Hi John and all,
> 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. >>
I have no doubt even the "worst" ear among us could clearly hear problems
caused by group delay errors.
I just took a moment to measure group delay in a 250Hz wide crystal filter.
The swept pattern looks like a 45-degree tilted Z. Group delay error delta
totaled 18 mS within the -6dB attenuation points of the filter.
There is one point in the passband, near the flat top of the passband, where
a sudden spike in delay time occurs. This spike results in a 10mS time delta
in a frequency span of only a few Hz!
When a signal is near "rough" noise, such changes are absolutely
devastating. Sharp noise pulses smear out to cover weak signals, and the
rise and fall of the signal are distorted and blended with the noise, so I
never used this filter. Now I know why!!
After looking at this filter, from a receiver that really stinks on high
selectivity with weak signals when noise is present, I'm convinced filter
design is a major player in weak signal work in the presence of noise.
73 Tom
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