Topband: Group delay distortion in receiver filters

[John Kaufmann]

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