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From: hedstrom@sirius.UVic.CA (Brad  Hedstrom)
Newsgroups: comp.dsp
Subject: Re: Why do FIR filters always have odd tap counts?
Message-ID: <HEDSTROM.90Dec6095652@pollux.UVic.CA>
Date: 6 Dec 90 09:56:52 GMT
References: <18193@netcom.UUCP>
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Organization: University of Victoria, Victoria, BC, Canada
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In-Reply-To: mcmahan@netcom.UUCP's message of 6 Dec 90 07:02:05 GMT
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In article <18193@netcom.UUCP> mcmahan@netcom.UUCP (Dave Mc Mahan) writes:
> I have done some playing around with FIR filters for various purposes, and
> always find something peculiar.  FIR filters always have odd tap counts.
> Why is that?  I have looked in various DSP books, but can find no hint as
> to why that fact is true.  The books I have looked in, however, always use
> odd tap counts.  Can somebody give me a good, clear explaination?  I'm more
> interested in a real explaination rather than pointers to books or
> articles, but will try to find the articles if sited.

A.V. Oppenheim and R.W. Schafer, Digital Signal Processing, 1975, pg.
238.

	"We note that for the case of N [the filter length] odd, the
phase shift corresponds to an integer number of samples delay, while
for N even, the delay is an integer plus one-half sample."

This also assumes linear phase filter (the coefficients are
symmetric).
_____________________________________________________________________________
Brad Hedstrom, University of Victoria, ECE Dept.
Internet:  hedstrom@sirius.uvic.ca
UUCP:   ...!{uw-beaver,ubc-vision}!uvicctr!hedstrom