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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