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From: karsh@trifolium.esd.sgi.com (Bruce Karsh)
Newsgroups: comp.dsp
Subject: Re: 180 deg phase shift
Message-ID: <102387@sgi.sgi.com>
Date: 8 May 91 17:17:46 GMT
References: <1644@fs1.ee.ubc.ca> <1991May7.152409.3933@njitgw.njit.edu> <1991May8.022953.781@appmag.com>
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Reply-To: karsh@trifolium.sgi.com (Bruce Karsh)
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 18

In article <1991May8.022953.781@appmag.com> todd@appmag.com (Todd Day) writes:

>First of all, 180 deg phase shift is not the same as simply inverting
>the signal.  A phase shift implies a time delay of some sort.

Sorry, but a phase shift need not imply a time delay.  The amount of delay
is called the group delay and it is the slope of the graph of the delay
function.  It's really only meaningful to think of it as a delay if the
delay function is largely a straight line.

The problem is that sin waves are periodic, so you can't really say whether
on is delayed with respect to another, or advanced.  Since the signal
is composed of the sum of sin waves, if you delay (or advance) all the
sin waves by 180 degrees, then you've just inverted all their signs.  When
you sum them all together, you get -1 times the original signal.

			Bruce Karsh
			karsh@sgi.com