem the OP presented he is working with relatively small
integers and a relatively small boxcar. So there should be (barring
error) no danger that using floating point will produce an output result
that is not the very same set of integers that a fixed point
implementation would produce.

-jim




Jerry
--
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