Xref: utzoo comp.dsp:1641 sci.electronics:20053
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!bionet!raven.alaska.edu!milton!whit
From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: comp.dsp,sci.electronics
Subject: Re: 180 deg phase shift
Message-ID: <1991May10.003817.5593@milton.u.washington.edu>
Date: 10 May 91 00:38:17 GMT
References: <1991May5.233533.18783@ux1.cso.uiuc.edu> <5781@media-lab.media.mit.edu.MEDIA.MIT.EDU> <1991May8.222501.19572@syd.dms.CSIRO.AU>
Organization: University of Washington, Seattle
Lines: 28

In article <1991May8.222501.19572@syd.dms.CSIRO.AU> steveq@syd.dms.CSIRO.AU (Stephen Quigg) writes:
>In article <5781@media-lab.media.mit.edu.MEDIA.MIT.EDU> cas@media-lab.media.mit.edu (Scud) writes:
>>In article <1991May5.233533.18783@ux1.cso.uiuc.edu> frerichs@adsl.ece.uiuc.edu (dfRERICHS) writes:
>>>Does anyone know the algorithm that would take a stream of sampled sound
>>>,shift it 180 deg in phase and spit it back out again?
>>
>>Use an op-amp as as inverter (ie with a gain of -1). 

> Inverting is NOT the same as 180 deg phase shift. For a symetric waveform
>(eg a sine wave) it looks the same, but with something assymetric what you
>will see is the waveform upside-down, which is not the same as shifted 180
>deg. Phase shifting moves a waveform along the time axis: it stays the same
>way up. Try it on an oscilloscope.


	That's wrong.  The inverter is a perfectly good 180 degree
phase shifter, and if you test it at ANY frequency you will see
180 degrees of phase shift; the phase shift versus frequency is
EXACTLY what was asked for.
	Your 'assymmetric waveform' has a lot of Fourier components,
and time-shifting it as you seem to be describing is NOT a
well-defined operation.  You have to find some particular frequency,
derive a time delay from THAT ONE FREQUENCY COMPONENT, and apply
that time delay to get the time-shift, and that is NOT the correct
phase shift for any frequency component except the one you
chose as 'most significant'.

	John Whitmore