IIR and FIR filters
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From: fisico32 on 11 Nov 2009 12:08 Hello Forum, what is the main difference between infinite impulse response and finite impulse response filters? Clearly, in real life, all filters should have a finite response: after a while the response must die off. In digital processing, a FIR is also necessary since we deal with finite amounts of data.... So, what is the point of a IIR filter? Any example? Is it just an analog theoretical concept? Surely a Gaussian dies off to zero only at infinity by the area under it is finite..... thanks fisico32
From: Jason on 11 Nov 2009 12:20 On Nov 11, 12:08 pm, "fisico32" <marcoscipio...(a)gmail.com> wrote: > Hello Forum, > > what is the main difference between infinite impulse response and finite > impulse response filters? > Clearly, in real life, all filters should have a finite response: after a > while the response must die off. > > In digital processing, a FIR is also necessary since we deal with finite > amounts of data.... > > So, what is the point of a IIR filter? Any example? Is it just an analog > theoretical concept? Surely a Gaussian dies off to zero only at infinity by > the area under it is finite..... > > thanks > fisico32 IIR filters do have an impulse response that is infinite in duration. You're right that most IIR filters have an impulse response that decays to zero over time, and at some point, the response will decay below 1 LSB of precision if you're using fixed point. From an engineering perspective, for these examples and other "analog" situations, there often comes a point in time where the response decays to a point where it is negligible, although they are mathematically modeled as being infinite in duration. However, there are some IIR filters that don't decay to zero; consider a system with a pole on the unit circle; its impulse response is a sinusoid, whose frequency depends on where the pole lies. Jason
From: Darol Klawetter on 11 Nov 2009 12:29 On Nov 11, 11:08 am, "fisico32" <marcoscipio...(a)gmail.com> wrote: > Hello Forum, > > what is the main difference between infinite impulse response and finite > impulse response filters? > Clearly, in real life, all filters should have a finite response: after a > while the response must die off. > > In digital processing, a FIR is also necessary since we deal with finite > amounts of data.... > > So, what is the point of a IIR filter? Any example? Is it just an analog > theoretical concept? Surely a Gaussian dies off to zero only at infinity by > the area under it is finite..... > > thanks > fisico32 IIR filters are not mere theory. They are often used when linear phase response is not required and the desired magnitude response would require a much longer FIR filter than IIR filter. Darol Klaweter
From: Jerry Avins on 11 Nov 2009 12:33 fisico32 wrote: > Hello Forum, > > what is the main difference between infinite impulse response and finite > impulse response filters? > Clearly, in real life, all filters should have a finite response: after a > while the response must die off. > > In digital processing, a FIR is also necessary since we deal with finite > amounts of data.... > > So, what is the point of a IIR filter? Any example? Is it just an analog > theoretical concept? Surely a Gaussian dies off to zero only at infinity by > the area under it is finite..... The impulse response of a filter doesn't determine how much data can be run through it. You ought to learn what these structures are, rather than asking about differences between things you don't understand. Read up about them on http://dspguide.com/, Wikipadia, and http://www.bores.com/courses/intro/index.htm Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: Randy Yates on 11 Nov 2009 13:53
"fisico32" <marcoscipioni1(a)gmail.com> writes: > Hello Forum, > > what is the main difference between infinite impulse response and finite > impulse response filters? > Clearly, in real life, all filters should have a finite response: after a > while the response must die off. > > In digital processing, a FIR is also necessary since we deal with finite > amounts of data.... > > So, what is the point of a IIR filter? Any example? Is it just an analog > theoretical concept? Surely a Gaussian dies off to zero only at infinity by > the area under it is finite..... First you should separate the real-world from theory. There are no infinitely-precise real numbers in the real world either - does that mean they're not useful? They are significantly different, besides the impulse response. IIR has only poles (except for zeros at 0), while FIR has only zeros (except for poles at 0). Also an IIR is inherently based on feedback, while an FIR is not (necessarily). -- Randy Yates % "And all you had to say Digital Signal Labs % was that you were mailto://yates(a)ieee.org % gonna stay." http://www.digitalsignallabs.com % Getting To The Point', *Balance of Power*, ELO |