IIR and FIR filters
in [DSP]
|
Prev: Want to learn DSP
Next: Wav File
From: Randy Yates on 11 Nov 2009 13:57 Randy Yates <yates(a)ieee.org> writes: > [...] > IIR has only poles (except for zeros at 0), Wups. This was wrong - too quick of a response. An IIR has non-trivial poles (it could have non-trivial zeros as well). -- Randy Yates % "She tells me that she likes me very much, Digital Signal Labs % but when I try to touch, she makes it mailto://yates(a)ieee.org % all too clear." http://www.digitalsignallabs.com % 'Yours Truly, 2095', *Time*, ELO
From: Tim Wescott on 11 Nov 2009 14:27 On Wed, 11 Nov 2009 11:08:34 -0600, 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. Clearly, in real life, some filters should _not_ have a finite response: the integrator in a PID controller needs to have a sustained, non-zero response to a zero input to hold the process output at a desired DC value even when process needs a non-zero input to do so. > In digital processing, a FIR is also necessary since we deal with finite > amounts of data.... In digital processing we often deal with data that is practically infinite. Consider a digital cell phone, which uses DSP to decode the data that's turned into the sounds that you hear. Do you want a phone that stops working any time it gets 5 minutes into a call? 10 minutes? One hour? No, you don't want it to _ever_ stop because 'the input data is finite'. You want it to keep going, and going and going. FIR filters are _handy_ for a variety of reasons (and are used heavily in a cell phone type demod application), but not because of that. > So, what is the point of a IIR filter? To provide and infinitely long response where it is needed, or to provide desired filtering where the phase vs. amplitude characteristics of an IIR filter are negligible or desirable. > Any example? PID controllers. Any place where filtering with a cut-off frequency much lower than the sample rate is desired, and same phase distortion can be tolerated (or, in the case of a control loop, where a minimum phase filter is necessary). > Is it just an analog theoretical concept? No. The world is full of real systems that have infinitely long responses to real events. Fire, flood, plagues and drought are hardly remote theoretical concepts when you're caught up in one. > Surely a Gaussian dies off to zero only at infinity > by the area under it is finite..... Yea, so? -- www.wescottdesign.com
From: Chris Bore on 13 Nov 2009 12:54
On Nov 11, 5:08 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..... 1) An IIR filter can achieve, for example, a given frequency domain cutoff with less computation (coefficients) than an FIR. 2) An IIR filter can readily duplicate the response of an analog filter - and so for example may be used where such a filter is required either according to a standard or to duplicate the performance of an analog system that is being replaced by a digital implementation. 3) An IIR filter can have an impulse response that is infinite, which is useful when you want the impusle response to be infinite (for example to build an oscillator). Chris =========== Chris Bore BORES Signal Processing www.bores.com > > thanks > fisico32 |