why all-pass filter introducing inaudiable magnitude distortion (hmalik1 )

Subject: why all-pass filter introducing inaudiable magnitude distortion
From:    hmalik1  <hmalik1(at)UIC.EDU>
Date:    Thu, 6 Nov 2003 03:20:49 -0600

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 = +/- phi). 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. Regards Hafiz M.A. Malik, Multimedia Systems LAB. Elecrical & Computer Engineering Dept, UIC, Chicago, IL, 60607 1(312)4135499 1(312)9966007 hmalik1(at)uic.edu hmalik(at)uic.edu http//multimedia.ece.uic.edu/aslam

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