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From: fede@ethz.UUCP (F. Bonzanigo)
Newsgroups: comp.dsp
Subject: Re: Phase compensation for IIR filters
Message-ID: <5126@ethz.UUCP>
Date: 16 Jul 90 18:48:36 GMT
References: <3504@radio.oakhill.UUCP> <1990Jul10.122121.28209@funet.fi> <3187@phred.UUCP>
Reply-To: fede@bernina.ethz.ch.UUCP (F. Bonzanigo)
Organization: ETH Zuerich, Switzerland
Lines: 31

In article <3187@phred.UUCP> james@phred.UUCP (JAMES Taylor) writes:
>
>Also interesting (and of no personal design experience) is an article:
>Clements (and Pease), "On Causal Linear Phase IIR Digital Filters",
>	IEEE Trans. ASSP, Vol 37, No. 4, Apr 1989

From the Summary (on p. 483) of the above paper:

"The main focus of this paper has been of a theoretical nature.
(Although) the nonrationality of the system described render them
unrealizable,..."

Before someone tries again to invent the perpetuum mobile, it has been
shown long ago that a causal and stable IIR filter with a rational (and
therefore realizable) transfer function cannot have a (exactly) linear
phase. A linear phase IIR filter should have its poles in reciprocal
pairs and therefore cannot be stable. The original proof can be found
in: A.G. Deczky, "General Expression for the Group Delay of Digital
Filters", Electronics Letters, Vol.5, No. 25, Dec. 11, 1969.


Federico Bonzanigo
  Electrical Engineering Dept.
  Swiss Federal Institute of Technology (ETH)
  CH-8092 Zurich, Switzerland
E-mail:      bonzanigo@nimbus.ethz.ch
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