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Topband: Group delay distortion in receiver filters

To: <topband@contesting.com>
Subject: Topband: Group delay distortion in receiver filters
From: vze1t9xc at verizon.net (John Kaufmann)
Date: Wed Jun 25 12:33:43 2003
The discussion on group delay and phase distortion in filters needs to be 
placed in the context of what distortions are really audible.  It is well known 
in the field of psychoacoustics that the human auditory system is remarkably 
insensitive to phase distortions, particularly in random waveforms.  This is a 
complex subject, but the fact that a phase distortion is easily measured or 
observed, say on an oscilloscope, does not necessarily mean it will be audible. 
 The kinds of "bad" phase/group delay distortions associated with filter types 
such as Chebyshev are usually accompanied by distortions in frequency amplitude 
response (i.e. non-flat frequency response).  It is the amplitude response 
errors that can be quite audible.

When a phase distortion is followed by a nonlinear element, it IS possible for 
the phase distortion to be converted to an audible amplitude distortion.  This 
is a well known problem in FM broadcast receiver IF filter design and nonlinear 
modulations like FM are particularly susceptible to this.  I'm not so sure this 
applies to CW or SSB detection in amateur receivers, although it is 
theoretically possible that some receiver stage that is downstream from the IF 
filter may be sufficiently nonlinear to convert phase distortion to amplitude 
distortion.  

73, John W1FV

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