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why all-pass filter introducing inaudiable magnitude distortion

Dear Experts and Friends,
I need guidance for understanding the theory behind distortion(inaudible)
introduced by all-pass filter.....

I'm using 10-14th order all-pass filter(cascade form) with complex pole-zero
pair close to unit circle i.e.( pole at .9 and zero at 1/.9 and theta = +/-
when a frame of 128-256 audio samples passed through the all-pass filter and
power-spectrum of the output frame is estimated using
a) Moving Average signal model
b) Autoregressive signal model

this is observed that all-pass filter has introduced a local minima in the
estimated power spectrum at frequency  = phi for both models, but minima in
model (a) is stronger than model (b).

this distortion introduced by all-pass filter become more stronger as
pole-zero pair moves close to unit circle or by increasing the order of the
filter used. (simulation is done using MatLab)

I would appreciate any suggestion or reference that'll help me to understand.
this behavior .

thanks in advance.


Hafiz M.A. Malik,
Multimedia Systems LAB.
Elecrical & Computer Engineering Dept,
UIC, Chicago, IL, 60607