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Re: freqz question (clarified)

From: Julius Smith
Subject: Re: freqz question (clarified)
Date: Thu, 17 Sep 2009 15:47:03 -0600

I agree that freqz should simply plot the magnitude and phase response of the filter. Since the arguments to freqz are merely filter coefficients B and A, there is no guarantee that the concepts of "pass band" or "stop band" make any sense for the given filter. For example, what if B/A is an allpass filter?

We ARE guaranteed, on the other hand, that every filter B/A (with A(1)==1) has a frequency response, and the most commonly desired frequency-response display, in my experience, is magnitude and phase versus frequency.

I would be happy implement this change (magnitude in dB, phase in degrees), but freqz appears to be in the base Octave distribution, while I can only change things in octave-forge, as far as I know.

- jos

At 02:03 PM 9/12/2009, Scott.Raevsky wrote:
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Just to clarify my previous question, Matlab's output of freqz is as follows:

freqz(b,a,...) plots the magnitude and unwrapped phase of the frequency response of the filter. The plot is displayed in the current figure window.

Why does Octave's freqz not plot magnitude and phase? Instead it plots 1) pass band, 2) stop band, and 3) phase. Plotting a separate pass band and stop band doesn't make sense to me.


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Julius O. Smith III <address@hidden>
Prof. of Music and Assoc. Prof. (by courtesy) of Electrical Engineering
CCRMA, Stanford University

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