Re: IIR vs. FIR filters
Re: IIR vs. FIR filters
- Subject: Re: IIR vs. FIR filters
- From: Chris Rogers <email@hidden>
- Date: Tue, 10 Dec 2002 11:53:36 -0800
Hi Ian,
Yes, and Apple has a highly altivec optimized convolution function
(conv() in vecLib.framework) which is amazingly fast.
There definitely are ways to approximate the frequency response of a
given IIR filter with an FIR. One of the simplest ways, but often not the
best solution for an Nth order FIR filter is to take the first
N samples of the impulse response from the IIR filter
(these first N samples can be easily generated in C code by passing
an impulse - first sample 1.0, following samples 0.0 - through
the IIR code). Improvements can be made by windowing this result.
It may be better to forget the original IIR filter, and start with
the exact desired frequency response, since the IIR filter itself
is probably just an approximation of some ideal desired response. Then
there are mathematical FIR filter design techniques (window design,
frequency sampling, etc.) which are
described in almost any good college text-book on DSP.
Chris Rogers
Core Audio
Apple Computer
Hi Friends,
I've noticed that there is a very large difference between
calculation efficiency between IIR and FIR filters. Many simple IIR
filters cant take advantage of pipelining and are also not vectorizable.
So, take into account only one multiply-add per 5 cyles and no
vectorization, and one quickly arrives at the conclusion that you can do
20 times as many multiply-adds per unit time in the context of a FIR
filter compared to what you can do in an IIR filter! The difference is
just going to get bigger as the FPU pipelines get longer.
So, given all of that, I am a bit curious whether there are any
technologies for converting IIR filters to FIR filters with similar
frequency response.
Cheers,
Ian Ollmann
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Ian Ollmann, Ph.D. email@hidden
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